Site-Directed Mutagenesis Method Mediated by Cas9

This study presents an in vitro CRISPR/Cas9-mediated mutagenic (ICM) system that allows rapid construction of designed mutants or site-saturation mutagenesis libraries in a PCR-independent manner. The plasmid DNA is double digested with Cas9 bearing specific single guide RNAs to remove the target nucleotides. Next, T5 exonuclease excises both 5'-ends of the linearized plasmid to generate homologous regions of approximately 15 nt. Subsequently, a short dsDNA of approximately 30-50 bp containing the desired mutation cyclizes the plasmid through base pairing and introduces the mutation into the plasmid. The gaps are repaired in Escherichia coli host cells after transformation. This method is highly efficient and accurate. Both single and multiple site-directed mutagenesis can be successfully performed, especially to large sized plasmids. This method demonstrates the great potential for creating high-quality mutant libraries in directed evolution as an alternative to PCR-based saturation mutagenesis, thus facilitating research on synthetic biology.