Modified terminal deoxynucleotidyl transferase (TdT) enzymes

Abstract

A modified terminal deoxynucleotidyl transferase (TdT) enzyme comprising amino acid modifications when compared to a wild type sequence SEQ ID NO 1 or a truncated version thereof or the homologous ami

Description

20 01 25 FIELD OF THE INVENTION The invention relates to the use of specific terminal deoxynucleotidyl transferase (TdT) enzymes or 5 the homologous amino acid sequence of Polp, Poip, PoIX, and PolQ of any species or the homologous amino acid sequence of X family polymerases of any species in a method of nucleic acid synthesis, to methods of synthesizing nucleic acids, and to the use of kits comprising said enzymes in a method of nucleic acid synthesis. The invention also relates to the use of terminal deoxynucleotidyl transferases or homologous enzymes in a method of template independent nucleic acid synthesis. 10 BACKGROUND OF THE INVENTION Nucleic acid synthesis is vital to modern biotechnology. The rapid pace of development in the biotechnology arena has been made possible by the scientific community's ability to artificially synthesise DNA, RNAand proteins. 15 Artificial DNA synthesis allows biotechnology and pharmaceutical companies to develop a range of peptide therapeutics, such as insulin for the treatment of diabetes. It allows researchers to characterise cellular proteins to develop new small molecule therapies for the treatment of diseases our aging population faces today, such as heart disease and cancer. It even paves the way forward to 20 creating life, as the Venter Institute demonstrated in 2010 when they placed an artificially synthesised genome into a bacterial cell. However, current DNA synthesis technology does not meet the demands of the biotechnology industry. Despite being a mature technology, it is highly challenging to synthesise a DNA strand greater 25 than 200 nucleotides in length in viable yield, and most DNA synthesis companies only offer up to 120 nucleotides routinely. In comparison, an average protein-coding gene is of the order of 2000-3000 contiguous nucleotides, a chromosome is at least a million contiguous nucleotides in length and an average eukaryotic genome numbers in the billions of nucleotides. In order to prepare nucleic acid strands thousands of base pairs in length, all major gene synthesis companies today rely on variations 30 of a 'synthesise and stitch' technique, where overlapping 40-60-mer fragments are synthesised and stitched together by enzymatic copying and extension. Current methods generally allow up to 3 kb in length for routine production. 20 01 25 The reason DNA cannot be synthesised beyond 120-200 nucleotides at a time is due to the current methodology for generating DNA, which uses synthetic chemistry (i.e., phosphoramidite technology) to couple a nucleotide one at a time to make DNA. Even if the efficiency of each nucleotide-coupling step is 99% efficient, it is mathematically impossible to synthesise DNA longer than 200 nucleotides in 5 acceptable yields. The Venter Institute illustrated this laborious process by spending 4 years and 20 million USD to synthesise the relatively small genome of a bacterium. Known methods of DNA sequencing use template-dependent DNA polymerases to add 3'-reversibly terminated nucleotides to a growing double-stranded substrate. In the 'sequencing-by-synthesis' 10 process, each added nucleotide contains a dye, allowing the user to identify the exact sequence of the template strand. Albeit on double-stranded DNA, this technology is able to produce strands of between 500-1000 bps long. However, this technology is not suitable for de novo nucleic acid synthesis because of the requirement for an existing nucleic acid strand to act as a template. 15 Various attempts have been made to use a terminal deoxynucleotidyl transferase for de novo singlestranded DNA synthesis. Uncontrolled de novo single-stranded DNA synthesis, as opposed to controlled, takes advantage of TdT's deoxynucleoside 5'-triphosphate (dNTP) 3'- tailing properties on single-stranded DNA to create, for example, homopolymeric adaptor sequences for next-generation sequencing library preparation. In controlled extensions, reversible deoxynucleoside 5'-triphosphate 20 termination technology needs to be employed to prevent uncontrolled addition of dNTPs to the 31-end of a growing DNA strand. The development of a controlled single-stranded DNA synthesis process through TdT would be invaluable to in situ DNA synthesis for gene assembly or hybridization microarrays as it removes the need for an anhydrous environment and allows the use of various polymers incompatible with organic solvents. 25 However, TdT has not been shown to efficiently add nucleoside triphosphates containing 3'-O-reversibly terminating moieties for building up a nascent single-stranded DNA chain necessary for a de novo synthesis cycle. A 3'-O- reversible terminating moiety would prevent a terminal transferase like TdT from catalysing the nucleotide transferase reaction between the 3'-end of a growing DNA 30 strand and the 5'-triphosphate of an incoming nucleoside triphosphate. There is therefore a need to identify modified terminal deoxynucleotidyl transferases that readily incorporate 3'-O- reversibly terminated nucleotides. Said modified terminal deoxynucleotidyl transferases can be used to incorporate 3'-O- reversibly terminated nucleotides in a fashion useful for 20 01 25 biotechnology and single-stranded DNA synthesis processes in order to provide an improved method of nucleic acid synthesis that is able to overcome the problems associated with currently available methods. The applicants have previously identified novel enzymes in application PCT / GB2020 / 050247. Described herein are further improved enzymes. 5 BRIEF DESCRIPTION OF THE FIGURES Figure 1. Sequence alignment of selected orthologs of wild-type terminal deoxynucleotidyl transferases using the Clustal Omega multiple sequence alignment program provided by the European Molecular Biology Laboratory (EMBL) multiple sequence alignment site. 10 Figure 2. Tailing experiment (Cacodylate buffer containing 1 mM CoCk) with enzyme variant (all equimolar)) at 37°C for 15 min. Wild-type bovine TdT (WT B.TdT), bovine TdT E456G N473G, and GenlOC are compared for their incorporation activity of the dNTPs as indicated on the gel. 15 Figure 3. dATP (1 mM dATP) tailing experiment (IxCutSmart buffer + 1 mM CoCk) with enzyme variant (3 pM) at indicated temperature(°C) for 15 min. D09 is Gar R3-F1TSA Variant 1 and Dll is Gar R3-F1 TSA Variant 2. Figure 4. The dNTP terminal transferase capability of R3-F1 TdT parent (left gel, SEQ 6.1) was 20 compared to R3-F1 TdT TSA variant 1 (right gel, SEQ. 6.2). Reactions were incubated for 10 min at the specified temperature (°C) indicated on the figure with a 5'-fluorophore labelled single-stranded DNA primer (0.1 pM) in IxCutSmart buffer (New England Biolabs), 1 mM CoCI2, 0.5 mM indicated dNTP on the figure, 0.05 pl thermostable inorganic pyrophosphatase (New England Biolabs, 2,000 units / ml stock). Reactions were first pre-incubated at the specified temperature for 5 min with all of the 25 reaction components assembled except the indicated dNTP; the indicated dNTP was added to initiate the reaction. After the 10 min reaction, reactions were killed with stop buffer consisting of sodium hydroxide, EDTA, and 50% v / v formamide. Reactions were directly loaded on a 20% denaturing PAGE gel and imaged by virtue of the 5'-fluorophore on the reacted ssDNA primer. From Figure 4, it is clear that the 6.2 Gar R3-F1 TSA mutant 1 (bovine E456G / N473G) (truncated gar 30 E301G and R318G) convey both thermostability and enhanced terminal transferase activity at all temperatures tested when compared to the 6.1 Gar R3-F1 parent. The 6.1 Gar TdT, like WT TdTs, disfavors dATP tailing as indicated by the shorter tail lengths in Figure 4 compared with the other dNTPs. Remarkably, the 6.2 Gar R3-F1 TSA mutant 1 demonstrates a drastic increase in terminal transferase activity for dATP; indeed, 6.2 demonstrates higher activity for dATP than any other dNTP. SUMMARY OF THE INVENTION Described herein are modified terminal deoxynucleotidyl transferase (TdT) enzymes or the homologous amino acid sequence of Polp, Poip, PoIX, and Pol0 of any species or the homologous amino acid sequence of X family polymerases of any species. Terminal transferase enzymes are 5 ubiquitous in nature and are present in many species. Many known TdT sequences have been reported in the NCBI database http: / / www.ncbi.nim.nih.gov / . 20 01 25 Gl Number Species http: / / www.ncbi.nlm.nih.gov / gi 768 Bos taurus 10 gi 460163 Gallus gallus gi 494987 Xenopus laevis gi 1354475 Oncorhynchus mykiss gi 2149634 Monodelphis domestica gi 12802441 Mus musculus 15 gi 28852989 Ambystoma mexicanum gi 38603668 Takifugu rubripes gi 40037389 Raja eglanteria gi 40218593 Ginglymostoma cirratum gi 46369889 Danio rerio 20 gi 73998101 Canis lupus familiaris gi 139001476 Lemur catta gi 139001490 Microcebus murinus gi 139001511 Otolemur garnettii gi 148708614 Mus musculus 25 gi 149040157 Rattus norvegicus gi 149704611 Equus caballus gi 164451472 Bos taurus gi 169642654 Xenopus (Silurana) tropicalis gi 291394899 Oryctolagus cuniculus 30 gi 291404551 Oryctolagus cuniculus gi 301763246 Ailuropoda melanoleuca gi 311271684 Sus scrofa gi 327280070 Anolis carolinensis gi 334313404 Monodelphis domestica 20 01 25 gi 344274915 Loxodonta africana gi 345330196 Ornithorhynchus anatinus gi 348588114 Cavia porcellus gi 351697151 Heterocephalus glaber 5 gi 355562663 Macaca mulatta gi 395501816 Sarcophilus harrisii gi 395508711 Sarcophilus harrisii gi 395850042 Otolemur garnettii gi 397467153 Pan paniscus 10 gi 403278452 Saimiri boliviensis boliviensis gi 410903980 Takifugu rubripes gi 410975770 Felis catus gi 432092624 Myotis davidii gi 432113117 Myotis davidii 15 gi 444708211 Tupaia chinensis gi 460417122 Pleurodeles waltl gi 466001476 Orcinus orca gi 471358897 Trichechus manatus latirostris gi 478507321 Ceratotherium simum simum 20 gi 478528402 Ceratotherium simum simum gi 488530524 Dasypus novemcinctus gi 499037612 Maylandia zebra gi 504135178 Ochotona princeps gi 505844004 Sorex araneus 25 gi 505845913 Sorex araneus gi 507537868 Jaculus jaculus gi 507572662 Jaculus jaculus gi 507622751 Octodon degus gi 507640406 Echinops telfairi 30 gi 507669049 Echinops telfairi gi 507930719 Condylura cristata gi 507940587 Condylura cristata gi 511850623 Mustela putorius furo gi 512856623 Xenopus (Silurana) tropicalis 20 01 25 gi 512952456 Heterocephalus glaber gi 524918754 Mesocricetus auratus gi 527251632 Melopsittacus undulatus gi 528493137 Danio rerio 5 gi 528493139 Danio rerio gi 529438486 Falco peregrinus gi 530565557 Chrysemys picta bellii gi 532017142 Microtus ochrogaster gi 532099471 Ictidomys tridecemlineatus 10 gi 533166077 Chinchilla lanigera gi 533189443 Chinchilla lanigera gi 537205041 Cricetulus griseus gi 537263119 Cricetulus griseus gi 543247043 Geospiza fortis 15 gi 543351492 Pseudopodoces humilis gi 543731985 Columba livia gi 544420267 Macaca fascicularis gi 545193630 Equus caballus gi 548384565 Pundamilia nyererei 20 gi 551487466 Xiphophorus maculatus gi 551523268 Xiphophorus maculatus gi 554582962 Myotis brandtii gi 554588252 Myotis brandtii gi 556778822 Pantholops hodgsonii 25 gi 556990133 Latimeria chalumnae gi 1557297894 Alligator sinensis gi 1558116760 Pelodiscus sinensis gi 1558207237 Myotis lucifugus gi 1560895997 Camelus ferus 30 gi 1560897502 Camelus ferus gi 1562857949 Tupaia chinensis gi 1562876575 Tupaia chinensis gi 1564229057 gi 1564236372 Alligator mississippiensis Alligator mississippiensis 20 01 25 gi 1564384286 Rattus norvegicus gi 1573884994 Lepisosteus oculatus The sequences of the various described terminal transferases show some regions of highly conserved 5 sequence, and some regions which are highly diverse between different species. A sequence alignment for sequences from a selection of species is shown in Figure 1. The inventors have modified the terminal transferase from Bos Taurus (shown as SED ID 1). However the corresponding modifications can be introduced into the analogous or homologous terminal 10 transferases from any other species, including the sequences listed above in the various NCBI entries, including those shown in Figure 2 or truncated versions thereof. The amino acid sequence of Bos Taurus is shown below (SEQ ID 1) 15 MDPLCTASSGPRKKRPRQVGASMASPPHDIKFQNLVLFILEKKMGTTRRNFLMELARRKGFRVENELS DSVTHIVAENNSGSEVLEWLQVQNIRASSQLELLDVSWLIESMGAGKPVEITGKHQLWRTDYSATPN PGFQKTPPLAVKKISQYACQRKTTLNNYNHIFTDAFEILAENSEFKENEVSYVTFMRAASVLKSLPFT IISMKDTEGIPCLGDKVKCIIEEIIEDGESSEVKAVLNDERYQSFKLFTSVFGVGLKTSEKWFRMGFR SLSKIMSDKTLKFTKMQKAGFLYYEDLVSCVTRAEAEAVGVLVKEAVWAFLPDAFVTMTGGFRRGKKI 20 GHDVDFLITSPGSAEDEEQLLPKVINLWEKKGLLLYYDLVESTFEKFKLPSRQVDTLDHFQKCFLILK LHHQRVDSSKSNQQEGKTWKAIRVDLVMCPYENRAFALLGWTGSRQFERDIRRYATHERKMMLDNHAL YDKTKRVFLKAESEEEIFAHLGLDYIEPWERNA All amino acid numbering is in reference to sequence ID 1, the full length sequence of 509 amino acids. 25 Applicants use truncations of the full length sequence which retain activity, and thus the truncations, being fewer amino acids, will have different numbering. The inventors have identified various amino acids modifications in the amino acid sequence having improved properties. The modifications described herein improve the ability to incorporate native 30 nucleotides, particularly dATP. The enzymes have a higher catalytic activity than the unmodified enzymes and operate at higher temperatures. Described herein are modified terminal deoxynucleotidyl transferase (TdT) enzymes comprising amino acid modifications when compared to a wild type sequence SEQ ID NO 1 or a truncated version 35 thereof or the homologous amino acid sequence of a terminal deoxynucleotidyl transferase (TdT) 20 01 25 enzyme in other species or the homologous amino acid sequence of Polp, Polp, PolA, and Pol0 of any species or the homologous amino acid sequence of X family polymerases of any species. Amino acid locations having improved activity include R193, E456, R457, or N473. 5 Described is a modified terminal deoxynucleotidyl transferase (TdT) enzyme wherein the modification is at R193. Described is a modified terminal deoxynucleotidyl transferase (TdT) enzyme wherein the modification 10 is at E456. Described is a modified terminal deoxynucleotidyl transferase (TdT) enzyme wherein the modification is at R457. 15 Described is a modified terminal deoxynucleotidyl transferase (TdT) enzyme wherein the modification is at N473. Described is a modified terminal deoxynucleotidyl transferase (TdT) enzyme wherein the amino acid modifications are selected from 20 a. R193 and / or b. Two or more of the amino acid positions E456, R457 and / or N473. Described is a modified terminal deoxynucleotidyl transferase (TdT) enzyme wherein modifications are at the three amino acid positions E456, R457, and N473. 25 Described is a modified terminal deoxynucleotidyl transferase (TdT) enzyme wherein modifications are at the three amino acid positions R193, E456, N473. Described is a modified terminal deoxynucleotidyl transferase (TdT) enzyme wherein modifications 30 are at the four amino acid positions R193, E456, R457, and N473. Modifications include one or more of R193H, E456G, R457S, and / or N473G. Modifications include R193H, E456G, R457S, and N473G. Modifications include any combination of the above mutations. Modifications include both E456G and N473G. Modifications include E456G, R457S, and N473G. 20 01 25 Modifications which improve the incorporation of nucleotides can be at one or more of the selected positions shown below. MDPLCTASSGPRKKRPRQVGASMASPPHDIKFQNLVLFILEKKMGTTRRNFLMELARRKGFRVENELS 5 DSVTHIVAENNSGSEVLEWLQVQNIRASSQLELLDVSWLIESMGAGKPVEITGKHQLWRTDYSATPN pgfqktpplavkkisqyacqrkttlnnynhiftdafeilaensefkenevsyvtfm|aasvlkslpft IISMKDTEGIPCLGDKVKCIIEEIIEDGESSEVKAVLNDERYQSFKLFTSVFGVGLKTSEKWFRMGFR SLSKIMSDKTLKFTKMQKAGFLYYEDLVSCVTRAEAEAVGVLVKEAVWAFLPDAFVTMTGGFRRGKKI GHDVDFLITSPGSAEDEEQLLPKVINLWEKKGLLLYYDLVESTFEKFKLPSRQVDTLDHFQKCFLILK 10 lhhqrvdssksnqqegktwkairvdlvmcpyenrafallgwtgsrqf®dirryatherkmmld|hal YDKTKRVFLKAESEEEIFAHLGLDYIEPWERNA References to particular sequences include truncations thereof. Included herein are modified terminal deoxynucleotidyl transferase (TdT) enzyme comprising at least one amino acid modification when 15 compared to a wild type sequence SEQ ID NO 1 or a truncated version thereof, or the homologous amino acid sequence of a terminal deoxynucleotidyl transferase (TdT) enzyme in other species, wherein the modification is selected from one or more of the amino acid of the sequence of SEQ ID NO 1 or the homologous regions in other species. 20 Truncated proteins may include at least the region shown below including one or more of the relevant modifications. KISQYACQRKTTLNNYNHIFTDAFEILAENSEFKENEVSYVTFMRAASVLKSLPFTIISMKDTEGIPC LGDKVKCIIEEIIEDGESSEVKAVLNDERYQSFKLFTSVFGVGLKTSEKWFRMGFRSLSKIMSDKTLK 25 FTKMQKAGFLYYEDLVSCVTRAEAEAVGVLVKEAVWAFLPDAFVTMTGGFRRGKKIGHDVDFLITSPG SAEDEEQLLPKVINLWEKKGLLLYYDLVESTFEKFKLPSRQVDTLDHFQKCFLILKLHHQRVDSSKSN QQEGKTWKAIRVDLVMCPYENRAFALLGWTGSRQFERDIRRYATHERKMMLDNHALYDKTKRVFLKAE SEEEIFAHLGLDYIEPWERNA 30 Described herein is a modified terminal deoxynucleotidyl transferase (TdT) enzyme comprising at least the Sequence ID 2: KISQYACQRKTTLNNYNHIFTDAFEILAENSEFKENEVSYVTFMRAASVLKSLPFTIISMKDTEGIPC LGDKVKCIIEEIIEDGESSEVKAVLNDERYQSFKLFTSVFGVGLKTSEKWFRMGFRSLSKIMSDKTLK 35 FTKMQKAGFLYYEDLVSCVTRAEAEAVGVLVKEAVWAFLPDAFVTMTGGFRRGKKIGHDVDFLITSPG SAEDEEQLLPKVINLWEKKGLLLYYDLVESTFEKFKLPSRQVDTLDHFQKCFLILKLHHQRVDSSKSN 20 01 25 QQEGKTWKAIRVDLVMCPYENRAFALLGWTGSRQFERDIRRYATHERKMMLDNHALYDKTKRVFLKAE SEEEIFAHLGLDYIEPWERNA or the homologous regions in other species, wherein the sequence has one or more amino acid 5 modifications in one or more of the amino acid positions R193, E456, R457, and / or N473 of the full length sequence. Homologous refers to protein sequences between two or more proteins that possess a common evolutionary origin, including proteins from superfamilies in the same species of organism as well as 10 homologous proteins from different species. Such proteins (and their encoding nucleic acids) have sequence homology, as reflected by their sequence similarity, whether in terms of percent identity or by the presence of specific residues or motifs and conserved positions. A variety of protein (and their encoding nucleic acid) sequence alignment tools may be used to determine sequence homology. For example, the Clustal Omega multiple sequence alignment program provided by the European 15 Molecular Biology Laboratory (EMBL) can be used to determine sequence homology or homologous regions. Improved sequences as described herein can contain two or more of the aforementioned modifications, namely, for example, 20 a. a first modification at position E456 of the sequence of SEQ ID NO 1 or the homologous region in other species; and b. a second modification at position N473 of the sequence of SEQ ID NO 1 or the homologous regions in other species or a truncated sequence. 25 or a. a first modification at position E456 of the sequence of SEQ ID NO 1 or the homologous region in other species; and b. a second modification at position R457 of the sequence of SEQ ID NO 1 or the homologous 30 region in other species; and c. a third modification at position N473 of the sequence of SEQ ID NO 1 or the homologous regions in other species or a truncated sequence. or 20 01 25 a. a first modification at position R193 of the sequence of SEQ ID NO 1 or the homologous region in other species; and b. a second modification at position E456 of the sequence of SEQ ID NO 1 or the homologous region in other species; and 5 c. a third modification at position R457ofthe sequence of SEQ ID NO 1 or the homologous region in other species; and d. a fourth modification at position N473 of the sequence of SEQ ID NO 1 or the homologous regions in other species or a truncated sequence. 10 As a comparison with other species, the sequence of Lepisosteus oculatus (spotted gar) TdT is shown below (Seq ID 3): MLHIPIFPPIKKRQKLPESRNSCKYEVKFSEVAIFLVERKMGSSRRKFLTNLARSKGFRIEDVLSDAVTHVVAEDNSAD ELWQWLQNSSLGDLSKIEVLDISWFTECMGAGKPVQVEARHCLVKSCPVIDQYLEPSTVETVSQYACQRRTTMEN 15 HNQIFTDAFAILAENAEFNESEGPCLAFMRAASLLKSLPHAISSSKDLEGLPCLGDQTKAVIEDILEYGQCSKVQDVLC DDRYQTIKLFTSVFGVGLKTAEKWYRKGFHSLEEVQADNAIHFTKMQKAGFLYYDDISAAVCKAEAQAIGQIVEET VRLIAPDAIVTLTGGFRRGKECGHDVDFLITTPEMGKEVWLLNRLINRLQNQGILLYYDIVESTFDKTRLPCRKFEAM DHFQKCFAIIKLKKELAAGRVQKDWKAIRVDFVAPPVDNFAFALLGWTGSRQFERDLRRFARHERKMLLDNHALY DKTKKIFLPAKTEEDIFAHLGLDYIDPWQRNA The amino acid positions are highlighted below MLHIPIFPPIKKRQKLPESRNSCKYEVKFSEVAIFLVERKMGSSRRKFLTNLARSKGFRIEDVLSDAVTHVVAEDNSAD ELWQWLQNSSLGDLSKIEVLDISWFTECMGAGKPVQVEARHCLVKSCPVIDQYLEPSTVETVSQYACQRRTTMEN hnqiftdafailaenaefnesegpclafm|aasllkslphaissskdleglpclgdqtkaviedileygqcskvqdvlc 25 DDRYQTIKLFTSVFGVGLKTAEKWYRKGFHSLEEVQADNAIHFTKMQKAGFLYYDDISAAVCKAEAQAIGQIVEET VRUAPDAIVTLTGGFRRGKECGHDVDFUTTPEMGKEVWLLNRUNRLQNQGILLYYDIVESTFDKTRLPCRKFEAM dhfqkcfaiiklkkelaagrvqkdwkairvdfvappvdnfafallgwtgsrqf||dlrrfarherkmlld|haly DKTKKIFLPAKTEEDIFAHLGLDYIDPWQRNA 30 The amino acid sequence of truncated Lepisosteus oculatusTdT is shown below: TVSQYACQRRTTMENHNQIFTDAFAILAENAEFNESEGPCLAFMRAASLLKSLPHAISSSKDLEGLPC LGDQTKAVIEDILEYGQCSKVQDVLCDDRYQTIKLFTSVFGVGLKTAEKWYRKGFHSLEEVQADNAIH FTKMQKAGFLYYDDISAAVCKAEAQAIGQIVEETVRLIAPDAIVTLTGGFRRGKECGHDVDFLITTPE 35 MGKEVWLLNRLINRLQNQGILLYYDIVESTFDKTRLPCRKFEAMDHFQKCFAIIKLKKELAAGRVQKD 20 01 25 WKAIRVDFVAPPVDNFAFALLGWTGSRQFERDLRRFARHERKMLLDNHALYDKTKKIFLPAKTEEDIF AHLGLDYIDPWQRNA ...wherein the sequence has one or more amino acid modifications in one or more of the amino acid 5 positions R193, E441, E442, and / or N458 of the full length sequence. The aforementioned positions or homologous to full length Bos taurus TdT amino acid positions R193, E456, R457, and N473, respectively. As a comparison with other species, the sequence of Mus musculus (mouse) TdT is shown below ()SE 10 ID 4): MDPLQAVHLGPRKKRPRQLGTPVASTPYDIRFRDLVLFILEKKMGTTRRAFLMELARRKGFRVENELS DSVTHIVAENNSGSDVLEWLQLQNIKASSELELLDISWLIECMGAGKPVEMMGRHQLVVNRNSSPSPV PGSQNVPAPAVKKISQYACQRRTTLNNYNQLFTDALDILAENDELRENEGSCLAFMRASSVLKSLPFP 15 ITSMKDTEGIPCLGDKVKSIIEGIIEDGESSEAKAVLNDERYKSFKLFTSVFGVGLKTAEKWFRMGFR TLSKIQSDKSLRFTQMQKAGFLYYEDLVSCVNRPEAEAVSMLVKEAVVTFLPDALVTMTGGFRRGKMT GHDVDFLITSPEATEDEEQQLLHKVTDFWKQQGLLLYCDILESTFEKFKQPSRKVDALDHFQKCFLIL KLDHGRVHSEKSGQQEGKGWKAIRVDLVMCPYDRRAFALLGWTGSRQFERDLRRYATHERKMMLDNHA LYDRTKGKTVTISPLDGKVSKLQKALRVFLEAESEEEIFAHLGLDYIEPWERNA 20 Modifications which improve the incorporation of modified nucleotides can be at one or more of selected positions shown below. MDPLQAVHLGPRKKRPRQLGTPVASTPYDIRFRDLVLFILEKKMGTTRRAFLMELARRKGFRVENELS 25 DSVTHIVAENNSGSDVLEWLQLQNIKASSELELLDISWLIECMGAGKPVEMMGRHQLVVNRNSSPSPV pgsqnvpapavkkisqyacqrrttlnnynqlftdaldilaendelrenegsclafm||assvlkslpfp ITSMKDTEGIPCLGDKVKSIIEGIIEDGESSEAKAVLNDERYKSFKLFTSVFGVGLKTAEKWFRMGFR TLSKIQSDKSLRFTQMQKAGFLYYEDLVSCVNRPEAEAVSMLVKEAVVTFLPDALVTMTGGFRRGKMT GHDVDFLITSPEATEDEEQQLLHKVTDFWKQQGLLLYCDILESTFEKFKQPSRKVDALDHFQKCFLIL 30 kldhgrvhseksgqqegkgwkairvdlvmcpydrrafallgwtgsrqferdlrryatherkmmld|ha LYDRTKGKTVTISPLDGKVSKLQKALRVFLEAESEEEIFAHLGLDYIEPWERNA Thus by a process of aligning sequences, it is immediately apparent which regions in the sequences of terminal transferases from other species correspond to the sequences described herein with respect 35 to the bovine sequence shown in SEQ ID NO 1. 20 01 25 Sequence homology extends to all modified or wild-type members of family X polymerases, such as DNA Polp (also known as DNA polymerase mu or POLM), DNA Pol(3 (also known as DNA polymerase beta or POLB), and DNA PoIX (also known known as DNA polymerase lambda or POLL). It is well known in the art that all family X member polymerases, of which TdT is a member, either have terminal 5 transferase activity or can be engineered to gain terminal transferase activity akin to terminal deoxynucleotidyl transferase (Biochim Biophys Acta. 2010 May; 1804(5): 1136-1150). For example, when the following human TdT loopl amino acid sequence ...ESTFEKLRLPSRKVDALDHF... 10 was engineered to replace the following human Polp amino acid residues ...HSCCESPTRLAQQSHMDAF..., 15 the chimeric human Polp containing human TdT loopl gained robust terminal transferase activity (N.yg.j.gjg...Ag,j.gj.gRgs. 2006 Sep; 34(16): 4572-4582). Furthermore, it was generally demonstrated in US patent application no. 2019 / 0078065 that family X polymerases when engineered to contain TdT loopl chimeras could gain robust terminal transferase 20 activity. Additionally, it was demonstrated that TdT could be converted into a template-dependent polymerase through specific mutations in the loopl motif (Nucleic Acids Research, Jun 2009, 37(14):4642-4656). As it has been shown in the art, family X polymerases can be trivially modified to either display template-dependent or template-independent nucleotidyl transferase activities. Therefore, all motifs, regions, and mutations demonstrated in this patent can be trivially extended to 25 modified X family polymerases to enable modified X family polymerases to incorporate 3'-modified nucleotides, reversibly terminated nucleotides, and modified nucleotides in general to effect methods of nucleic acid synthesis. As a comparison with other family X polymerases, the human Polp sequence is shown below (Seq ID 30 5): MLPKRRRARVGSPSGDAASSTPPSTRFPGVAIYLVEPRMGRSRRAFLTGLARSKGFRVLDACSSEATH VVMEETSAEEAVSWQERRMAAAPPGCTPPALLDISWLTESLGAGQPVPVECRHRLEVAGPRKGPLSPA WMPAYACQRPTPLTHHNTGLSEALEILAEAAGFEGSEGRLLTFCRAASVLKALPSPVTTLSQLQGLPH 35 FGEHSSRVVQELLEHGVCEEVERVRRSERYQTMKLFTQIFGVGVKTADRWYREGLRTLDDLREQPQKL 20 01 25 TQQQKAGLQHHQDLSTPVLRSDVDALQQVVEEAVGQALPGATVTLTGGFRRGKLQGHDVDFLITHPKE GQEAGLLPRVMCRLQDQGLILYHQHQHSCCESPTRLAQQSHMDAFERSFCIFRLPQPPGAAVGGSTRP CPSWKAVRVDLVVAPVSQFPFALLGWTGSKLFQRELRRFSRKEKGLWLNSHGLFDPEQKTFFQAASEE DIFRHLGLEYLPPEQRNA 5 Thus by a process of aligning sequences, it is immediately apparent which positions in the sequences of all family X polymerases from any species correspond to the sequences described herein with respect to the bovine sequence shown in SEQ ID NO 1. 10 Furthermore, the A family polymerase, DNA Pol0 (also known as DNA polymerase theta or POLO.) was demonstrated to display robust terminal transferase capability (eljfe. 2016; 5: e 13740). DNA Pol0 was also demonstrated to be useful in methods of nucleic acid synthesis (GB patent application no. 2553274). In US patent application no. 2019 / 0078065, it was demonstrated that chimeras of DNA Pol0 and family X polymerases could be engineered togain robust terminal transferase activity and become 15 competent for methods of nucleic acid synthesis. Therefore, all motifs, regions, and mutations demonstrated in this patent can be trivially extended to modified A family polymerases, especially DNA Pol0, to enable modified A family polymerases. 20 DETAILED DESCRIPTION OF THE INVENTION The inventors have evolved for a TdT enzyme that will tail canonical dATP at temperatures higher than 37 °C. They obtained a group of candidates that tail significantly more than the parent at 37 °C and higher, with one candidate showing residual activity at 67 °C (Figure 3). Inventors have shown that the modifications can be applied to enzyme sequences from varying species. 25 The enzymes described herein have a greater catalytic efficiency and utilise nucleotide reagents at a lower concentration than wild type sequences. Where used in conjunction with terminated nucleotide reagents, enzymes having a high efficiency for 30 incorporation of native nucleotides without a terminator modification can be used to deplete the concentration of native reagents and improve the efficiency of nucleic acid synthesis. Described herein are modified terminal deoxynucleotidyl transferase (TdT) enzymes. The enzymes have an improved catalytic efficiency for nucleotide incorporation. Terminal transferase enzymes are 35 ubiquitous in nature and are present in many species. Many known TdT sequences have been reported 20 01 25 in the NCBI database. The sequences described herein are modified from the sequence of the cow, but the corresponding changes can be introduced into the homologous sequences from other species. Homologous amino acid sequences of Polp, Poip, PoIX, and Pol0 or the homologous amino acid sequence of X family polymerases also possess terminal transferase activity. References to terminal 5 transferase also include homologous amino acid sequences of Polp, Poip, PoIX, and PolQ or the homologous amino acid sequence of X family polymerases where such sequences possess terminal transferase activity. Disclosed herein is a modified terminal deoxynucleotidyl transferase (TdT) enzyme comprising at least 10 one amino acid modification when compared to a wild type sequence, wherein the modification is selected from one or more of the amino acid positions of the sequence of SEQ ID NO 1 or the homologous regions in other species or a truncated portion thereof. Described herein is a modified terminal deoxynucleotidyl transferase (TdT) enzyme comprising at 15 least the sequence ID No 2: KISQYACQRKTTLNNYNHIFTDAFEILAENSEFKENEVSYVTFMRAASVLKSLPFTIISMKDTEGIPCLGDKVKCIIEEIIE DGESSEVKAVLNDERYQSFKLFTSVFGVGLKTSEKWFRMGFRSLSKIMSDKTLKFTKMQKAGFLYYEDLVSCVTRAE AEAVGVLVKEAVWAFLPDAFVTMTGGFRRGKKIGHDVDFUTSPGSAEDEEQLLPKVINLWEKKGLLLYYDLVESTF EKFKLPSRQVDTLDHFQKCFLILKLHHQRVDSSKSNQQEGKTWKAIRVDLVMCPYENRAFALLGWTGSRQFERDIR 20 RYATHERKMMLDNHALYDKTKRVFLKAESEEEIFAHLGLDYIEPWERNA or the equivalent homologous region in other species, wherein the sequence has one or more amino acid modifications in one or more of the amino acid positions E456 or N473 of the full length sequence. The sequence above of 355 amino acids can be attached to other amino acids without affecting the 25 function of the enzyme. For example there can be a further N-terminal sequence that is incorporated simply as a protease cleavage site, for example the sequence MENLYFQG. Further disclosed is a modified terminal deoxynucleotidyl transferase (TdT) enzyme comprising at least two amino acid modifications when compared to a wild type sequence SEQ ID NO 1 or the 30 homologous amino acid sequence of a terminal deoxynucleotidyl transferase (TdT) enzyme in other species, wherein the modifications are selected from modifications at the amino acid positions E456 and N473 of the sequence of SEQ ID NO 1 or the homologous region in other species. 20 01 25 The modifications can be chosen from any amino acid that differs from the wild type sequence. The amino acid can be a naturally occurring amino acid. The modified amino acid can be selected from ala, arg, asn, asp, cys, gin, glu, gly, his, ile, leu, lys, met, phe, pro, ser, thr, trp, val, and sec. 5 For the purposes of brevity, the modifications are further described in relation to SEQ ID NO 1, but the modifications are applicable to the sequences from other species, for example those sequences listed above having sequences in the NCBI database. The sequence modifications also apply to truncated versions of SEQ ID NO 1. 10 The sequences can be modified at positions in addition to those regions described. Embodiments on the invention may include for example sequences having modifications to amino acids outside the defined positions, providing those sequences retain terminal transferase activity. Embodiments of the invention may include for example sequences having truncations of amino acids outside the defined positions, providing those sequences retain terminal transferase activity. For example the sequences 15 may be BRCT truncated as described in application WO2018215803 where amino acids are removed from the N-terminus whilst retaining or improving activity. Alterations, additions, insertions or deletions or truncations to amino acid positions outside the claimed regions are therefore within the scope of the invention, providing that the claimed regions as defined are modified as claimed. The sequences described herein refer to TdT enzymes, which are typically at least 300 amino acids in 20 length. All sequences described herein can be seen as having at least 300 amino acids. The claims do not cover peptide fragments or sequences which do not function as terminal transferase enzymes. Modifications disclosed herein contain at least one modification at the defined positions. In certain locations, mutations can be preferentially combined. 25 Specific amino acid changes can include any one of E456G and N473G. Specific amino acid changes can include both of E456G and N473G. Specific amino acid changes can include 30 a. R193H and / or b. Two or more of the amino acid positions E456G, R457S and / or N473G. Specific amino acid changes can include R193H, E456G, R457S and N473G. Also disclosed is a method of nucleic acid synthesis, which comprises the steps of: 20 01 25 (a) providing an initiator oligonucleotide; and (b) adding a nucleotide to said initiator oligonucleotide in the presence of a terminal deoxynucleotidyl transferase (TdT) as defined herein. 5 The method can add greater than 1 nucleotide. The nucleotides are not blocked at the 3'- position. The enzymes can preferentially incorporate one of the four nucleotides. Terminal transferases typically do not incorporate dATP as efficiently as other nucleotides, such as dGTP, dTTP, and dCTP (Basu et al., Synthesis of Compositionally Unique DNA By Terminal Deoxynucleotidyl Transferase 10 Biochem. Biophys. Res. Commun., 1983). Modifications described herein convey both thermostability and enhanced terminal transferase activity at all temperatures tested when compared to the parent. Remarkably, the enzymes demonstrate a drastic increase in terminal transferase activity for dATP; indeed they demonstrate higher activity for dATP than any other dNTP. 15 The enzymes are especially well suited to terminator-free enzymatic DNA synthesis for de novo template-independent nucleic acid synthesis. Methods of terminator-free enzymatic DNA synthesis control include pH control such as that described in U.S. Provisional Application No. 62 / 333,501; apyrase control such as that described in U.S. Provisional Application No. 62 / 575,017; reduced activity control such as that described in U.S. Provisional Application No. 62 / 741,143; caging control of 20 necessary co-factors such as that described in bioRxiv doi 10,1101 / 1020.02.19.9.¾ dNTP concentration, reaction time, and dNTP ratio control such as that described in U.S. Provisional Application No. 62 / 513,111; and tethered nucleoside triphosphate control such as that described in U.S. Provisional Application No. 62 / 354,635. 25 The enzyme can also be used to remove nucleotides containing a 3'-OH moiety in a solution of nucleotides containing a 3'-block or 3'-reversible block or 3'-terminator moiety as the enzyme will preferentially incorporate 3'-OH nucleotides onto the 3'-end of a single-stranded DNA molecule. The enzyme can also be used for other molecular biology applications, including the TUNEL assay for 30 identifying and quantifying apoptotic cells. References herein to ’nucleoside triphosphates’ refer to a molecule containing a nucleoside (i.e. a base attached to a deoxyribose or ribose sugar molecule) bound to three phosphate groups. Examples of nucleoside triphosphates that contain deoxyribose are: deoxyadenosine triphosphate (dATP), 20 01 25 deoxyguanosine triphosphate (dGTP), deoxycytidine triphosphate (dCTP) or deoxythymidine triphosphate (dTTP). Examples of nucleoside triphosphates that contain ribose are: adenosine triphosphate (ATP), guanosine triphosphate (GTP), cytidine triphosphate (CTP) or uridine triphosphate (UTP). Other types of nucleosides may be bound to three phosphates to form nucleoside 5 triphosphates, such as naturally occurring modified nucleosides and artificial nucleosides. References herein to an 'initiator oligonucleotide' or 'initiator sequence' refer to a short oligonucleotide with a free 3'-end which the nucleotide can be attached to. In one embodiment, the initiator sequence is a DNA initiator sequence. In an alternative embodiment, the initiator sequence is an RNA initiator sequence. 10 References herein to a 'DNA initiator sequence' refer to a small sequence of DNA which the nucleotide can be attached to, i.e., DNA will be synthesised from the end of the DNA initiator sequence. In one embodiment, the initiator sequence is between 5 and 50 nucleotides long, such as between 5 15 and 30 nucleotides long (i.e. between 10 and 30), in particular between 5 and 20 nucleotides long (i.e., approximately 20 nucleotides long), more particularly 5 to 15 nucleotides long, for example 10 to 15 nucleotides long, especially 12 nucleotides long. In one embodiment, the initiator sequence is single-stranded. In an alternative embodiment, the 20 initiator sequence is double-stranded. It will be understood by persons skilled in the art that a 3'-overhang (i.e., a free 3'-end) allows for efficient addition. In one embodiment, the initiator sequence is immobilised on a solid support. This allows TdT and the cleaving agent to be removed (in steps (c) and (e), respectively) without washing away the synthesised 25 nucleic acid. The initiator sequence may be attached to a solid support stable under aqueous conditions so that the method can be easily performed via a flow setup. In one embodiment, the initiator sequence is immobilised on a solid support via a reversible interacting moiety, such as a chemically-cleavable linker, an antibody / immunogenic epitope, a 30 biotin / biotin binding protein (such as avidin or streptavidin), or glutathione-GST tag. Therefore, in a further embodiment, the method additionally comprises extracting the resultant nucleic acid by removing the reversible interacting moiety in the initiator sequence, such as by incubating with proteinase K. 20 01 25 In one embodiment, the initiator sequence contains a base or base sequence recognisable by an enzyme. A base recognised by an enzyme, such as a glycosylase, may be removed to generate an abasic site which may be cleaved by chemical or enzymatic means. A base sequence may be recognised and cleaved by a restriction enzyme. The initiator may contain a uracil or 8-oxoguanine 5 nucleobase. In a further embodiment, the initiator sequence is immobilised on a solid support via a chemically-cleavable linker, such as a disulfide, allyl, or azide-masked hemiaminal ether linker. Therefore, in one embodiment, the method additionally comprises extracting the resultant nucleic acid by cleaving the 10 chemical linker through the addition of tris(2-carboxyethyl)phosphine (TCEP) or dithiothreitol (DTT) for a disulfide linker; palladium complexes or an allyl linker; or TCEP for an azide-masked hemiaminal ether linker. In one embodiment, the resultant nucleic acid is extracted and amplified by polymerase chain reaction 15 using the nucleic acid bound to the solid support as a template. The initiator sequence could therefore contain an appropriate forward primer sequence and an appropriate reverse primer could be synthesised. In one embodiment, the terminal deoxynucleotidyl transferase (TdT) of the invention is added in the 20 presence of an extension solution comprising one or more buffers (e.g., Tris or cacodylate), one or more salts (e.g., Na+, K+, Mg2+, Mn2+, Cu2+, Zn2+, Co2+, etc. all with appropriate counterions, such as Cl) and inorganic pyrophosphatase (e.g., the Saccharomyces cerevisiae homolog). It will be understood that the choice of buffers and salts depends on the optimal enzyme activity and stability. The use of an inorganic pyrophosphatase helps to reduce the build-up of pyrophosphate due to nucleoside 25 triphosphate hydrolysis by TdT. Therefore, the use of an inorganic pyrophosphatase has the advantage of reducing the rate of (1) backwards reaction and (2) TdT strand dismutation. In one embodiment, step (b) is performed at a pH range between 5 and 10. Therefore, it will be understood that any buffer with a buffering range of pH 5-10 could be used, for example cacodylate, 30 Tris, HEPES or Tricine, in particular cacodylate or Tris. In one embodiment, step (b) is performed at a temperature greater than 37 °C. Step (b) can be performed at greater than 50 °C, or greater than 60 °C. In one embodiment, the reagents used in step (b) are removed by applying a wash solution. In one embodiment, the wash solution comprises the same buffers and salts as used in the extension solution described herein. This has the advantage of allowing the wash solution to be collected and recycled as extension solution in step (b) when the method steps are repeated. 5 Also disclosed is a kit comprising a terminal deoxynucleotidyl transferase (TdT) as defined herein in combination with an initiator sequence and one or nucleoside triphosphates, for example dATP. The invention includes the nucleic acid sequence used to express the modified terminal transferase. 10 Included within the invention are the codon-optimized cDNA sequences which express the modified terminal transferase. Included are the codon-optimized cDNA sequences for each of the protein variants. The invention includes a cell line producing the modified terminal transferase. 15 20 01 25 20 01 25

Claims

1. A modified terminal deoxynucleotidyl transferase (TdT) enzyme comprising amino acid modifications when compared to wild type sequences SEQ ID NOs 1, 3 or 4 or truncated versions thereof, wherein the modified terminal deoxynucleotidyl transferase (TdT) enzyme5 includes amino acid modifications at positions E456 and N473 of SEQ ID NO 1, positions E441and N458 of SEQ ID NO 3, or positions E457 and N474 of SEQ ID NO 4 or truncated versions thereof, wherein the enzyme is N473G of SEQ ID NO 1, N458G of SEQ ID NO 3, or N474G of SEQ ID NO 4.

2. The modified terminal deoxynucleotidyl transferase (TdT) enzyme according to claim 1, 10 wherein the modifications are E456G and N473G of SEQ ID NO 1, E441G and N458G of SEQ IDNO 3, or E457G and N474G of SEQ ID NO 4.

3. A method of nucleic acid synthesis, which comprises the steps of:(a) providing an initiator oligonucleotide;(b) adding dATP, dCTP, dGTP, or dTTP to said initiator oligonucleotide in the presence of a15 terminal deoxynucleotidyl transferase (TdT) as defined in claim 1 or claim 2; and(c) removal of all reagents from the initiator oligonucleotide.

4. The method as defined in claim 3, wherein greater than 1 nucleotide is added.

5. The method according to claim 3, wherein the dNTP is added at greater than 37 °C.

6. The method according to claim 3, wherein the dNTP is added at greater than 50 °C.20 7. A kit comprising a terminal deoxynucleotidyl transferase (TdT) as defined in claim 1 or claim 2in combination with an initiator oligonucleotide and dNTP.

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