Rules of Donor Preference in Saccharomyces Mating-Type Gene Switching Revealed by a Competition Assay Involving Two Types of Recombination

[摘要] Mating type ( MAT ) switching in Saccharomyces cerevisiae is initiated by a double-strand break (DSB) created at MAT by HO endonuclease. MAT a cells activate the entire left arm of chromosome III; thus MAT a preferentially recombines with the silent donor HML . In contrast, MAT α cells inactivate the left arm, including HML , and thus preferentially recombine with HMR , 100 kb to the right of MAT . We present a novel competition assay, in which the DSB at MAT can be repaired either by MAT switching or by single-strand annealing (SSA) between two URA3 genes flanking MAT . With preferred donors, MAT a or MAT α switching occurs 65–70% of the time in competition with SSA. When HML is deleted, 40% of MAT a cells recombine with the “wrong” donor HMR ; however, when HMR is deleted, only 18% of MAT α cells recombine with HML . In interchromosomal switching, with donors on chromosome III and MAT on chromosome V, MAT a retains its strong preference for HML and switching is efficient, when the chromosome III recombination enhancer is present. However, MAT α donor preference is lost and interchromosomal switching is very inefficient. These experiments demonstrate the utility of using competition between two outcomes to measure the relative efficiency of recombination.

[学科分类] 医学（综合）