Methods and compositions for DNA base editing

Abstract

To provide methods and compositions for modifying a target site in the genome of a cell.SOLUTION: Provided are fusion proteins including one or more DNA binding domains and one or more heterologous domains, for example DNA modifying domains, connected by improved linker sequences. Provided are codon optimized polynucleotides encoding fusion proteins including one or more DNA binding domains and one or more heterologous domains connected by improved linker sequences.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to methods and compositions for targeted nucleotide base editing in the genome of a cell.

[0002] STATEMENT REGARDING ELECTRONIC SUBMISSION OF SEQUENCE LISTINGS This specification is accompanied by a Sequence Listing in ASCII text format entitled "81945_ST25," which was created on September 18, 2020, and submitted under 37 CFR § 1.821, of approximately 702 kilobytes, and which is hereby incorporated by reference. [Background technology]

[0003] In agriculture, the ability to edit plant genomes to create advantageous alleles is highly desirable. This has the potential to increase yield and prevent disease. Genome editing is an emerging field that has made slow progress in plants. Furthermore, modifications to the genome beyond the intended modification present challenges, limiting the application of desired modifications. CRISPR-CAS9 works by creating double-strand breaks in DNA. These breaks can be repaired by non-homologous end joining or homology-dependent repair, resulting in the insertion or deletion of DNA bases. This strategy, termed base editing, alters DNA without creating breaks to create insertions and deletions. In one version, an enzyme called cytidine deaminase targets specific bases with the CAS9 enzyme (Shimatani et al., 2017. Nat. Biotechnol. 35, 441-443) or CAS12a enzyme (Li et al., 2018. Nat. Biotechnol. 36, 324-327), which have been modified to prevent DNA cleavage. The cytidine deaminase and the nuclease-deficient CAS9 or CAS12a are fused to each other via an amino acid linker. Improved linker connections can improve the accuracy of cleavage by, for example, reducing off-target base modifications, thereby improving the function of the fusion protein. Summary of the Invention [Means for solving the problem]

[0004] To meet this need, the present inventors provide an optimized and improved Cas12a enzyme and construct. Specifically, the present inventors provide a fusion protein comprising a heterologous domain, a first linker sequence, and a type V CRISPR-Cas enzyme. The first linker sequence comprises a repeated GGGGS sequence. The heterologous domain can be a deaminase, polymerase, nuclease, relaxase, alkyltransferase, methyltransferase, adenosine deaminase, cytidine deaminase, oxidase, thymine alkyltransferase, adenine oxidase, adenosine methyltransferase, glycosylase, or nuclear localization signal. For base editing, the heterologous domain is a deaminase domain (e.g., cytidine deaminase or adenine deaminase). The cytidine deaminase domain can be an activation-induced cytidine deaminase ("AID") or apolipoprotein B mRNA editing complex ("APOBEC") domain, for example, from the APOBEC1 family of deaminases. In some circumstances, the APOBEC domain comprises a sequence at least 70% identical to SEQ ID NO: 1. In cases where an adenine deaminase is required, the adenine deaminase can be a TadA domain comprising an amino acid sequence at least 70% identical to SEQ ID NO: 92.

[0005] When the Type V CRISPR-Cas enzyme is a Type VA ("Cas12a") enzyme, the Cas12a is selected from the group consisting of SEQ ID NO:3, SEQ ID NO:6, SEQ ID NO:22, SEQ ID NO:45, SEQ ID NO:46, SEQ ID NO:47, and SEQ ID NO:48. The Cas12a domain can be catalytically inactive but still bind to the target DNA, allowing the heterologous domain to operate. When the Cas12a is inactive, the sequence is SEQ ID NO:3, SEQ ID NO:6, or SEQ ID NO:22.

[0006] The first linker sequence between the heterologous domain and the Cas12a enzyme may comprise at least three repeated GGGGS residues. In other applications, the first linker sequence may comprise at least six repeated GGGGS residues.

[0007] The fusion protein may comprise SEQ ID NO:11, 12, 13, or 44 and may also comprise a uracil DNA glycosylase inhibitor ("UGI") domain (represented by SEQ ID NO:8). The UGI domain may be linked to the Cas12a enzyme by a second linker comprising the sequence SGGS. The fusion protein may comprise SEQ ID NO:17, SEQ ID NO:24, SEQ ID NO:35, SEQ ID NO:39, SEQ ID NO:43, SEQ ID NO:50, SEQ ID NO:52, SEQ ID NO:54, SEQ ID NO:56, SEQ ID NO:81, SEQ ID NO:83, SEQ ID NO:85, SEQ ID NO:87, or SEQ ID NO:89. When contacted with DNA, these fusion proteins produce a higher frequency of on-target editing and a lower frequency of off-target editing compared to prior art fusion proteins lacking the first linker sequence of repetitive GGGGS sequences.

[0008] The inventors also provide a method of editing plant genomic DNA by contacting the plant genomic DNA with (a) a fusion protein described according to one of the above embodiments, optionally comprising a UGI domain, and (b) a guide RNA ("gRNA") that targets the fusion protein of step (a) to a target DNA sequence in the plant genomic DNA, wherein the edited plant genomic DNA has less off-target editing compared to plant genomic DNA edited with a fusion protein having a first linker other than a repetitive GGGGS sequence.

[0009] The inventors also provide a method for editing plant genomic DNA with reduced off-target editing by contacting plant genomic DNA with (a) a fusion protein described according to one of the above embodiments, optionally comprising a UGI domain, and (b) a guide RNA ("gRNA") that targets the fusion protein of step (a) to a target DNA sequence in the plant genomic DNA, wherein the edited plant genomic DNA has reduced off-target editing compared to plant genomic DNA edited with a fusion protein having a first linker other than a repetitive GGGGS sequence. In one embodiment, the fusion protein comprises SEQ ID NO:24.

[0010] The inventors also provide a method for obtaining a population of edited plants that has low off-target editing by: (a) obtaining a population of plant cells that comprises the genomic DNA to be edited; (b) obtaining a nucleotide sequence encoding a fusion protein and optional UGI domain described by one of the above embodiments; (c) transforming the population of plant cells with the nucleotide sequence of step (b), thereby expressing in the population of plant cells the fusion protein encoded by the nucleic acid sequence; (d) growing the population of transformed plant cells into plants, at least one of which plants has been edited; and (e) selecting at least one edited plant from the product of step (d), thereby obtaining a population of edited plants, wherein the population of edited plants has reduced off-target editing compared to plants edited with a fusion protein having a first linker other than a repetitive GGGGS sequence. In one aspect, the nucleotide sequence encoding the fusion protein comprises SEQ ID NO:17, SEQ ID NO:24, SEQ ID NO:35, SEQ ID NO:39, SEQ ID NO:43, SEQ ID NO:50, SEQ ID NO:52, SEQ ID NO:54, SEQ ID NO:56, SEQ ID NO:81, SEQ ID NO:83, SEQ ID NO:85, SEQ ID NO:87, or SEQ ID NO:89. [Brief explanation of the drawings]

[0011] [Figure 1]Schematic diagrams of three versions of the DNA construct for Cas12aBE are shown. (1) represents the promoter, (2) is a nuclear localization signal, (3) is a deaminase (e.g., an APOBEC deaminase), (4) is an XTEN linker, (5) is LbCas12a, (6) is an SGGS linker, (7) is a uracil glycosylase inhibitor, (8) is a long linker (e.g., a (G4S)6 linker), (9) is Mb2Cas12a, and (10) is a guide RNA coding element. Figure 1A shows the LbCas12aBE plus guide RNA construct in the 5' to 3' direction, with the deaminase (3) operably linked to LbCas12a (5) by the XTEN linker (4). Figure 1B shows the LbCas12aBE plus guide RNA construct in a 5' to 3' orientation, where the deaminase (3) is operably linked to LbCas12a (5) by a (G4S)6 linker (8). Figure 1C shows the Mb2Cas12aBE plus guide RNA construct in a 5' to 3' orientation, where the deaminase (3) is operably linked to Mb2Cas12a (9) by a (G4S)6 linker (8). [Figure 2] 1 shows a schematic diagram of a DNA construct containing a Cas12aBE and multiple guide RNAs in the 5' to 3' direction. (1) represents a promoter, (2) is a nuclear localization signal, (3) is a deaminase (e.g., APOBEC deaminase), (6) is an SGGS linker, (7) is a uracil glycosylase inhibitor, (8) is a long linker (e.g., a (G4S)6 linker), (9) is Cas12a, (10) is a first guide RNA coding element, (11) is a second guide RNA coding element, and (12) is a third guide RNA coding element. Each guide RNA coding element contains a crRNA segment and a target sequence segment that can hybridize to a genomic target DNA sequence. DETAILED DESCRIPTION OF THE INVENTION

[0012] Brief description of the sequences in the sequence listing SEQ ID NO: 1 is the amino acid sequence of Apobec1. SEQ ID NO: 2 is the nucleotide sequence of Apobec1. SEQ ID NO: 3 is the amino acid sequence of catalytically inactive Mb2Cas12a. SEQ ID NO: 4 is the nucleotide sequence of catalytically inactive Mb2Cas12a. SEQ ID NO: 5 is the nucleotide sequence of catalytically inactive cLbCas12aBE. SEQ ID NO: 6 is the amino acid sequence of catalytically inactive cLbCas12aBE. SEQ ID NO:7 is the nucleotide sequence of uracil DNA glycosylase inhibitor (UGI). SEQ ID NO: 8 is the amino acid sequence of uracil DNA glycosylase inhibitor (UGI). SEQ ID NO: 9 is the nucleotide sequence containing the expression cassette prSoUbi4:SV40NLS:cLbCas12aBE:GS6Linker:SV40NLS:SGGSLinker:UGI:SGGSLinker:SV40NLS:tNOS. SEQ ID NO: 10 is the nucleotide sequence of the optimized (G4S)x6 linker. SEQ ID NO: 11 is the amino acid sequence of the optimized (G4S)x6 linker. SEQ ID NO: 12 is the amino acid sequence of the 18 aa linker-SX. SEQ ID NO: 13 is the amino acid sequence of 15 aa linker-(G4S)×3. SEQ ID NO: 14 is the nucleotide sequence containing the fusion protein cLBCas12aBE-07 from construct 25057. SEQ ID NO: 15 is the amino acid sequence containing the fusion protein cLBCas12aBE-07 from construct 25057. SEQ ID NO: 16 is the nucleotide sequence containing the fusion protein cLBCas12aBE-08 from construct 25058. SEQ ID NO: 17 is the amino acid sequence containing the fusion protein cLBCas12aBE-08 from construct 25058. SEQ ID NO: 18 is the nucleotide sequence containing the fusion protein cLBCas12aBE-01 from construct 24524. SEQ ID NO: 19 is the amino acid sequence containing the fusion protein cLBCas12aBE-01 from construct 24524. SEQ ID NO: 20 is the nucleotide sequence of cCas9BE-02. SEQ ID NO: 21 is the amino acid sequence of cCas9BE-02. SEQ ID NO: 22 is the amino acid sequence of catalytically inactive AsCas12a. SEQ ID NO: 23 is the nucleotide sequence containing the fusion protein cLBCas12aBE-06 from construct 24904. SEQ ID NO: 24 is the amino acid sequence containing the fusion protein cLBCas12aBE-06 from construct 24904. SEQ ID NO: 25 is the nucleotide sequence containing the promoter prSoUbi4-02. SEQ ID NO: 26 is a nucleotide sequence containing the Cas12a gRNA waxy1 target sequence. SEQ ID NO: 27 is a nucleotide sequence containing the Cas9 gRNA waxy1 target sequence. SEQ ID NO: 28 is the nucleotide sequence containing exon 4 of the ZmWaxy1 gene. SEQ ID NO: 29 is the forward primer of ZmWaxy1. SEQ ID NO: 30 is the reverse primer of ZmWaxy1. SEQ ID NO: 31 is the sequencing primer for ZmWaxy1. SEQ ID NO: 32 is the nucleotide sequence containing the fusion protein cLbCpf1-02 from construct 24523. SEQ ID NO: 33 is the amino acid sequence comprising the fusion protein cLbCpf1-02 from construct 24523. SEQ ID NO: 34 is the nucleotide sequence containing the fusion protein cLbCas12a-05 from construct 25181. SEQ ID NO: 35 is the amino acid sequence containing the fusion protein cLbCas12a-05 from construct 25181. SEQ ID NO: 36 is the nucleotide sequence containing the fusion protein cLbCas12a-02 from construct 25205. SEQ ID NO: 37 is the amino acid sequence containing the fusion protein cLbCas12a-02 from construct 25205. SEQ ID NO: 38 is the nucleotide sequence containing the fusion protein cLbCas12a-25 from construct 25513. SEQ ID NO: 39 is the amino acid sequence containing the fusion protein cLbCas12a-25 from construct 25513. SEQ ID NO: 40 is the nucleotide sequence containing the fusion protein cMb2Cas12a-01 from construct 25220. SEQ ID NO: 41 is the amino acid sequence containing the fusion protein cMb2Cas12a-01 from construct 25220. SEQ ID NO: 42 is the nucleotide sequence containing the fusion protein cMb2Cas12a-02 from construct 25382. SEQ ID NO: 43 is the amino acid sequence containing the fusion protein cMb2Cas12a-02 from construct 25382. SEQ ID NO: 44 is the amino acid sequence of an optimized (G4SG)x6 linker. SEQ ID NO: 45 is the amino acid sequence of active LbCas12a. SEQ ID NO: 46 is the amino acid sequence of active Mb2Cas12a. SEQ ID NO: 47 is the amino acid sequence of active AsCas12a. SEQ ID NO: 48 is the amino acid sequence of active FnCas12a. SEQ ID NO: 49 is the nucleotide sequence containing the fusion protein cMb2Cas12a-BE-01 from construct 25457. SEQ ID NO: 50 is the amino acid sequence containing the fusion protein cMb2Cas12a-BE-01 from construct 25457. SEQ ID NO: 51 is the nucleotide sequence containing the fusion protein cLbCas12a-BE-08 from construct 25268. SEQ ID NO: 52 is the amino acid sequence containing the fusion protein cLbCas12a-BE-08 from construct 25268. SEQ ID NO: 53 is the nucleotide sequence containing the fusion protein cLbCas12a-05 from construct 25173. SEQ ID NO: 54 is the amino acid sequence containing the fusion protein cLbCas12a-05 from construct 25173. SEQ ID NO: 55 is the nucleotide sequence containing the fusion protein cLbCas12a-05 from construct 25175. SEQ ID NO: 56 is the amino acid sequence containing the fusion protein cLbCas12a-05 from construct 25175. SEQ ID NO: 57 is the amino acid sequence of catalytically inactive LbCas12a with an optimized (G4SG)6 linker. SEQ ID NO: 58 is the amino acid sequence of active Mb2Cas12a with an optimized (G4S)6 linker. SEQ ID NO: 59 is the amino acid sequence of catalytically inactive Mb2Cas12a with an XTEN linker. SEQ ID NO: 60 is the amino acid sequence of active AsCas12a with an XTEN linker. SEQ ID NO: 61 is the amino acid sequence of catalytically inactive AsCas12a with an XTEN linker. SEQ ID NO: 62 is the amino acid sequence of active FnCas12a with an XTEN linker. SEQ ID NO: 63 is the amino acid sequence of active AsCas12a with an optimized (G4S)6 linker. SEQ ID NO: 64 is the amino acid sequence of catalytically inactive AsCas12a with an optimized (G4S)6 linker. SEQ ID NO: 65 is the amino acid sequence of active FnCas12a with an optimized (G4S)6 linker. SEQ ID NO: 66 is the amino acid sequence of catalytically inactive Mb2Cas12a with an optimized (G4SG)6 linker. SEQ ID NO: 67 is the amino acid sequence of active AsCas12a with an optimized (G4SG)6 linker. SEQ ID NO: 68 is the amino acid sequence of catalytically inactive AsCas12a with an optimized (G4SG)6 linker. SEQ ID NO: 69 is the amino acid sequence of active FnCas12a with an optimized (G4SG)6 linker. SEQ ID NO: 70 is the amino acid sequence of the XTEN linker. SEQ ID NO: 71 is a nucleotide sequence containing the Cas12a gRNA SBEII target sequence. SEQ ID NO: 72 is a nucleotide sequence containing the Cas12a gRNA GL2 target sequence. SEQ ID NO: 73 is a nucleotide sequence containing the Cas12a gRNA Fad2 target sequence. SEQ ID NO: 74 is a nucleotide sequence containing the Cas12a crRNA sequence used by the waxy1 target sequence, the SBEII target sequence, and the Fad2 target sequence. SEQ ID NO: 75 is a nucleotide sequence containing the Cas12a crRNA sequence used by the GL2 target sequence. SEQ ID NO: 76 is the nucleotide sequence containing the fusion protein cCas9ABE-01 from construct 24785. SEQ ID NO: 77 is the amino acid sequence containing the fusion protein cCas9ABE-01 from construct 24785. SEQ ID NO: 78 is the nucleotide sequence containing the fusion protein cLbCas1aABE-01 from construct 25459. SEQ ID NO: 79 is the amino acid sequence containing the fusion protein cLbCas1aABE-01 from construct 25459. SEQ ID NO: 80 is the nucleotide sequence containing the fusion protein cLbCas12aABE-02 from construct 25504. SEQ ID NO: 81 is the amino acid sequence containing the fusion protein cLbCas12aABE-02 from construct 25504. SEQ ID NO: 82 is the nucleotide sequence containing the fusion protein cLbCas12aBE-09 from construct 25289. SEQ ID NO: 83 is the amino acid sequence containing the fusion protein cLbCas12aBE-09 from construct 25289. SEQ ID NO: 84 is the nucleotide sequence containing the fusion protein cdLbCas12a-ABE-CBE-01 from construct 25658. SEQ ID NO: 85 is the amino acid sequence containing the fusion protein cdLbCas12a-ABE-CBE-01 from construct 25658. SEQ ID NO: 86 is the nucleotide sequence containing the fusion protein cdLbCas12a-ABE-CBE-02 from construct 25701. SEQ ID NO: 87 is the amino acid sequence containing the fusion protein cdLbCas12a-ABE-CBE-02 from construct 25701. SEQ ID NO: 88 is the nucleotide sequence containing the fusion protein cdLbCas12a-ABE-CBE-03 from construct 25702. SEQ ID NO: 89 is the amino acid sequence containing the fusion protein cdLbCas12a-ABE-CBE-03 from construct 25702. SEQ ID NO: 90 is a nucleotide sequence containing the Cas12a gRNA ADH1 target sequence. SEQ ID NO: 91 is the nucleotide sequence containing the TadA dimer. SEQ ID NO: 92 is the amino acid sequence containing the TadA dimer.

[0013] This description is not intended to be a detailed catalog of all the different ways in which the invention may be implemented or all the features that may be added to the invention. For example, features shown in connection with one embodiment may be incorporated in other embodiments, and features shown in connection with a particular embodiment may be omitted from that embodiment. Additionally, numerous variations and additions to the various embodiments suggested herein will be apparent to those of skill in the art in light of this disclosure without departing from the invention. Accordingly, the following description is intended to illustrate some particular embodiments of the invention, but is not intended to exhaustively identify all permutations, combinations, and variations thereof.

[0014] definition Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terms used in the description of the invention herein are for the purpose of describing particular embodiments only and are not intended to limit the invention. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0015] The following definitions and methods are provided to better define the present invention and to guide those of ordinary skill in the art in the practice of the present invention. Unless otherwise specified, terms used herein should be understood according to conventional usage by those of ordinary skill in the relevant art. Definitions of common terms in molecular biology are found in Rieger et al., Glossary of Genetics: Classical and Molecular, 5 th edition, Springer-Verlag: New York, 1994.

[0016] As used herein, the term "long linker" refers to a polypeptide chain of at least 10 amino acids used to link a heterologous domain to a protein of interest. For example, without limitation, a long linker can include the sequence GGGGSGGGGSGGGGSGGGGGSGGGGGSGGGGS (SEQ ID NO: 11), otherwise designated (G4S)6, or (G4S)x6, or (G4S)*6. A long linker can include GGGGSGGGGGSGGGGGSGGGGGSGGGGGSGGGGGSGGGGGSG (SEQ ID NO: 44), otherwise designated (G4SG)6, or (G4SG)x6, or (G4SG)*6. Heterologous domains linked to proteins by long linkers include cytidine deaminase, guanine deaminase, uracil glycosylase inhibitor ("UGI"), nuclease, and any other proteinaceous domain that can be operably linked in a heterologous manner to a protein of interest. Such proteins of interest include, but are not limited to, site-specific nucleases (e.g., Cas9, Cas12a, Cas12b, Cas12i, Cas12j, or other CRISPR nucleases), zinc fingers, meganucleases, transcription activator-like effector nucleases ("TALENs"), and the like.

[0017] As used in describing the embodiments of this invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly dictates otherwise.

[0018] As used herein, "and / or" refers to and includes any and all possible combinations of one or more of the associated listed items.

[0019] The term "about," as used herein, when referring to a measurable value such as an amount of a compound, a dosage, a time, a temperature, etc., is meant to encompass variations of 20%, 10%, 5%, 1%, 0.5%, or even 0.1% of the particular amount.

[0020] The terms "comprise" and / or "comprising" as used herein specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0021] As used herein, the transitional phrase "consisting essentially of" means that the scope of the claim is to be construed to include the specific materials or steps recited in the claim, as well as those that do not materially affect the basic and novel characteristics of the claimed invention. Thus, the term "consisting essentially of," when used in the claims of the present invention, is not intended to be construed as the equivalent of "comprising."

[0022] As used herein, the term "amplified" refers to the construction of multiple copies of a nucleic acid molecule or multiple copies of a complementary nucleic acid molecule using at least one of the nucleic acid molecules as a template. See, e.g., Diagnostic Molecular Microbiology: Principles and Applications, D.H. Persing et al., Ed., American Society for Microbiology, Washington, DC (1993). The product of amplification is called an amplicon.

[0023] A "coding sequence" is a nucleic acid sequence that is transcribed into RNA, such as mRNA, rRNA, tRNA, snRNA, sense RNA, or antisense RNA. In some embodiments, the RNA is then translated in an organism to produce a protein.

[0024] As used herein, the term transgenic "event" refers to a recombinant plant produced by transformation and regeneration of a single plant cell with heterologous DNA, e.g., an expression cassette containing one or more genes of interest (e.g., transgenes). The term "event" refers to the original transformant and / or progeny of the transformant containing the heterologous DNA. The term "event" also refers to progeny produced by sexual outcrossing between a transformant and another line. Even after repeated backcrossing to the recurrent parent, the inserted DNA and the adjacent DNA from the transformed parent are present in the progeny of the cross at the same chromosomal location. Transformation of plant tissue typically produces multiple events, each representing the insertion of a DNA construct into a different location in the genome of the plant cell. Specific events are selected based on transgene expression or other desirable characteristics. Thus, as used herein, "event MIR604," "MIR604," or "MIR604 event" refers to the original MIR604 transformant and / or progeny of the MIR604 transformant (U.S. Pat. Nos. 7,361,813, 7,897,748, 8,354,519, and 8,884,102, which are incorporated herein by reference).

[0025] As used herein, "expression cassette" refers to a nucleic acid molecule capable of directing the expression of a particular nucleotide sequence in a suitable host cell. It includes a promoter operably linked to a nucleotide sequence of interest (typically a coding region), which is operably linked to a termination signal. Expression cassettes also typically include sequences necessary for proper translation of the nucleotide sequence. The coding region usually encodes a protein of interest, but may also encode a functional RNA of interest, such as an antisense RNA, or a non-translated RNA, in either the sense or antisense orientation. Expression cassettes may also include sequences that are not necessary for the direct expression of the nucleotide sequence of interest but are present for convenient restriction sites for removal of the cassette from the expression vector. Expression cassettes containing a nucleotide sequence of interest may be chimeric, meaning that at least one of its components is heterologous with respect to at least one of the other components. Expression cassettes may also be naturally occurring but obtained in a recombinant form useful for heterologous expression. Typically, however, the expression cassette is heterologous to the host, i.e., the particular nucleic acid sequence of the expression cassette does not naturally occur in the host cell but must have been introduced into the host cell or an ancestor of the host cell by a transformation process known in the art. Expression of the nucleotide sequence in the expression cassette may be under the control of a constitutive promoter or an inducible promoter that initiates transcription only when the host cell is exposed to some particular external stimulus. In the case of multicellular organisms such as plants, the promoter may also be specific to a particular tissue, organ, or developmental stage. An expression cassette or a fragment thereof, when transformed into a plant, may also be referred to as an "inserted sequence" or "inserted sequence."

[0026] A "gene" is a defined region located within a genome that contains, in addition to the aforementioned coding nucleic acid sequence, other primarily regulatory nucleic acid sequences involved in the control of expression, i.e., transcription and translation, of the coding portion. A gene can contain both coding and non-coding regions (e.g., introns, regulatory elements, promoters, enhancers, termination sequences, and 5' and 3' untranslated regions). A gene typically expresses mRNA, functional RNA, or a specific protein containing regulatory sequences. A gene may or may not be used to produce a functional protein. In some embodiments, a gene refers to only the coding region. The term "native gene" refers to a gene as found in nature. The term "chimeric gene" refers to any gene that: 1) contains regulatory and coding sequences that are not found together in nature, or 2) encodes portions of a protein that are not naturally adjacent, or 3) contains portions of a promoter that are not naturally adjacent. Thus, a chimeric gene can contain regulatory and coding sequences from different sources, or regulatory and coding sequences from the same source but arranged in a manner different from that found in nature. A gene may be "isolated," which refers to a nucleic acid molecule that is substantially or essentially free from components normally found associated with the nucleic acid molecule in its natural state, including other cellular material, culture medium from recombinant production, and / or various chemicals used to chemically synthesize the nucleic acid molecule.

[0027] The terms "express" or "expression" of a polynucleotide coding sequence means that the sequence is transcribed and, optionally, translated.

[0028] A "gene of interest," "nucleotide sequence of interest," or "sequence of interest" refers to any gene that, when transferred into a plant, confers a desired characteristic to the plant, such as antibiotic resistance, viral resistance, insect resistance, disease resistance, or resistance to other pests, herbicide resistance, improved nutritional value, improved performance in an industrial process, or altered reproductive ability. A "gene of interest" may also be transferred to a plant to produce a commercially valuable enzyme or metabolite in the plant.

[0029] As used herein, "heterologous" refers to a nucleic acid molecule or nucleotide sequence that is not naturally associated with the host cell into which it is introduced, whether it is derived from another species or from the same species or organism but modified from its original form or the form primarily expressed in the cell (including non-naturally occurring multiple copies of a naturally occurring nucleic acid sequence). Thus, a nucleotide sequence derived from an organism or species different from that of the cell into which it is introduced is heterologous with respect to that cell and its progeny. In addition, heterologous nucleotide sequences include nucleotide sequences derived from and inserted into the same native cell type of origin, but present in a non-natural state (e.g., present in a different copy number) and / or under the control of regulatory sequences different from those found in the nucleic acid molecule's native state. A nucleic acid sequence can also be heterologous to other nucleic acid sequences with which it may be associated, for example, in a nucleic acid construct such as an expression vector. As a non-limiting example, a promoter can be present in a nucleic acid construct in combination with one or more regulatory elements and / or coding sequences that are not naturally occurring in association with that particular promoter, i.e., heterologous to the promoter.

[0030] A "homologous" nucleic acid sequence is a nucleic acid sequence that is naturally associated with a host cell into which it is introduced. A homologous nucleic acid sequence can also be a nucleic acid sequence that is naturally associated with another nucleic acid sequence that may be present, for example, in a nucleic acid construct. As a non-limiting example, a promoter can be present in a nucleic acid construct in combination with one or more regulatory elements and / or coding sequences that are naturally associated with that particular promoter, i.e., that are homologous to the promoter.

[0031] "Operably linked" refers to the association of nucleic acid sequences on a single nucleic acid sequence such that the function of one affects the function of the other. For example, a promoter is operably linked to a coding sequence or functional RNA if it is capable of affecting the expression of that coding sequence or functional RNA (i.e., the coding sequence or functional RNA is under the transcriptional control of the promoter). A coding sequence in a sense or antisense orientation can be operably linked to a regulatory sequence. Thus, a regulatory or control sequence (e.g., a promoter) operably associated with a nucleotide sequence can affect the expression of the nucleotide sequence. For example, a promoter operably linked to a nucleotide sequence encoding GFP can affect the expression of that GFP nucleotide sequence.

[0032] Control sequences need not be contiguous with the nucleotide sequence of interest, so long as they function to direct its expression. Thus, for example, intervening untranslated but transcribed sequences can be present between the promoter and coding sequence and the promoter sequence can still be considered "operably linked" to the coding sequence.

[0033] As used herein, a "primer" is an isolated nucleic acid that is annealed to a complementary target DNA strand by nucleic acid hybridization to form a hybrid between the primer and the target DNA strand, and then extended along the target DNA strand by a polymerase, such as a DNA polymerase. Primer pairs or sets can be used, for example, for amplification of nucleic acid molecules by polymerase chain reaction (PCR) or other nucleic acid amplification methods.

[0034] A "probe" is an isolated nucleic acid molecule that is complementary to a portion of a target nucleic acid molecule and is typically used to detect and / or quantify the target nucleic acid molecule. Thus, in some embodiments, a probe can be an isolated nucleic acid molecule to which a detectable moiety or reporter molecule (e.g., a radioisotope, a ligand, a chemiluminescent agent, a fluorescent agent, or an enzyme) is attached. Probes according to the present invention can include not only deoxyribonucleic acid or ribonucleic acid, but also polyamide and other probe materials that can specifically bind to a target nucleic acid sequence and be used to detect the presence and / or quantify the amount of that target nucleic acid sequence.

[0035] TaqMan probes are designed to anneal within the DNA region amplified by a specific primer set. As Taq polymerase extends the primers and synthesizes a complementary strand 3' to 5' from the single-stranded template, the polymerase's 5' to 3' exonuclease extends the nascent strand through the probe, thereby degrading the probe annealed to the template. Degradation of the probe then releases the fluorophore and destroys its proximity to the quencher, thus relieving the quenching effect and allowing the fluorophore to fluoresce. Therefore, the fluorescence detected in a quantitative PCR thermal cycler is directly proportional to the amount of fluorophore released and the amount of DNA template present in the PCR.

[0036] Primers and probes are generally 5 to 100 nucleotides or more in length. In some embodiments, primers and probes can be at least 20 nucleotides or more in length, or at least 25 nucleotides or more, or at least 30 nucleotides or more in length. Such primers and probes specifically hybridize to the target sequence under optimal hybridization conditions known in the art. Primers and probes according to the present invention can have perfect sequence complementarity with the target sequence, but probes that differ from the target sequence and retain the ability to hybridize to the target sequence can be designed by conventional methods according to the present invention.

[0037] Methods for preparing and using probes and primers are described, for example, in Molecular Cloning: A Laboratory Manual, 2nd ed., vol. 1-3, ed. Sambrook et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 1989. PCR-primer pairs can be derived from known sequences, for example, by using computer programs designed for that purpose.

[0038] Polymerase chain reaction (PCR) is a technique for "amplifying" specific segments of DNA. To perform PCR, at least a portion of the nucleotide sequence of the DNA molecule to be replicated must be known. Generally, primers or short oligonucleotides are used that are complementary (e.g., substantially complementary or completely complementary) to the nucleotide sequence at the 3' end of each strand of the DNA to be amplified (the known sequence). The DNA sample is heated to separate its strands and then mixed with the primers. The primers hybridize to their complementary sequences in the DNA sample. Synthesis begins (5' to 3' direction) using the original DNA strand as a template. The reaction mixture must contain all four deoxynucleotide triphosphates (dATP, dCTP, dGTP, and dTTP) and DNA polymerase. Polymerization continues until each newly synthesized strand contains the sequence recognized by the other primer. When this occurs, two DNA molecules identical to the original molecule are produced. These two molecules are heated to separate their strands, and the process is repeated. Each cycle doubles the number of DNA molecules. Using automated equipment, each cycle of replication can be completed in less than 5 minutes. After 30 cycles, what began as a single molecule of DNA has been amplified to over 1 billion copies (2 30 =1.02×10 9 ).

[0039] The oligonucleotides of the oligonucleotide primer pair are located on opposite DNA strands and are complementary to DNA sequences flanking the region to be amplified. The annealed primers hybridize to the newly synthesized DNA strand. The first amplification cycle generates two new DNA strands whose 5' ends are fixed by the position of the oligonucleotide primers but whose 3' ends are variable (ragged 3' ends). The two new strands can then serve as templates for the synthesis of complementary strands of the desired length (the 5' ends are defined by the primers, and the 3' ends are fixed because synthesis cannot proceed beyond the end of the opposing primer). After several cycles, products of the desired fixed length begin to predominate.

[0040] Quantitative polymerase chain reaction (qPCR), also known as real-time polymerase chain reaction, monitors the accumulation of DNA products from a PCR reaction in real time. qPCR is a molecular biology laboratory technique based on the polymerase chain reaction (PCR) and is used to amplify and simultaneously quantify target DNA molecules. Even a single copy of a specific sequence can be amplified and detected in PCR. The PCR reaction exponentially generates copies of the DNA template, resulting in a quantitative relationship between the amount of starting target sequence and the amount of PCR product accumulated in any particular cycle. Due to inhibitors of the polymerase reaction found in the template, reagent limitation, or accumulation of pyrophosphate molecules, the PCR reaction eventually stops generating template at an exponential rate (i.e., plateau phase), making end-point quantification of PCR product unreliable. Therefore, overlapping reactions can generate variable amounts of PCR product. Only during the exponential phase of the PCR reaction is it possible to extrapolate back to determine the starting amount of template sequence. Measurement of PCR products as they accumulate (i.e., real-time quantitative PCR) allows for quantification during the exponential phase of the reaction, thus eliminating the variability associated with conventional PCR. In real-time PCR assays, positive reactions are detected by the accumulation of a fluorescent signal. Quantitative PCR allows for both detection and quantification of one or more specific sequences in a DNA sample. This amount can be either the absolute number of copies, or the relative amount when normalized to the DNA input or an additional normalizing gene. Since the first documentation of real-time PCR, it has been used in an increasing and diverse number of applications, including mRNA expression studies, DNA copy number measurements in genomic or viral DNA, allelic discrimination assays, expression analysis of specific splice variants of genes, and gene expression in paraffin-embedded tissues and laser capture microdissected cells.

[0041] As used herein, the term "Ct value" refers to the "threshold cycle," which is defined as the fractional cycle number at which the amount of amplified target reaches a fixed threshold. In some embodiments, this represents the intersection of the amplification curve with a threshold line. The amplification curve is typically "S"-shaped, showing the change in relative fluorescence (Y-axis) of each reaction at a given cycle (X-axis), which, in some embodiments, is recorded during PCR by a real-time PCR instrument. In some embodiments, the threshold line is the level of detection at which the reaction reaches a fluorescence intensity above background. See Livak & Schmittgen (2001) 25 Methods 402-408. This is a relative measure of the concentration of the target in PCR. Generally, a good Ct value for a quantitative assay such as qPCR is, in some embodiments, in the range of 10-40 for a given reference gene. The Ct level is inversely proportional to the amount of target nucleic acid in the sample (i.e., the lower the Ct level, the greater the amount of detectable target nucleic acid in the sample). Additionally, a good Ct value for a quantitative assay such as qPCR exhibits a linear response range with proportional dilutions of the target gDNA.

[0042] In some embodiments, qPCR is carried out under conditions that allow Ct values ​​to be collected in real time for quantitative analysis.For example, in a typical qPCR experiment, DNA amplification is monitored at each cycle of PCR during the extension stage.When DNA is in the log-linear phase of amplification, the amount of fluorescence generally increases above background.In some embodiments, Ct values ​​are collected at this point.

[0043] As used herein, the term "cell" refers to any living cell. A cell may be a prokaryotic or eukaryotic cell. A cell may be isolated. A cell may or may not be capable of regenerating into an organism. A cell may be in the form of a tissue, callus, culture, organ, or part. In some embodiments, a cell may be a plant cell. A plant cell of the present invention may be in the form of an isolated single cell, or may be a cultured cell, or may be part of a more highly organized unit, such as a plant tissue or plant organ. A plant cell may be derived from or part of an angiosperm or gymnosperm. In further embodiments, a plant cell may be a monocotyledonous plant cell or a dicotyledonous plant cell. A monocotyledonous plant cell may be, for example, a corn, rice, sorghum, sugarcane, barley, wheat, oat, turfgrass, or ornamental grass cell. The dicotyledonous plant cell may be, for example, a tobacco, pepper, eggplant, sunflower, crucifer, flax, potato, cotton, soybean, sugar beet, or rapeseed cell.

[0044] The term "plant part," as used herein, includes, but is not limited to, embryos, pollen, ovules, seeds, leaves, stems, shoots, flowers, branches, fruits, grains, ears, cobs, husks, stalks, roots, root tips, anthers, plant cells, including plant cells that are intact in a plant and / or plant parts, plant protoplasts, plant tissues, plant cell tissue cultures, plant callus, plant mass, etc. As used herein, "shoot" refers to the above-ground part, including leaves and stems. Furthermore, as used herein, "plant cell" refers to the structural and physiological unit of a plant, which includes a cell wall and may also be referred to as a protoplast.

[0045] The terms "introducing" or "introducing" in the context of a cell, prokaryotic cell, bacterial cell, eukaryotic cell, plant cell, plant, and / or plant part means contacting a nucleic acid molecule with a cell, eukaryotic cell, plant, plant part, and / or plant cell such that the nucleic acid molecule gains access to the interior of the cell, eukaryotic cell, plant cell, and / or plant and / or plant part. When two or more nucleic acid molecules are introduced, the nucleic acid molecules can be assembled as part of a single polynucleotide or nucleic acid construct, or as separate polynucleotides or nucleic acid constructs, and can be located on the same or different nucleic acid constructs. Thus, the polynucleotides can be introduced into the plant cell in a single transformation event, in separate transformation events, or, for example, as part of a breeding protocol.

[0046] As used herein, the terms "transformed" and "transgenic" refer to any cell, prokaryotic cell, eukaryotic cell, plant, plant cell, callus, plant tissue, or plant part that contains all or part of at least one recombinant (e.g., heterologous) polynucleotide. In some embodiments, all or part of the recombinant polynucleotide is stably integrated into a chromosome or a stable extrachromosomal element, so that it can be passed on to successive generations. For purposes of the present invention, the term "recombinant polynucleotide" refers to a polynucleotide that has been altered, rearranged, or modified by genetic engineering. Examples include any cloned polynucleotide, or a polynucleotide that has been joined or linked to a heterologous sequence. The term "recombinant" does not refer to changes in a polynucleotide that result from naturally occurring events, such as spontaneous mutation, or from non-naturally occurring mutagenesis followed by selective breeding.

[0047] The term "transformation," as used herein, refers to the introduction of heterologous nucleic acid into a cell. Cellular transformation can be stable or transient. Thus, the transgenic cells, plant cells, plants, and / or plant parts of the present invention can be stably transformed or transiently transformed. Transformation can refer to the transfer of a nucleic acid molecule into the genome of a host cell, resulting in genetically stable inheritance. In some embodiments, introduction into the plant, plant part, and / or plant cell is via bacterial-mediated transformation, particle bombardment transformation, calcium phosphate-mediated transformation, cyclodextrin-mediated transformation, electroporation, liposome-mediated transformation, nanoparticle-mediated transformation, polymer-mediated transformation, virus-mediated nucleic acid delivery, whisker-mediated nucleic acid delivery, microinjection, sonication, infiltration, polyethylene glycol-mediated transformation, protoplast transformation, or any other electrical, chemical, physical, and / or biological mechanism that results in the introduction of nucleic acid into the plant, plant part, and / or cell thereof, or any combination thereof.

[0048] The procedures for transforming plants are well known and routine in the art and are described throughout the literature.Non-limiting examples of methods for transforming plants include bacterial-mediated nucleic acid delivery (for example, via bacteria from the genus Agrobacterium), virus-mediated nucleic acid delivery, silicon carbide or nucleic acid whisker-mediated nucleic acid delivery, liposome-mediated nucleic acid delivery, microinjection, microparticle bombardment, calcium phosphate-mediated transformation, cyclodextrin-mediated transformation, electroporation, nanoparticle-mediated transformation, sonication, infiltration, PEG-mediated nucleic acid uptake, and transformation via any other electrical, chemical, physical (mechanical) and / or biological mechanism (including any combination thereof) that results in the introduction of nucleic acid into plant cells. General guides to various plant transformation methods known in the art include Miki et al. ("Procedures for Introducing Foreign DNA into Plants" in Methods in Plant Molecular Biology and Biotechnology, Glick, BR and Thompson, JE, Eds. (CRC Press, Inc., Boca Raton, 1993), pages 67-88) and Rakowoczy-Trojanowska (Cell Mol Biol Lett 7:849-858 (2002)).

[0049] Agrobacterium-mediated transformation is a commonly used method for transforming plants due to its high transformation efficiency and its wide utility with many different species. Agrobacterium-mediated transformation typically involves introducing a binary vector carrying the foreign DNA of interest into an appropriate Agrobacterium strain, which can rely on the complement of vir genes carried by the host Agrobacterium strain either on a coexisting Ti plasmid or on the chromosome (Uknes et al. 1993, Plant Cell 5:159-169). Introduction of the recombinant binary vector into Agrobacterium can be achieved by a triparental mating procedure using Escherichia coli carrying the recombinant binary vector and a helper E. coli strain carrying a plasmid capable of mobilizing the recombinant binary vector into the target Agrobacterium strain. Alternatively, the recombinant binary vector can be introduced into Agrobacterium by nucleic acid transformation (Hoefgen and Willmitzer 1988, Nucleic Acids Res 16:9877).

[0050] Transformation of plants with recombinant Agrobacterium usually involves co-cultivation of Agrobacterium with explants from plants and follows methods well known in the art. Transformed tissue is typically regenerated on selective media carrying an antibiotic or herbicide resistance marker between binary plasmid T-DNA borders.

[0051] Another method for transforming plants, plant parts, and plant cells involves propelling inert or biologically active particles into plant tissues and cells. See, e.g., U.S. Patent Nos. 4,945,050, 5,036,006, and 5,100,792. Generally, this method involves propelling inert or biologically active particles into plant cells under conditions effective to penetrate the outer surface of the cells and incorporate them into their interior. When inert particles are used, vectors can be introduced into cells by coating the particles with the vector containing the nucleic acid of interest. Alternatively, one or more cells can be surrounded by the vector, such that the vector is carried into the cells by the particle's wake. Biologically active particles (e.g., dried yeast cells, dried bacteria, or bacteriophage, each containing one or more nucleic acids desired to be introduced) can also be propelled into plant tissue.

[0052] "Transient transformation" in the context of a polynucleotide means that the polynucleotide is introduced into a cell and does not integrate into the genome of the cell.

[0053] As used herein, in the context of a polynucleotide introduced into a cell, "stably introduce," "stably introduced," "stable transformation," or "stably transformed" means that the introduced polynucleotide is stably integrated into the genome of the cell, and thus the cell is stably transformed with the polynucleotide. The integrated polynucleotide can thus be inherited by its progeny, and more particularly, by progeny for multiple successive generations. As used herein, "genome" includes the nuclear and / or plastid genome, and thus includes, for example, integration of a polynucleotide into a chloroplast genome. As used herein, stable transformation also refers to a polynucleotide that is maintained extrachromosomally (e.g., a minichromosome).

[0054] Transient transformation can be detected, for example, by enzyme-linked immunosorbent assay (ELISA) or Western blot, which can detect the presence of peptides or polypeptides encoded by one or more nucleic acid molecules introduced into an organism. Stable transformation of a cell can be detected, for example, by Southern blot hybridization assay of the cell's genomic DNA using a nucleic acid sequence that specifically hybridizes with the nucleotide sequence of the nucleic acid molecule introduced into the organism (e.g., a plant). Stable transformation of a cell can be detected, for example, by Northern blot hybridization assay of the cell's RNA using a nucleic acid sequence that specifically hybridizes with the nucleotide sequence of the nucleic acid molecule introduced into a plant or other organism. Stable transformation of a cell can also be detected, for example, by polymerase chain reaction (PCR) or other amplification reactions well known in the art, using specific primer sequences that hybridize with target sequences in the nucleic acid molecule, resulting in amplification of the target sequence, which can be detected according to standard methods. Transformation can also be detected by direct sequencing and / or hybridization protocols well known in the art.

[0055] Thus, in certain embodiments of the present invention, plant cells can be transformed by any method known in the art and as described herein, and intact plants can be regenerated from these transformed cells using any of a variety of known techniques. Plant regeneration from plant cells, plant tissue cultures, and / or cultured protoplasts is described, for example, in Evans et al. (Handbook of Plant Cell Cultures, Vol. 1, MacMilan Publishing Co., New York (1983)); and Vasil IR (ed.) (Cell Culture and Somatic Cell Genetics of Plants, Acad. Press, Orlando, Vol. I (1984), and Vol. II (1986)). Methods for selecting transformed transgenic plants, plant cells, and / or plant tissue cultures are routine in the art and can be used in the methods of the present invention provided herein.

[0056] "Transformation and regeneration process" refers to the process of stably introducing a transgene into a plant cell and regenerating a plant from the transgenic plant cell. As used herein, transformation and regeneration includes a selection process in which the transgene contains a selectable marker and the transformed cells incorporate and express the transgene so that the transformed cells survive and developmentally reproduce in the presence of a selection agent. "Regeneration" refers to the growth of a whole plant from a plant cell, a group of plant cells, or a plant piece, such as a protoplast, callus, or tissue portion.

[0057] The terms "nucleotide sequence," "nucleic acid," "nucleic acid sequence," "nucleic acid molecule," "oligonucleotide," and "polynucleotide" are used interchangeably herein to refer to heteropolymers of nucleotides and encompass both RNA and DNA, including cDNA, genomic DNA, mRNA, synthetic (e.g., chemically synthesized) DNA or RNA, and chimeras of RNA and DNA. The term nucleic acid molecule refers to a chain of nucleotides regardless of chain length. A nucleotide comprises a sugar, a phosphate, and a base, which may be either a purine or a pyrimidine. A nucleic acid molecule can be double-stranded or single-stranded. If single-stranded, the nucleic acid molecule can be the sense strand or the antisense strand. Nucleic acid molecules can be synthesized using oligonucleotide analogs or derivatives (e.g., inosine or phosphorothioate nucleotides). Such oligonucleotides can be used, for example, to prepare nucleic acid molecules with altered base-pairing abilities or increased resistance to nucleases. The nucleic acid sequences provided herein are presented in a 5' to 3' orientation from left to right and are represented using the standard code for representing nucleotide letters as set forth in the United States Sequencing Regulations, 37 CFR §§ 1.821-1.825 and the World Intellectual Property Organization (WIPO) Standard ST.25.

[0058] A "nucleic acid fragment" is a fragment of a given nucleic acid molecule. An "RNA fragment" is a fragment of a given RNA molecule. A "DNA fragment" is a fragment of a given DNA molecule. A "nucleic acid fragment" is a fragment of a given nucleic acid molecule, not isolated from the molecule. An "RNA fragment" is a fragment of a given RNA molecule, not isolated from the molecule. A "DNA fragment" is a fragment of a given DNA molecule, not isolated from the molecule. A polynucleotide segment can be any length, for example, at least 5, 10, 15, 20, 25, 30, 40, 50, 75, 100, 150, 200, 300, or 500 or more nucleotides in length. A segment or portion of a guide sequence can be about 50%, 40%, 30%, 20%, 10% of the guide sequence, for example, one-third or less of the guide sequence, for example, 7, 6, 5, 4, 3, or 2 nucleotides in length.

[0059] The term "derived from" in the context of a molecule refers to a molecule that is isolated or created using the parent molecule or information from that parent molecule. For example, the Cas9 single mutant nickase and the Cas9 double mutant null-nuclease are each derived from the wild-type Cas9 protein.

[0060] In higher plants, deoxyribonucleic acid (DNA) is the genetic material, while ribonucleic acid (RNA) is involved in the transfer of the information contained in DNA into proteins. A "genome" is the totality of genetic material contained in each cell of an organism. Unless otherwise indicated, a particular nucleic acid sequence of the present invention also implicitly encompasses conservatively modified variants thereof (e.g., degenerate codon substitutions) and complementary sequences as well as the explicitly indicated sequence. Specifically, degenerate codon substitutions can be achieved by generating sequences in which the third position of one or more selected (or all) codons is substituted with mixed-base and / or deoxyinosine residues (Batzer et al., Nucleic Acid Res. 19:5081 (1991); Ohtsuka et al., J. Biol. Chem. 260:2605-2608 (1985); and Rossolini et al., Mol. Cell. Probes 8:91-98 (1994)). The term nucleic acid molecule is used interchangeably with gene, cDNA, and mRNA encoded by a gene.

[0061] As used herein, "sequence identity" refers to the degree to which two optimally aligned polynucleotide or peptide sequences remain invariant over the window of alignment of the components (e.g., nucleotides or amino acids). "Identity" can be readily calculated by known methods, including, but not limited to, those described in Computational Molecular Biology (Lesk, AM, ed.) Oxford University Press, New York (1988); Biocomputing: Informatics and Genome Projects (Smith, DW, ed.) Academic Press, New York (1993); Computer Analysis of Sequence Data, Part I (Griffin, AM, and Griffin, HG, eds.) Humana Press, New Jersey (1994); Sequence Analysis in Molecular Biology (von Heinje, G., ed.) Academic Press (1987); and Sequence Analysis Primer (Gribskov, M. and Devereux, J., eds.) Stockton Press, New York (1991).

[0062] As used herein, the term "percent sequence identity" or "percent identity" refers to the percentage of identical nucleotides in a linear polynucleotide sequence of a reference ("query") polynucleotide molecule (or its complementary strand) compared to a test ("subject") polynucleotide molecule (or its complementary strand) when the two sequences are optimally aligned. In some embodiments, "percent identity" can refer to the percentage of identical amino acids in an amino acid sequence.

[0063] As used herein, the phrase "substantially identical," in the context of two nucleic acid molecules, nucleotide sequences, or protein sequences, refers to two or more sequences or subsequences that have at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, or at least about 99% nucleotide or amino acid residue identity when compared and aligned for maximum correspondence as determined using one of the following sequence comparison algorithms or by visual inspection. In some embodiments of the invention, substantial identity exists over a region of the sequence that is at least about 50 to about 150 residues in length. Thus, in some embodiments of the invention, substantial identity exists over a region of the sequence that is at least about 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, or more residues in length. In some particular embodiments, the sequences are substantially identical over at least about 150 residues. In further embodiments, the sequences are substantially identical over the entire length of the coding region. Moreover, in representative embodiments, substantially identical nucleotide or protein sequences perform substantially the same function (e.g., targeting to a specific genomic target, endonucleolytic cleavage of a specific genomic target site).

[0064] For sequence comparison, typically, one sequence serves as the reference sequence to which test sequence is compared.When using sequence comparison algorithm, test sequence and reference sequence are input into computer, and subsequence coordinates are designated as needed, and sequence algorithm program parameters are designated.Then, sequence comparison algorithm calculates the percent sequence identity of test sequence to reference sequence based on designated program parameters.

[0065] Optimal alignment of sequences to align a comparison window is well known to those skilled in the art and can be performed by tools such as the Smith and Waterman local homology algorithm, the Needleman and Wunsch homology alignment algorithm, the Pearson and Lipman search for similarity method, and optionally by computerized implementations of these algorithms, such as GAP, BESTFIT, FASTA, and TFASTA, available as part of the GCG® Wisconsin Package® (Accelrys Inc., San Diego, CA). The "fractional identity" for an aligned segment of a test sequence and a reference sequence is the number of identical components shared by the two aligned sequences divided by the total number of components in the reference sequence segment (i.e., the entire reference sequence or a smaller, defined portion of the reference sequence). Percent sequence identity is expressed as the fractional identity multiplied by 100. Comparison of one or more polynucleotide sequences can be to a full-length polynucleotide sequence or a portion thereof, or to a longer polynucleotide sequence. For purposes of the present invention, "percent identity" may also be determined using BLASTX version 2.0 for translated nucleotide sequences and BLASTN version 2.0 for polynucleotide sequences.

[0066] Software for performing BLAST analysis is publicly available through the National Center for Biotechnology Information. The algorithm involves first identifying high-scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence that, when aligned with words of the same length in a database sequence, match or meet some positive threshold score, T. T is referred to as the neighborhood word score threshold (Altschul et al., 1990). These initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are then extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using the parameters M (reward score for a pair of matching residues; always >0) and N (penalty score for mismatching residues; always <0) for nucleotide sequences. For amino acid sequences, a scoring matrix is ​​used to calculate the cumulative score. Extension of word hits in each direction is stopped when the cumulative alignment score falls below its maximum achieved value by an amount X, or when the cumulative score falls below zero due to the accumulation of one or more negative-scoring residue alignments, or when the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLASTN program (for nucleotide sequences) uses as defaults a word length (W) of 11, an expectation (E) of 10, a cutoff of 100, M=5, N=4, and a comparison of both strands. For amino acid sequences, the BLASTP program uses as defaults a word length (W) of 3, an expectation (E) of 10, and the BLOSUM62 scoring matrix (see Henikoff & Henikoff, Proc. Natl. Acad. Sci. USA 89:10915 (1989)).

[0067] In addition to calculating percent sequence identity, the BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin & Altschul, Proc. Nat'l. Acad. Sci. USA 90:5873-5787 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability that a match between two nucleotide or amino acid sequences would occur by chance. For example, a test nucleic acid sequence is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleotide sequence with the reference nucleotide sequence is less than about 0.1 to less than about 0.001. Thus, in some embodiments of the present invention, the smallest sum probability in a comparison of the test nucleotide sequence with the reference nucleotide sequence is less than about 0.001.

[0068] Two nucleotide sequences can also be considered substantially identical if the two sequences hybridize to each other under stringent conditions. In some representative embodiments, two nucleotide sequences considered to be substantially identical hybridize to each other under highly stringent conditions.

[0069] "Stringent hybridization conditions" and "stringent hybridization wash conditions" in the context of nucleic acid hybridization experiments, such as Southern and Northern hybridization, are sequence-dependent and vary under different environmental parameters. An extensive guide to nucleic acid hybridization can be found in Tijssen Laboratory Techniques in Biochemistry and Molecular Biology—Hybridization with Nucleic Acid Probes, part I, chapter 2, "Overview of principles of hybridization and the strategy of nucleic acid probe assays," Elsevier, New York (1993). In general, highly stringent hybridization and wash conditions are those that achieve the thermal melting point (T) for a specific sequence at a defined ionic strength and pH. m ) is chosen to be approximately 5°C lower than

[0070] T m is the temperature (under defined ionic strength and pH) at which 50% of the target sequence hybridizes to a perfectly matched probe. Very stringent conditions are defined as the T mAn example of stringent hybridization conditions for hybridization of complementary nucleotide sequences with more than 100 complementary residues on a filter in a Southern or Northern blot is 50% formamide and 1 mg heparin at 42°C, with hybridization occurring overnight. An example of highly stringent wash conditions is 0.15 M NaCl at 72°C for approximately 15 minutes. An example of stringent wash conditions is a 0.2x SSC wash at 65°C for 15 minutes (see Sambrook, below, for a description of SSC buffers). Often, a low stringency wash precedes a high stringency wash to remove background probe signal. For example, an example of a moderate stringency wash for a duplex of more than 100 nucleotides is 1x SSC at 45°C for 15 minutes. For example, an example of a low stringency wash for a duplex of more than 100 nucleotides is 4-6x SSC at 40°C for 15 minutes. For short probes (e.g., about 10-50 nucleotides), stringent conditions typically include a salt concentration of less than about 1.0 M Na ion, typically about 0.01-1.0 M Na ion (or other salt) at pH 7.0-8.3, and a temperature typically of at least about 30°C. Stringent conditions can also be achieved by the addition of destabilizing agents such as formamide. Generally, a signal-to-noise ratio of 2-fold (or higher) than that observed for an unrelated probe in a particular hybridization assay indicates detection of specific hybridization. Nucleotide sequences that do not hybridize to each other under stringent conditions are still substantially identical if the proteins they encode are substantially identical. This can occur, for example, when copies of nucleotide sequences are generated using the maximum codon degeneracy permitted by the genetic code.

[0071] The following are exemplary sets of hybridization / wash conditions that can be used to clone homologous nucleotide sequences that are substantially identical to a reference nucleotide sequence of the present invention. In one embodiment, the reference nucleotide sequence is hybridized to a "test" nucleotide sequence in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO, 1 mM EDTA at 50° C. and washed in 2× SSC, 0.1% SDS at 50° C. In another embodiment, the reference nucleotide sequence is hybridized to a "test" nucleotide sequence in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO, 1 mM EDTA at 50° C. and washed in 1× SSC, 0.1% SDS at 50° C., or hybridized to a "test" nucleotide sequence in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO, 1 mM EDTA at 50° C. and washed in 0.5× SSC, 0.1% SDS at 50° C. In yet a further embodiment, the reference nucleotide sequence is hybridized to a "test" nucleotide sequence in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO, 1 mM EDTA at 50°C and washed in 0.1X SSC, 0.1% SDS at 50°C, or hybridized to a "test" nucleotide sequence in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO, 1 mM EDTA at 50°C and washed in 0.1X SSC, 0.1% SDS at 65°C.

[0072] An "isolated" nucleic acid molecule or nucleotide sequence or an "isolated" polypeptide is a nucleic acid molecule, nucleotide sequence, or polypeptide that exists apart from its natural environment by the hand of man and / or has a different, modified, regulated, and / or altered function compared to its function in its natural environment, and is therefore not a product of nature. An isolated nucleic acid molecule or isolated polypeptide can exist in purified form or can exist in a non-native environment (e.g., a recombinant host cell). Thus, for example, with respect to a polynucleotide, the term isolated means that it is separated from the chromosome and / or cell in which it naturally occurs. A polynucleotide is also isolated if it is separated from the chromosome and / or cell in which it naturally occurs and then loses its genetic context, chromosome, chromosomal location, and / or is inserted into a cell in which it does not naturally occur. Recombinant nucleic acid molecules and nucleotide sequences of the present invention can be considered "isolated" as defined above.

[0073] Thus, an "isolated nucleic acid molecule" or "isolated nucleotide sequence" is a nucleic acid molecule or nucleotide sequence that is not immediately adjacent to the nucleotide sequences to which it is immediately adjacent (one at the 5' end and one at the 3' end) in the naturally occurring genome of the organism from which it originates. Thus, in one embodiment, an isolated nucleic acid includes some or all of the 5' non-coding (e.g., promoter) sequences immediately adjacent to the coding sequence. Thus, the term includes recombinant nucleic acids that are incorporated into, for example, a vector, an autonomously replicating plasmid, or virus, or the genomic DNA of a prokaryote or eukaryote, or that exist as a separate molecule independent of other sequences (e.g., a cDNA or genomic DNA fragment produced by PCR or restriction endonuclease treatment). It also includes recombinant nucleic acids that are part of a hybrid nucleic acid molecule that encodes an additional polypeptide or peptide sequence. An "isolated nucleic acid molecule" or "isolated nucleotide sequence" can also include nucleotide sequences that are derived from and inserted into the same natural cell type of origin, but that exist in a non-natural state, e.g., in different copy numbers and / or under the control of regulatory sequences that differ from those found in the nucleic acid molecule's natural state.

[0074] The term "isolated" can further refer to a nucleic acid molecule, nucleotide sequence, polypeptide, peptide, or fragment (e.g., when produced by recombinant DNA technology), or chemical precursor or other chemical (e.g., when chemically synthesized), that is substantially free of cellular material, viral material, and / or culture medium. Furthermore, an "isolated fragment" is a fragment of a nucleic acid molecule, nucleotide sequence, or polypeptide that is not naturally occurring as a fragment and, as such, would not be found in the natural state. "Isolated" does not necessarily mean that the preparation is technically pure (homogeneous), but is sufficiently pure to provide the polypeptide or nucleic acid in a form that can be used for its intended purpose.

[0075] In representative embodiments of the present invention, "isolated" nucleic acid molecules, nucleotide sequences, and / or polypeptides are at least about 5%, 10%, 15%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 75%, 80%, 85%, 90%, 95%, 97%, 98%, 99% (w / w) or more pure. In other embodiments, "isolated" nucleic acids, nucleotide sequences, and / or polypeptides indicate that at least about a 5-fold, 10-fold, 25-fold, 100-fold, 1000-fold, 10,000-fold, 100,000-fold or more enrichment of nucleic acid (w / w) has been achieved relative to the starting material.

[0076] A "wild-type" nucleotide sequence or amino acid sequence refers to a naturally occurring ("native") or endogenous nucleotide sequence or amino acid sequence. Thus, for example, a "wild-type mRNA" is an mRNA that occurs naturally in or is endogenous to an organism. A "homologous" nucleotide sequence is a nucleotide sequence that is naturally associated with a host cell into which it is introduced.

[0077] The terms "open reading frame" and "ORF" refer to the amino acid sequence encoded between the translation initiation and termination codons of a coding sequence. The terms "initiation codon" and "termination codon" refer to the unit of three adjacent nucleotides ("codon") in a coding sequence that specify the initiation and chain termination of protein synthesis (mRNA translation), respectively.

[0078] A "promoter" refers to a nucleotide sequence, usually upstream (5') of a coding sequence, that controls expression of the coding sequence by providing recognition for RNA polymerase and other factors necessary for proper transcription. A "promoter regulatory sequence" consists of proximal and more distal upstream elements. Promoter regulatory sequences affect transcription, RNA processing or stability, or translation of the associated coding sequence. Regulatory sequences include enhancers, promoters, untranslated leader sequences, introns, and polyadenylation signal sequences. These include natural and synthetic sequences, as well as sequences that may be a combination of natural and synthetic sequences. An "enhancer" is a DNA sequence that can stimulate promoter activity and may be a native element of the promoter or a heterologous element inserted to increase the level or tissue specificity of the promoter. Enhancers can operate in both orientations (normal or inverted) and can function when moved either upstream or downstream from the promoter. The term "promoter" includes "promoter regulatory sequence."

[0079] "Primary transformants" and "T0 generation" refer to transgenic plants that are the same genetic generation as the tissue originally transformed (i.e., that have not undergone meiosis and fertilization since transformation). "Secondary transformants" and "T1, T2, T3, etc. generations" refer to transgenic plants derived from the primary transformant through one or more meiotic and fertilization cycles. These may be derived by self-fertilization of the primary or secondary transformants or by crossing the primary or secondary transformants with other transformed or untransformed plants.

[0080] A "transgene" refers to a nucleic acid molecule that has been introduced into a genome by transformation and is stably maintained. A transgene may contain at least one expression cassette, typically contains at least two expression cassettes, and may contain ten or more expression cassettes. A transgene may include, for example, a gene that is heterologous or homologous to a gene in a particular plant being transformed. Additionally, a transgene may include a native gene inserted into a non-native organism, or a chimeric gene. The term "endogenous gene" refers to a native gene in its natural location in the genome of an organism. A "foreign" gene refers to a gene not normally found in the host organism but that is introduced into an organism by gene transfer.

[0081] "Intron" refers to an intervening segment of DNA that is present almost exclusively in eukaryotic genes but is not translated into an amino acid sequence in the gene product. Introns are removed from premature mRNA through a process called splicing, which leaves the exons intact to form the mRNA. For purposes of the present invention, the definition of the term "intron" includes modifications to the nucleotide sequence of an intron derived from a target gene, provided that the modified intron does not significantly reduce the activity of its associated 5' regulatory sequence.

[0082] "Exon" refers to a segment of DNA that contains the coding sequence for a protein or portion thereof. Exons are separated by intervening non-coding sequences (introns). For purposes of the present invention, the definition of the term "exon" includes modifications to the nucleotide sequence of an exon derived from a target gene, provided that the modified exon does not significantly reduce the activity of its associated 5' regulatory sequence.

[0083] The term "cleavage" or "cleaving" refers to the cleavage of a covalent phosphodiester bond in the ribosylphosphodiester backbone of a polynucleotide. The term "cleavage" or "cleaving" encompasses both single-strand and double-strand breaks. Double-strand breaks can occur as a result of two different single-strand cleavage events. Cleavage can result in the generation of either blunt ends or staggered ends. A "nuclease cleavage site" or "genomic nuclease cleavage site" is a region of nucleotides containing a nuclease cleavage sequence recognized by a specific nuclease, which acts to cleave the nucleotide sequence of genomic DNA on one or both strands. Such cleavage by a nuclease enzyme initiates DNA repair mechanisms within the cell, which establish an environment for homologous recombination to occur.

[0084] The present invention provides fusion proteins having an improved linker between the deaminase domain and the site-specific DNA binding domain, thereby increasing editing efficiency and reducing mutation frequency. In some embodiments of the present invention, the deaminase domain is a cytidine deaminase. In other embodiments of the present invention, the deaminase domain is an adenine deaminase. In some embodiments, the cytidine deaminase domain is an activation-induced cytidine deaminase ("AID"). In some embodiments of the present invention, the cytidine deaminase domain is an apolipoprotein B mRNA editing complex ("APOBEC") domain. In some embodiments, the APOBEC domain is an APOBEC1 family deaminase.

[0085] "Cytidine deaminase" refers to an enzyme that catalyzes the irreversible hydrolytic deamination of cytidine and deoxycytidine to uridine and deoxyuridine, respectively. Cytidine deaminases maintain intracellular pyrimidine pools. One family of cytidine deaminases is APOBEC ("apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like"). Members of this family are C-to-U editing enzymes. The N-terminal domain of APOBEC-like proteins is the catalytic domain, and the C-terminal domain is the pseudocatalytic domain. More specifically, this catalytic domain is a zinc-dependent cytidine deaminase domain, which is important for cytidine deamination. RNA editing by APOBEC1 requires homodimerization, and this complex interacts with RNA-binding proteins to form an editosome. Non-limiting examples of APOBEC proteins include APOBEC1, APOBEC2, APOBEC3A, APOBEC3B, APOBEC3C, APOBEC3D, APOBEC3F, APOBEC3G, APOBEC3H, APOBEC4, and activation-induced (cytidine) deaminase. Various mutants of APOBEC proteins are also known that confer different editing properties on base editors. For example, in the case of human APOBEC3A, certain mutants (e.g., Y130F, Y132D, W104A, and D131Y) are superior to wild-type human APOBEC3A in terms of editing efficiency. Thus, the term APOBEC and each of its family members also encompasses variants and mutants that share a certain level of sequence identity (e.g., 70%, 75%, 80%, 85%, 90%, 95%, 98%, or 99%) with the corresponding wild-type APOBEC protein and retain cytidine deamination activity. These variants and mutants may be derived by amino acid addition, deletion, and / or substitution, which in some embodiments are conservative substitutions.

[0086] The "cytosine base editor" ("CBE") converts C·G base pairs to T·A base pairs.

[0087] "Adenine deaminase" refers to an enzyme that catalyzes the hydrolytic deamination of adenosine to inosine, which pairs with C and is therefore read or replicated as G. An exemplary enzyme is TadA from E. coli, which functions as a homodimer.

[0088] The "adenine base editor" ("ABE") converts A·T base pairs to G·C base pairs.

[0089] Lachnospiraceae bacterium Cpf1 (LbCpf1) is one of many Cpf1 proteins in a large family. The terms "Cpf1" and "Cas12a" are used interchangeably throughout. Cpf1 is a Cas protein. The term "Cas protein" or "clustered regularly interspaced short palindromic repeats (CRISPR)-associated (Cas) protein" refers to an RNA-guided DNA endonuclease enzyme associated with CRISPR (clustered regularly interspaced short palindromic repeats), an adaptive immune system found in, for example, Streptococcus pyogenes and other bacteria. Cas proteins include Cas9, Cas12, Cas12b, Cas12i, and Cas12j. In some embodiments of the invention, the site-specific DNA-binding domain is a catalytically inactive Cas12a from a Lachnospiraceae bacterium ("dLbCas12a"). In other embodiments, the site-specific DNA-binding domain is a catalytically active one from a Lachnospiraceae bacterium ("LbCas12a") or a catalytically active one from Moraxella bovoculi AAX08_00205 ("Mb2Cas12a"). In some embodiments of the present invention, the site-specific DNA-binding domain of the fusion protein is a Cas12a protein derived from a Lachnospiraceae bacterium, Acidaminococcus sp., Moraxella bovoculi, Thiomicrospira sp., Moraxella lacunata, Methanomethylophilus alvus, Butyrivibrio sp., or Bacteroidetesoral sp.

[0090] The fusion protein may include other fragments such as a uracil DNA glycosylase inhibitor (UGI) and a nuclear localization sequence (NLS).

[0091] The "uracil glycosylase inhibitor" (UGI), which can be prepared from Bacillus subtilis bacteriophage PBS1, is a small protein (9.5 kDa) that inhibits E. coli uracil-DNA glycosylase (UDG) and UDG from other species. Inhibition of UDG occurs through reversible protein binding with a 1:1 UGD:UGI stoichiometry. UGI can dissociate the UDG-DNA complex. A non-limiting example of a UGI is found in Bacillus phage AR9 (YP_009283008.1). In some embodiments, UGI comprises the amino acid sequence of SEQ ID NO:8 or has at least 70%, 75%, 80%, 85%, 90%, or 95% sequence identity to SEQ ID NO:8 and retains uracil glycosylase inhibitory activity.

[0092] In some embodiments, the UGI is located C-terminal to the cytidine deaminase-Cpf1 moiety. In some embodiments, the fusion protein comprises at least two UGIs.

[0093] In some embodiments, at least one nuclear localization signal ("NLC") is located at the C-terminus of the first fragment and the second fragment (the cytidine deaminase-Cpf1 portion), e.g., between the second fragment (comprising Cpf1) and UGI. In some embodiments, at least two NLSs are located between the second fragment and UGI. In some embodiments, at least three NLSs are located between the second fragment and UGI. In some embodiments, at least one NLS is located at the N-terminus of the first fragment and the second fragment (the cytidine deaminase-Cpf1 portion).

[0094] Non-limiting examples of the arrangement of components in the fusion proteins include, from N-terminus to C-terminus, (a) NLS, cytidine deaminase, Cas12a, NLS, UGI, NLS, 2A, and UGI; (b) NLS, cytidine deaminase, Cas12a, NLS, NLS, UGI, NLS, 2A, and UGI; (c) NLS, cytidine deaminase, Cas12a, NLS, UGI, NLS, 2A, UGI, 2A, and UGI; (d) NLS, cytidine deaminase, Cas12a, NLS, UGI, NLS, 2A, UGI, 2A, UGI, 2A, and UGI.

[0095] In some embodiments, the fusion protein optionally includes a peptide linker between each of the fragments. In some embodiments, the peptide linker has 1-100 amino acid residues (or, without limitation, 3-20, 4-15). In some embodiments, at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% of the amino acid residues in the peptide linker are selected from the group consisting of alanine, glycine, cysteine, and serine.

[0096] The present invention also provides a nucleic acid molecule comprising a nucleic acid sequence encoding a guide RNA of the present invention. The nucleic acid molecule can be a DNA or RNA molecule. In some embodiments, the nucleic acid molecule is circularized. In other embodiments, the nucleic acid molecule is linear. In some embodiments, the nucleic acid molecule is single-stranded, partially double-stranded, or double-stranded. In some embodiments, the nucleic acid molecule is complexed with at least one polypeptide. The polypeptide can have a nucleic acid recognition domain or a nucleic acid binding domain. In some embodiments, the polypeptide is a shuttle, for example, for mediating delivery of the chimeric RNA of the present invention and, optionally, a nuclease. In some embodiments, the polypeptide is a Feldan shuttle (US Patent Application Publication No. 20160298078, incorporated herein by reference).

[0097] "On-target editing" refers to a cytosine-to-thymine substitution in the region following the PAM site targeted by the gRNA. The primary editing window is 8-13 bases following the PAM site. "Off-target editing" refers to an indel or base change other than C to T within the gRNA target region, or a base change or indel outside the gRNA target region.

[0098] A "site-specific modifying polypeptide" modifies a target DNA (e.g., cleavage or methylation of the target DNA) and / or a polypeptide associated with the target DNA (e.g., methylation or acetylation of a histone tail). A site-specific modifying polypeptide is also referred to herein as a "site-directed polypeptide" or an "RNA-binding site-specific modifying polypeptide." A site-specific modifying polypeptide is either a single RNA molecule or an RNA duplex of at least two RNA molecules, and interacts with a guide RNA that is guided to a DNA sequence (e.g., a chromosomal sequence or an extrachromosomal sequence, e.g., an episomal sequence, a minicircle sequence, a mitochondrial sequence, a chloroplast sequence, etc.) due to its association with the guide RNA.

[0099] In some cases, the site-directed modified polypeptide is a naturally occurring modified polypeptide. In other cases, the site-directed modified polypeptide is a non-naturally occurring polypeptide (e.g., a chimeric polypeptide or a naturally occurring polypeptide that has been modified, e.g., mutated, deleted, or inserted). Exemplary naturally occurring site-directed modified polypeptides are known in the art (see, e.g., Makarova et al., 2017, Cell 168:328-328.e1, and Shmakov et al., 2017, Nat Rev Microbiol 15(3):169-182, both of which are incorporated herein by reference). These naturally occurring polypeptides bind to DNA-targeting RNAs, are thereby guided to specific sequences within the target DNA, and cleave the target DNA to generate double-strand breaks.

[0100] A site-directed modified polypeptide comprises two portions: an RNA-binding portion and an active portion. In some embodiments, a site-directed modified polypeptide comprises: (i) an RNA-binding portion that interacts with a DNA-targeting RNA, where the DNA-targeting RNA comprises a nucleotide sequence complementary to a sequence in the target DNA; and (ii) an active portion that exhibits site-specific enzymatic activity (e.g., activity for DNA methylation, activity for DNA cleavage, activity for histone acetylation, activity for histone methylation, etc.), where the site of enzymatic activity is determined by the DNA-targeting RNA. In other embodiments, a site-directed modified polypeptide comprises: (i) an RNA-binding portion that interacts with a DNA-targeting RNA, where the DNA-targeting RNA comprises a nucleotide sequence complementary to a sequence in the target DNA; and (ii) an active portion that modulates transcription within the target DNA (e.g., to increase or decrease transcription), where the site of modulated transcription within the target DNA is determined by the DNA-targeting RNA.

[0101] In some cases, the site-directed modifying polypeptide has an operably linked heterologous domain. This heterologous domain can be an enzyme or a signal peptide. In embodiments where the heterologous domain is an enzymatic domain, this domain has an enzymatic activity that modifies a target nucleic acid (e.g., nuclease activity, methyltransferase activity, demethylase activity, DNA repair activity, DNA damage activity, deamination activity, reverse transcriptase activity, dismutase activity, alkylation activity, methylation activity, depurination activity, oxidation activity, pyrimidine dimer formation activity, integrase activity, transposase activity, recombinase activity, polymerase activity, ligase activity, helicase activity, photolyase activity, or glycosylase activity). In other cases, the site-directed modifying polypeptide has an operably linked enzymatic domain whose enzymatic activity modifies a polypeptide (e.g., a histone) associated with the target DNA (e.g., methyltransferase activity, demethylase activity, acetyltransferase activity, deacetylase activity, kinase activity, phosphatase activity, ubiquitin ligase activity, deubiquitinating activity, adenylating activity, deadenylating activity, sumoylating activity, desumoylating activity, ribosylation activity, deribosylation activity, myristoylating activity, or demyristoylating activity). Exemplary enzymatic domains include adenosine deaminase, oxidase, thymine alkyltransferase, adenine oxidase, adenosine methyltransferase, adenosine deaminase, and glycosylase, either alone or in combination with other enzymatic domains. In embodiments in which the heterologous domain is a signal peptide, the signal peptide can be a nuclear localization signal ("NLS," eg, SV40 NLS).

[0102] In some cases, different site-specific modifying polypeptides, e.g., different Cas9 proteins (i.e., Cas9 proteins from various species), may be advantageous for use in the various provided methods of the present invention to take full advantage of the different enzymatic properties of different Cas9 proteins (e.g., due to different PAM sequence preferences; increased or decreased enzymatic activity; increased or decreased levels of cytotoxicity; altered balance between NHEJ, homology-directed repair, single-strand breaks, and double-strand breaks, etc.). Cas9 proteins from various species (e.g., those disclosed in Shmakov et al., 2017, or polypeptides derived therefrom) may require different PAM sequences in target DNA. Thus, for a particular Cas9 enzyme selected, the PAM sequence requirements may differ from the 5'-N GG-3' sequence (where N is either A, T, C, or G) known to be necessary for Cas9 activity. Numerous Cas9 orthologs from a wide variety of species have been identified herein, and the proteins share few identical amino acids. All identified Cas9 orthologs share the same domain architecture, with a central HNH endonuclease domain and a split RuvC / RNaseH domain. Cas9 proteins share four key motifs with conserved structures: motifs 1, 2, and 4 are RuvC-like motifs, while motif 3 is an HNH motif. In contrast, Cas12a proteins from various species may have different PAM sequence requirements compared to the canonical PAM of TTTV LbCas12a.

[0103] The site-specific modifying polypeptide can also be a chimeric or modified CRISPR / Cas nuclease. For example, the site-specific modifying polypeptide can be a modified Cas9 "base editor." Base editing allows for the direct and irreversible conversion of one target DNA base to another in a programmable manner without the need for DNA cleavage or a donor DNA molecule. For example, Komor et al. (2016, Nature, 533:420-424) teach a Cas9-cytidine deaminase fusion, in which Cas9 is also engineered to be inactive and not induce double-stranded DNA breaks. In addition, Gaudelli et al. (2017, Nature, doi:10.1038 / nature24644) teach a catalytically impaired Cas9 fused to tRNA adenosine deaminase, which can mediate the conversion of A / T to G / C in a target DNA sequence. Another class of engineered Cas9 nucleases that can act as site-specific modifying polypeptides in the methods and compositions of the invention are variants that can recognize a wide range of PAM sequences, including NG, GAA, and GAT (Hu et al., 2018, Nature, doi:10.1038 / nature26155).

[0104] Embodiment In one embodiment, the inventors provide a fusion protein comprising, from N- to C-terminal, a heterologous domain, a first linker sequence, and a type V CRISPR-Cas enzyme, wherein the first linker sequence comprises a repeated GGGGS sequence. In one aspect, the heterologous domain is a deaminase, polymerase, nuclease, relaxase, alkyltransferase, methyltransferase, adenosine deaminase, cytidine deaminase, oxidase, thymine alkyltransferase, adenine oxidase, adenosine methyltransferase, glycosylase, or nuclear localization signal. In another aspect, the heterologous domain is a deaminase domain. In yet another aspect, the deaminase domain is a cytidine deaminase. In another aspect, the cytidine deaminase domain is activation-induced cytidine deaminase ("AID"). In yet another embodiment, the cytidine deaminase domain is an apolipoprotein B mRNA editing complex ("APOBEC") domain. In another embodiment, the APOBEC domain is a deaminase of the APOBEC1 family. In yet another embodiment, the APOBEC domain comprises a sequence at least 70% identical to SEQ ID NO:1. In another embodiment, the deaminase domain is an adenine deaminase. In yet another embodiment, the adenine deaminase is a TadA domain comprising a sequence at least 70% identical to SEQ ID NO:92.

[0105] In one embodiment, the Type V CRISPR-Cas enzyme is a Type VA ("Cas12a") enzyme. In another embodiment, the Cas12a domain is selected from the group consisting of SEQ ID NO:3, SEQ ID NO:6, SEQ ID NO:22, SEQ ID NO:45, SEQ ID NO:46, SEQ ID NO:47, and SEQ ID NO:48. In yet another embodiment, the Cas12a domain is catalytically inactive and is selected from the group consisting of SEQ ID NO:3, SEQ ID NO:6, and SEQ ID NO:22.

[0106] In one embodiment, the first linker sequence comprises at least 3 repeated GGGGS, hi another embodiment, the first linker sequence comprises at least 6 repeated GGGGS.

[0107] In one embodiment, the fusion protein comprises a sequence selected from the group consisting of SEQ ID NOs: 11, 12, 13, and 44. In another embodiment, the fusion protein further comprises a uracil DNA glycosylase inhibitor ("UGI") domain. In yet another embodiment, the UGI domain comprises SEQ ID NO: 8. In another embodiment, the UGI domain is linked to the Cas12a enzyme by a second linker comprising the sequence SGGS. In yet another embodiment, the fusion protein comprises a sequence selected from the group consisting of SEQ ID NO: 17, SEQ ID NO: 24, SEQ ID NO: 35, SEQ ID NO: 39, SEQ ID NO: 43, SEQ ID NO: 50, SEQ ID NO: 52, SEQ ID NO: 54, SEQ ID NO: 56, SEQ ID NO: 81, SEQ ID NO: 83, SEQ ID NO: 85, SEQ ID NO: 87, and SEQ ID NO: 89. In another embodiment, the fusion protein, when contacted with DNA, produces a higher frequency of on-target editing and a lower frequency of off-target editing compared to fusion proteins having a first linker sequence other than a repetitive GGGGS sequence.

[0108] In another embodiment, the inventors provide a method for editing plant genomic DNA, comprising contacting plant genomic DNA with (a) a fusion protein of the above aspect, optionally comprising a UGI domain, and (b) a guide RNA ("gRNA") that targets the fusion protein of step (a) to a target DNA sequence in the plant genomic DNA, wherein the edited plant genomic DNA has reduced off-target editing compared to plant genomic DNA edited with a fusion protein having a first linker other than repetitive GGGGS sequences.

[0109] In another embodiment, the inventors provide a method of editing plant genomic DNA with reduced off-target editing, comprising contacting plant genomic DNA with (a) a fusion protein of the above aspect, optionally comprising a UGI domain, and (b) a guide RNA ("gRNA") that targets the fusion protein of step (a) to a target DNA sequence in the plant genomic DNA, wherein the edited plant genomic DNA has reduced off-target editing compared to plant genomic DNA edited with a fusion protein having a first linker other than a repetitive GGGGS sequence. In an aspect, the fusion protein comprises SEQ ID NO:24.

[0110] In another embodiment, the present inventors provide a method for obtaining a population of edited plants with reduced off-target editing, the method comprising: (a) obtaining a population of plant cells comprising genomic DNA to be edited; (b) obtaining a nucleotide sequence encoding the fusion protein of the above aspect and any UGI domain; (c) transforming the population of plant cells with the nucleotide sequence of step (b), thereby expressing the fusion protein encoded by the nucleic acid sequence in the population of plant cells; (d) growing the population of transformed plant cells into plants, at least one of which is edited; and (e) selecting at least one edited plant from the product of step (d), thereby obtaining a population of edited plants, wherein the population of edited plants has reduced off-target editing compared to plants edited with a fusion protein having a first linker other than a repetitive GGGGS sequence. In one aspect, the nucleotide sequence encoding the fusion protein comprises SEQ ID NO: 17, SEQ ID NO: 24, SEQ ID NO: 35, SEQ ID NO: 39, SEQ ID NO: 43, SEQ ID NO: 50, SEQ ID NO: 52, SEQ ID NO: 54, SEQ ID NO: 56, SEQ ID NO: 81, SEQ ID NO: 83, SEQ ID NO: 85, SEQ ID NO: 87, and SEQ ID NO: 89. In some embodiments, codon-optimized polynucleotides are provided that encode fusion proteins comprising one or more DNA-binding domains and one or more DNA-modifying domains connected by an improved linker sequence. [Example]

[0111] The following examples provide illustrative embodiments. In view of the present disclosure and the general state of the art, those skilled in the art will understand that the following examples are intended to be illustrative only, and that numerous changes, modifications, and variations can be adopted without departing from the scope of the subject matter of the present disclosure.

[0112] Example 1. Construction of vectors for expression of dLbCas12a-BE and guide RNA We fused catalytically inactive Lachnospiraceae bacterium Cas12a (hereafter "dLbCas12a," previously known as dLbCpf1) containing the D832A / E925A / D1148A mutations, rat cytidine deaminase (APOBEC1), and uracil-DNA glycosylase inhibitor (UGI) into a single protein via an amino acid linker, enabling advantageous properties for base editing in plants. This fusion construct was optimized for Zea mays codons, commercially synthesized (GenScript, Nanjing, China), and cloned under the sugarcane ubiquitin-4 (SoUbi4) gene promoter to constitutively generate dLbCas12a-BE.

[0113] In construct 24524, dLbCas12a-BE contained a nuclear localization signal (SV40-NLS) followed by APOBEC1 linked to dLbCas12a by an XTEN protein linker, and then by SV40-NLS linked to UGI by an SGGS linker. SV40-NLS was also incorporated at the C-terminus of UGI by an SGGS linker, improving nuclear targeting of this fusion protein. The synthetic sequence of dLbCas12a-BE, generated using maize-optimized codons, is set forth in SEQ ID NO: 18.

[0114] In construct 24904, dLbCas12a-BE, the SV40-NLS was followed by APOBEC1 linked to dLbCas12a by a 30-amino acid linker, GGGGS GGGGS GGGGS GGGGS GGGGS GGGGS (SEQ ID NO: 11), which contains six GGGGS amino acid repeats, designated (G4S)x6, and then by the SV40NLS linked to UGI by an SGGS linker. The SV40-NLS was also incorporated at the C-terminus of UGI by an SGGS linker, improving targeting of this fusion protein to the nucleus. The synthetic sequence of dLbCas12a-BE, generated with maize-optimized codons, is set forth in SEQ ID NO: 23.

[0115] In construct 25057, dLbCas12a-BE contained an SV40-NLS followed by APOBEC1 linked to dLbCas12a by an XTEN protein linker, followed by an SV40-NLS linked to UGI by an 18-amino acid linker, GGSTG GGSGG GSGGG SSG (SEQ ID NO: 12), designated SX. SV40-NLS was also incorporated at the C-terminus of UGI by a 15-amino acid linker, GGGGS GGGGS, designated (G4S)x3, to improve targeting of dLbCas12a-BE into the nucleus. The synthetic sequence of dLbCas12a-BE generated with maize-optimized codons is set forth in SEQ ID NO: 14.

[0116] In construct 25058, dLbCas12a-BE contained an SV40-NLS followed by APOBEC1 linked to dLbCas12a by a 30-amino acid linker (G4S) × 6, followed by an SV40-NLS linked to UGI by an SX linker. An SV40-NLS was also incorporated at the C-terminus of UGI by a (G4S) × 3 linker, improving nuclear targeting of dLbCas12a-BE. The synthetic sequence of dLbCas12a-BE generated with maize-optimized codons is set forth in SEQ ID NO: 16.

[0117] In the dLbCas12a-BE construct, a CRISPR / Cas12a guide RNA transcript was expressed under the control of the SoUbi4 promoter. This CRISPR / Cas12a guide RNA transcript targeted the maize Waxy1 exon 4 region and changed C9, C10, or C22 following the PAM sequence in exon 4 to T. This CRISPR / Cas12a guide RNA transcript also contained direct repeats of LbCrRNA as a scaffold. The synthetic sequence of the guide RNA is set forth in SEQ ID NO:26.

[0118] In construct 24784, a nuclear localization signal (xSV40NLS-06) was followed by a cytidine deaminase (xAPOBEC1-01) gene linked to a maize-optimized Cas9 gene (cCas9BE-02) by xXTEN-02, followed by a nuclear localization signal (xSV40NLS-04) linked to a uracil DNA glycosylase inhibitor, xUGI-02, by xSGGS linker-02, which in turn was linked to the nuclear localization signal, xSV40NLS-07, by xSGGS linker-02. This fusion protein was driven under the control of the sugarcane ubiquitin-4 promoter (prSoUbi4-02) followed by the NOS terminator (tNOS-05-01). The Cas9 protein is a nickase Cas9 mutant with D10A and uracil DNA glycosylase inhibitor (UGI) fused to rat APOBEC1. A nuclear localization signal was also incorporated into the C-terminus of Cas9 to improve targeting of Cas9 to the nucleus. The synthetic sequence of cCas9BE-02 is set forth in SEQ ID NO:20.

[0119] Example 2. Agrobacterium-mediated transformation of maize embryos To generate potential editing events in maize Wx1, the elite maize transgenic variety NP2222 was selected for all experiments described (WO 16106121, incorporated herein by reference).

[0120] For maize transformation, maize variety NP2222 was employed. Maize ears were harvested from GH when the immature embryos were approximately 1.2 mm, and then the ears were sterilized with 20% Clorox solution for 20 minutes and rinsed three times with sterile water.

[0121] Agrobacterium tumefaciens strain LBA4404 17740 RecA carrying the vector by electroporation - The cultures were streaked onto YP medium containing Gent antibiotic (25 μg / ml) and Spec antibiotic (100 μg / ml) and grown for 2 days at 28°C. Prior to transformation, single colonies were selected, streaked onto fresh YP plates, and grown for 1 day at 28°C. Agrobacterium was resuspended using the inoculation medium. OD 660 was adjusted to 0.25.

[0122] We removed the endosperm, isolated the immature embryos with a sterile scalpel, and collected them en bloc. We then injected the infected immature embryos into an Agrobacterium suspension for 2-3 minutes. The infected embryos were transferred to co-cultivation medium at 22°C for 2-4 days.

[0123] After the co-cultivation step, the embryos were transferred to medium containing the selection agent in the dark at 28° C. for 4 weeks. Resistant embryogenic calli were transferred to regeneration medium and cultured at 28° C. under a 16 / 8 photoperiod. After approximately 3 weeks, the regenerated plantlets were transferred to growth containers containing rooting medium under the same incubation temperature and light conditions.

[0124] Example 3. Analysis of edited bases in target regions We used the Phire Plant Direct PCR Master Mix (Thermo Fisher, F160L) to amplify a 410-bp DNA fragment containing the target region directly from maize leaf samples. DNA purification was not required prior to PCR. Mutations at the target site were analyzed by Sanger sequencing of the amplified DNA fragment.

[0125] DNA extraction and PCR amplification were performed according to the manufacturer's recommendations. A portion of a young leaf (e.g., a punch approximately 2 mm in diameter) was placed in 30 μL of dilution buffer. The leaf sample was crushed with a 100 μL pipette tip by gently pressing it against the wall of the tube, and 20 μL of dilution buffer was added. After crushing the leaf, the solution was greenish. The plant material was spun down in a centrifuge, and 1 μL of the supernatant was used as a template for a 20 μL PCR reaction.

[0126] The PCR system consists of the following:

[0127] [Table 1]

[0128] PCR primers for ZmWaxy1: Forward primer: 5'-AGATGGGAGACGGGTACGAGACGG-3' (SEQ ID NO: 29) Reverse primer: 5'-GTATGGGTTGTTGTTGAGGCTCAGG-3' (SEQ ID NO: 30) DNA sequencing primer: 5'-GACCACCCACTGTTCCTGGAGAGGG-3' (SEQ ID NO: 31)

[0129] PCR conditions: 98°C for 5 min; 98°C for 5 seconds followed by 35 cycles of 60°C for 5 seconds; 72°C for 20 seconds; 72°C for 1 minute; and Hold at 4°C until ready for analysis.

[0130] Sequencing: PCR products were separated by agarose gel electrophoresis and purified before Sanger DNA sequencing with specific primers. In the case of heterozygous mutations, double peaks were observed at the target nucleotide position, whereas a unique single peak distinct from the control was considered a homozygous mutation. ZmWxy1 exon 4 was amplified and sequenced using transgenic events of constructs 24524, 24904, and 24784 to assess base editing.

[0131] [Table 2]

[0132] The edited nucleotides are shown in gray shading. As shown above, this version of the Cas12a base editor, which contains an XTEN linker between the APOBEC domain and the site-specific nuclease, most efficiently edited cysteine ​​to thiamine at positions 5 and 6. However, there are examples of guanine to adenine edits at positions -2, 7, and 49. Positions are determined by the number of nucleotides away from the start of the PAM site.

[0133] [Table 3]

[0134] The edited nucleotides are shown shaded gray. In this version, the Cas12a base editor, which contains an XTEN linker between the APOBEC domain and the inactivated site-specific nuclease, edited cysteines to thiamines at positions 9, 10, and 22, and guanines to adenines at positions 39, 44, 52, and especially 53. Where guanines are edited to adenines, editing occurs on the complementary strand.

[0135] [Table 4]

[0136] The edited nucleotides are shown in gray shading. In this version, the Cas12a base editor, which contains a long linker containing (G4S)6 between the APOBEC domain and the inactivated site-specific nuclease, edited cysteines to thiamines at positions 9 and 10, and guanines to adenines at positions 19 and 53. The guanine to adenine edits indicate editing on the complementary strand.

[0137] Example 4. Measurement of editing efficiency.

[0138] [Table 5]

[0139] Table 4 shows how the base editing efficiency of Cas12a with a long linker compares to that of Cas9. Without optimization, Cas12aBE has a low editing efficiency of approximately 5%, much lower than that of Cas9 (87%). However, the addition of a long linker operably linking the deaminase to catalytically inactive Cas12a improved editing efficiency by 12-fold.

[0140] [Table 6]

[0141] Table 5 shows a direct comparison of the editing efficiency of the LbCas12a base editor when operably linked to either an XTEN linker or a long linker. Editing efficiency of difficult targets is improved by nearly 5-fold when the deaminase is operably linked to a site-specific nuclease by a long linker (e.g., (G4S)6).

[0142] [Table 7]

[0143] [Table 8]

[0144] High editing efficiency can be achieved by using multiple guide RNA molecules in the same construct ("multiplexing" or "multiple editing") and by having a long linker between the nuclear localization signal and the active Cas12a. Even difficult targets such as SBEIIb have achieved acceptable editing efficiency when they are part of a multiplexed editing experimental design.

[0145] Example 5. Improved editing in soybean. Soybean editing using a combination of long linkers and Cas12a is also greatly improved: GmFAD2 editing with standard Cas12a and long linker-Cas12a is improved by nearly 7-fold.

[0146] [Table 9]

[0147] Example 6. Long linkers improved Mb2Cas12a editing in maize. This long linker also improves the editing efficiency of additional Cas12 enzymes, such as Mb2Cas12a.

[0148] [Table 10]

[0149] Without the long linker, Mb2Cas12a did not edit the target sequence at all, but the long linker significantly improved the editing efficiency.

[0150] Example 7. Other heterologous domains operably linked to Cas12a, connected by long linkers. It is within the scope of the present invention to tether heterologous domains (beyond just the APOBEC deaminase) to Cas12a via long linkers, including, but not limited to, deaminase, polymerase, nuclease, relaxase, alkyltransferase, methyltransferase, adenosine deaminase, cytidine deaminase, oxidase, thymine alkyltransferase, adenine oxidase, adenosine methyltransferase, glycosylase, or nuclear localization signal.

[0151] We operably linked adenine deaminase to Cas12a to create the Cas12a adenine base editor ("Cas12a-ABE"). We fused catalytically inactive LbCas12a (containing D832A, E925A, and D1148A mutations) to Escherichia coli (E. coli) wild-type adenine deaminase ("TadA," engineered to contain W23R, H36L, P48A, R51L, L84F, A106V, D108N, H123Y, S146C, D147Y, R152P, E155V, I156F, and K157N amino acid substitutions) operably linked by an amino acid linker. This fusion construct was optimized for Zea mays codons, commercially synthesized (GenScript, Nanjing, China), and cloned under the sugarcane ubiquitin-4 (SoUbi4) gene promoter to constitutively generate dLbCa12a-BE.

[0152] In construct 25459, dLbCas12a-ABE, a 189-bp potato intron was inserted into the TadA coding sequence following a TadA variant linked by an XTEN protein linker to create a TadA dimer. This was fused to dLbCas12a, and an SV40-NLS was also incorporated into the C-terminus of dLbCas12a via a GS linker to improve targeting of the fusion protein into the nucleus. The synthetic sequence of dLbCas12a-ABE, generated using maize-optimized codons, is set forth in SEQ ID NO:79.

[0153] In construct 25504, dLbCas12a-ABE, a 189-bp potato intron was inserted into the TadA coding sequence following the TadA variant to create a TadA dimer. This was linked to dLbCas12a by a 30-amino acid linker (G4S) × 6 protein linker, and an SV40-NLS was also incorporated into the C-terminus of dLbCas12a via a GS linker to improve targeting of the fusion protein into the nucleus. The synthetic sequence of dLbCas12a-ABE, generated with maize-optimized codons, is set forth in SEQ ID NO:81.

[0154] In the dLbCas12a-ABE construct, the CRISPR / Cas12a guide RNA transcript is expressed under the control of the SoUbi4 promoter, and this CRISPR / Cas12a guide RNA transcript targets the maize Waxy1 gene. This CRISPR / Cas12a guide RNA transcript also contains the direct repeat of LbCrRNA as a scaffold. The synthetic sequence of the guide RNA is set forth in SEQ ID NO:74.

[0155] Experiments with construct 25459 (in which adenine deaminase was linked to dLbCas12a by an XTEN linker) did not result in detectable editing when used in corn plants. Experiments with construct 25504, in which adenine deaminase was linked to dLbCas12a by a (G4S)*6 long linker, resulted in an editing efficiency of 7%, approximately half that of the Cas9ABE control (construct 24785). See Table 10.

[0156] [Table 11]

[0157] We believe this is the first time that a Cas12a ABE has been shown to function in plants, and we attribute this technical success to the use of a long linker to operably link adenine deaminase to Cas12a.

[0158] Example 8. Dual base editor in maize Dual base editors (cytidine deaminase and adenine deaminase domains fused to a Cas enzyme). This concept could apply targeted saturation mutagenesis of crop genes to generate gene variants with improved agronomic performance (e.g., C:G>T:A and A:T>G:C substitutions in the same target region). We multiplexed four guide RNAs: one targeting the ZmWaxy1 gene and three different guide RNAs targeting the ZmADH gene.

[0159] [Table 12]

[0160] [Table 13]

[0161] In total, dLbCas12a-based CBE-ABE resulted in 1% C-to-T and A-to-G mutations. The addition of an intron increases vector stability but can reduce enzyme activity due to inefficient splicing. This is believed to be the first example of double CBE-ABE editing in plants using Cas12a.

[0162] [Table 14]

[0163] In the table above, most Cas12aBE constructs follow the pattern of heterologous enzyme domain-linker-Cas enzyme. Exceptions to this pattern are 25702 [TadA dimer-linker-PmCDA-linker-Cas enzyme], 25701 [PmCDA-linker-TadA dimer-linker-Cas enzyme], and 25658 [TadA dimer-linker-Cas enzyme-PmCDA]. Additional nuclear localization sequences, uracil glycosylase inhibitors, and other components may be present but are not shown in this table. Such details are provided in the sequences listed in the attached sequence listing.

[0164] The examples and embodiments provided herein are non-limiting illustrations of the claims and should not be construed as the only practical examples. Further variations may be implemented by those skilled in the art.

Claims

1. 1. A fusion protein comprising, in N-terminal to C-terminal direction, a heterologous domain, a first linker sequence, and a V-type CRISPR-Cas enzyme, wherein the first linker sequence comprises repeated GGGGS sequences.

2. 2. The fusion protein of claim 1, wherein the heterologous domain is a deaminase, polymerase, nuclease, relaxase, alkyltransferase, methyltransferase, adenosine deaminase, cytidine deaminase, oxidase, thymine alkyltransferase, adenine oxidase, adenosine methyltransferase, glycosylase, or nuclear localization signal.

3. The fusion protein of claim 2 , wherein the heterologous domain is a deaminase domain.

4. The fusion protein of claim 3, wherein the deaminase domain is a cytidine deaminase.

5. The fusion protein of claim 4, wherein the cytidine deaminase domain is activation-induced cytidine deaminase ("AID").

6. The fusion protein of claim 4 , wherein the cytidine deaminase domain is an apolipoprotein B mRNA editing complex (“APOBEC”) domain.

7. 7. The fusion protein of claim 6, wherein the APOBEC domain is a deaminase of the APOBEC1 family.

8. The fusion protein of claim 7 , wherein the APOBEC domain comprises a sequence that is at least 70% identical to SEQ ID NO:

1.

9. The fusion protein of claim 3 , wherein the deaminase domain is an adenine deaminase.

10. The fusion protein of claim 9, wherein the adenine deaminase is a TadA domain.

11. The fusion protein of claim 10, wherein the TadA domain comprises a sequence at least 70% identical to SEQ ID NO:

92.

12. The fusion protein of claim 1, wherein the type V CRISPR-Cas enzyme is a type VA (Cas12a) enzyme.

13. 13. The fusion protein of claim 12, wherein the Cas12a domain is selected from the group consisting of SEQ ID NO:3, SEQ ID NO:6, SEQ ID NO:22, SEQ ID NO:45, SEQ ID NO:46, SEQ ID NO:47, and SEQ ID NO:

48.

14. 14. The fusion protein of claim 13, wherein the Cas12a domain is catalytically inactive and is selected from the group consisting of SEQ ID NO:3, SEQ ID NO:6, and SEQ ID NO:

22.

15. 2. The fusion protein of claim 1, wherein the first linker sequence comprises at least three repeated GGGGS.

16. 16. The fusion protein of claim 15, wherein the first linker sequence comprises at least six repeated GGGGS.

17. The fusion protein of any one of claims 1 to 16, wherein the fusion protein comprises a sequence selected from the group consisting of SEQ ID NOs: 11, 12, 13, and 44.

18. 18. The fusion protein of any one of claims 1 to 17, further comprising a uracil DNA glycosylase inhibitor ("UGI") domain.

19. 19. The fusion protein of claim 18, wherein the UGI domain comprises SEQ ID NO:

8.

20. 20. The fusion protein of Claim 19, wherein the UGI domain is linked to the Cas12a enzyme by a second linker comprising the sequence SGGS.

21. 2. The fusion protein of claim 1, wherein the fusion protein comprises a sequence selected from the group consisting of SEQ ID NO:17, SEQ ID NO:24, SEQ ID NO:35, SEQ ID NO:39, SEQ ID NO:43, SEQ ID NO:50, SEQ ID NO:52, SEQ ID NO:54, SEQ ID NO:56, SEQ ID NO:81, SEQ ID NO:83, SEQ ID NO:85, SEQ ID NO:87, and SEQ ID NO:

89.

22. 2. The fusion protein of Claim 1, wherein, when contacted with DNA, the fusion protein undergoes on-target editing at a higher frequency and off-target editing at a lower frequency compared to a fusion protein having a first linker sequence other than a repetitive GGGGS sequence.

23. A method for editing plant genomic DNA, comprising: plant genomic DNA; (a) a fusion protein according to any one of claims 1 to 17, optionally comprising a UGI domain; and (b) a guide RNA (“gRNA”) that targets the fusion protein of step (a) to a target DNA sequence in the plant genomic DNA. contacting the the edited plant genomic DNA has less off-target editing compared to plant genomic DNA edited by a fusion protein having a first linker other than a repetitive GGGGS sequence; method.

24. 1. A method for editing plant genomic DNA with low off-target editing, comprising: plant genomic DNA; (a) a fusion protein according to any one of claims 1 to 17, optionally comprising a UGI domain; and (b) a guide RNA (“gRNA”) that targets the fusion protein of step (a) to a target DNA sequence in the plant genomic DNA. contacting the the edited plant genomic DNA has less off-target editing compared to plant genomic DNA edited by a fusion protein having a first linker other than a repetitive GGGGS sequence; method.

25. 25. The method of claim 24, wherein the fusion protein comprises SEQ ID NO:

24.

26. 1. A method for obtaining a population of edited plants with low off-target editing, comprising: (a) obtaining a population of plant cells containing the genomic DNA to be edited; (b) obtaining a nucleotide sequence encoding the fusion protein of any one of claims 1 to 16 and any UGI domain; (c) transforming the population of plant cells with the nucleotide sequence of step (b), thereby expressing in the population of plant cells the fusion protein encoded by the nucleic acid sequence; (d) growing the population of transformed plant cells into plants, at least one of the plants being edited; and (e) selecting at least one edited plant from the product of step (d), thereby obtaining a population of edited plants. Including, the population of edited plants has reduced off-target editing compared to plants edited with a fusion protein having a first linker other than a repetitive GGGGS sequence. method.

27. 27. The method of claim 26, wherein the nucleotide sequence encodes a fusion protein selected from the group consisting of SEQ ID NO:17, SEQ ID NO:24, SEQ ID NO:35, SEQ ID NO:39, SEQ ID NO:43, SEQ ID NO:50, SEQ ID NO:52, SEQ ID NO:54, SEQ ID NO:56, SEQ ID NO:81, SEQ ID NO:83, SEQ ID NO:85, SEQ ID NO:87, and SEQ ID NO:89.

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