Single‐gene deletions that restore mating competence to diploid yeast

Using the Saccharomyces cerevisiae MAT a / MAT α ORF deletion collection, homozygous deletion strains were identified that undergo mating with MAT a or MAT α haploids. Seven homozygous deletions were identified that confer enhanced mating. Three of these, lacking CTF8 , CTF18 , and DCC1 , mate at a low frequency with either MAT a or MAT α haploids. The products of these genes form a complex involved in sister chromatid cohesion. Each of these strains also exhibits increased chromosome loss rates, and mating likely occurs due to loss of one copy of chromosome III, which bears the MAT locus. Three other homozygous diploid deletion strains, ylr193c Δ /ylr193c Δ, yor305w Δ/ yor305w Δ, and ypr170c Δ/ ypr170c Δ, mate at very low frequencies with haploids of either or both mating types. However, an ist3 Δ/ ist3 Δ strain mates only with MAT a haploids. It is shown that IST3 , previously linked to splicing, is required for efficient processing of the MAT a 1 message, particularly the first intron. As a result, the ist3 Δ/ ist3 Δ strain expresses unbalanced ratios of Matα to Mat a proteins and therefore mates with MAT a haploids. Accordingly, mating in this diploid can be repressed by introduction of a MAT a 1 cDNA. In summary, this study underscores and elaborates upon predicted pathways by which mutations restore mating function to yeast diploids and identifies new mutants warranting further study.