Antibiotic-Free Plasmid Selection via ThyA Nonsense Codon
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Solution Overview
Problem
Current methods for multiplying expression plasmids without antibiotic resistance genes are inefficient, leading to unsatisfactory yields and potential contamination by native plasmids or mini-plasmids carrying prokaryotic sequences.
Innovation Solution
A novel approach using a mutated E. coli strain with a nonsense codon in the thyA gene, supplemented by a suppressor tRNA, allows for the selection and multiplication of plasmids without antibiotic resistance genes, utilizing a rich medium for growth and standard purification techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If expression plasmids are multiplied in E. coli cells using traditional antibiotic selection methods, then plasmid retention in bacterial cultures is ensured, but antibiotic contamination risks and sensitization issues arise for clinical use
Solution Approach 1:
The invention extracts and removes the antibiotic resistance gene from the expression plasmid system. Instead of relying on antibiotic resistance markers for selection, the patent uses a different selection mechanism based on thymidine auxotrophy and suppressor tRNA, thereby eliminating the harmful antibiotic component while maintaining plasmid retention functionality
Solution Approach 2:
The invention introduces an intermediary selection mechanism using thymidine auxotrophy and suppressor tRNA. The E. coli cell's inability to synthesize thymidine (due to thyA gene inactivation) serves as an intermediary selection pressure that replaces the direct antibiotic selection, allowing plasmid retention without antibiotic contamination
2Object-affected harmful factors
If plasmids without antibiotic resistance genes are constructed using recombination methods, then antibiotic-free production is achieved, but contamination by native plasmids or mini-plasmids occurs
Solution Approach 1:
The invention enables the plasmid system to self-select against contamination. The thymidine auxotrophy of the E. coli host cell creates a selective environment where only cells containing the complete expression plasmid with functional suppressor tRNA can grow. This self-service mechanism automatically eliminates native plasmids and mini-plasmids that lack the necessary suppressor function
Solution Approach 2:
The system establishes a feedback loop where plasmid presence directly affects bacterial growth. The suppressor tRNA encoded by the plasmid compensates for the thyA mutation, creating a growth-dependent feedback mechanism that ensures only complete, functional plasmids are maintained while preventing contamination by incomplete plasmid forms
3Object-affected harmful factors
If the thyA gene is inactivated to create thymidine auxotrophy for plasmid selection, then antibiotic-free plasmid multiplication is enabled, but cell growth is impaired without thymidine supplementation
Solution Approach 1:
The invention converts the harmful effect of thymidine auxotrophy (inability to grow without thymidine) into a beneficial selection mechanism. The growth impairment caused by thyA inactivation is transformed into a selective advantage for plasmid-containing cells, which gain the ability to grow through the suppressor tRNA mechanism. This converts a metabolic disadvantage into a powerful selection tool that enables antibiotic-free plasmid multiplication while maintaining cell productivity in the presence of the plasmid
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method enables effective selection and high-yield production of plasmids without antibiotic resistance, ensuring purity and suitability for clinical use without the risks associated with antibiotic contamination.
Implementation Method 1
The said suppressor tRNA comprises an anticodon capable of pairing with the nonsense codon and being specific of an amino acid capable of restoring the translation of the thyA gene
Data Source
AI summary
The invention relates to a mutated cell such as a bacteria or yeast in which the thyA gene coding thymidylate synthase includes a nonsense codon, preferably the amber codon, said nonsense codon replacing a codon coding an amino acid and inducing the interruption of thyA gene translation and the auxotrophy of the cell for the thymidine. Advantageously, the endA gene coding the endonuclease 1 and/or the recA gene coding the recombinase is inactivated in said mutated cell. The invention also relates to an expression plasmid including a transgene and a sequence of a suppressing ARM structural gene containing an anticodon that can be paired with the nonsense codon of the thyA gene and is specific of an amino acid capable of restoring the translation of the mutated thyA gene and thereby obtaining a protein of the wild or mutated type having a thymidylate synthase activity. The invention also relates to a method for the multiplication of the expression plasmid.