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Identification of biologically active mutants by combinatorial cassette mutagenesis: exclusion of wild-type codon
1Department of Pharmaceutical Chemistry, University of California, San Francisco 94143-0448.
Abstract:
A degenerate codon (N)(N)(G+C) is often used in cassette mutagenesis to encode all 20 natural amino acids at the target mutation site. However, the presence of the wild-type codon in the degenerate codon presents some inconvenience in screening and identification of catalytically active mutants. The wild-type enzyme will always be identified as catalytically active in a screening process and in most cases can only be distinguished from active mutants by DNA sequencing. Sequencing of background wild-type enzyme represents wasted effort in the identification of active mutants. This paper describes a simple approach for exclusion of the wild-type codon in degenerate codons through the synthesis of two or three oligonucleotide mixtures. The minimum number of individual colonies required to achieve a high degree of certainty of including all possible codons for screening of catalytic activity can be estimated using a statistical procedure. The use of degenerate codons that exclude the wild-type amino acid facilitates the screening process and saves time and expense in DNA sequencing.
Insights
This study presents a method to exclude wild-type codons in degenerate codons for cassette mutagenesis. This approach simplifies screening and reduces costs by avoiding sequencing of non-mutated enzymes.
Area of Science:
- Molecular Biology
- Protein Engineering
- Biotechnology
Background:
- Degenerate codons (NNG+C) are used in cassette mutagenesis to encode all 20 amino acids.
- The presence of wild-type codons in degenerate codons complicates mutant screening and identification.
- Wild-type enzymes are often indistinguishable from active mutants without DNA sequencing, leading to wasted effort.
Purpose of the Study:
- To describe a method for excluding wild-type codons from degenerate codons used in mutagenesis.
- To improve the efficiency of screening for catalytically active mutants.
- To reduce the time and expense associated with DNA sequencing in mutant identification.
Main Methods:
- Synthesis of two or three oligonucleotide mixtures to create degenerate codons excluding the wild-type sequence.
- Application of a statistical procedure to estimate the minimum number of colonies needed for comprehensive codon coverage.
- Utilizing modified degenerate codons in cassette mutagenesis.
Main Results:
- A simplified screening process for catalytically active mutants.
- Reduced need for DNA sequencing to identify active mutants.
- Efficient generation and identification of desired enzyme variants.
Conclusions:
- Excluding wild-type codons from degenerate codons streamlines the mutant screening process.
- This method saves significant time and resources in enzyme engineering.
- The approach enhances the efficiency of identifying novel, catalytically active enzyme mutants.