Site-specific genomic (SSG) and random domain-localized (RDL) mutagenesis in yeast

Background: A valuable weapon in the arsenal available to yeast geneticists is the ability to introduce specific mutations into yeast genome. In particular, methods have been developed to introduce deletions into the yeast genome using PCR fragments. These methods are highly efficient because they do not require cloning in plasmids. Results: We have modified the existing method for introducing deletions in the yeast (S. cerevisiae) genome using PCR fragments in order to target point mutations to this genome. We describe two PCR-based methods for directing point mutations into the yeast genome such that the final product contains no other disruptions. In the first method, site-specific genomic (SSG) mutagenesis, a specific point mutation is targeted into the genome. In the second method, random domain-localized (RDL) mutagenesis, a mutation is introduced at random within a specific domain of a gene. Both methods require two sequential transformations, the first transformation integrates the URA3 marker into the targeted locus, and the second transformation replaces URA3 with a PCR fragment containing one or a few mutations. This PCR fragment is synthesized using a primer containing a mutation (SSG mutagenesis) or is synthesized by error-prone PCR (RDL mutagenesis). In SSG mutagenesis, mutations that are proximal to the URA3 site are incorporated at higher frequencies than distal mutations, however mutations can be introduced efficiently at distances of at least 500 bp from the URA3 insertion. In RDL mutagenesis, to ensure that incorporation of mutations occurs at approximately equal frequencies throughout the targeted region, this region is deleted at the same time URA3 is integrated. Conclusion: SSG and RDL mutagenesis allow point mutations to be easily and efficiently incorporated into the yeast genome without disrupting the native locus.