SWITCHING OF A MATING-TYPE a MUTANT ALLELE IN BUDDING YEAST SACCHAROMYCES CEREVISIAE

Aimed at investigating the recovery of a specific mutant allele of the mating type locus (MAT) by switching a defective MAT allele, these experiments provide information bearing on several models proposed for MAT interconversion in bakers yeast, Saccharomyces cerevisiae. Hybrids between heterothallic (ho) cells carrying a mutant MAT  a allele, designated mata-2, and MATα ho strains show a high capacity for mating with MAT  a strains. The MATα/mata-2 diploids do not sporulate. However, zygotic clones obtained by mating MATa homothallic (HO) cells with mata-2 ho cells are unable to mate and can sporulate. Tetrad analysis of such clones revealed two diploid (MATa/MATa) :two haploid segregants. Therefore, MAT switches occw in MATα/mata-2 HO/ho cells to produce MATα/MAT a cells capable of sporulation. In heterothallic strains, the mata-2 allele can be switched to a functional MATα and subsequently to a functional MATa. Among 32 MATα to MATa switches tested, where the MATα was previously derived from the mata-2 mutant, only one mata-2 like isolate was observed. However, the recovered allele, unlike the parental allele, conplements the matα stel-5 mutant, suggesting that these alleles are not identical and that the recovered allele presumably arose as a mutation of the MATα locus. No mata-2 was recovered by HO-mediated switching of MATα (previously obtained from mata-2 by HO) in 217 switches analyzed. We conclude that in homothallic and heterothallic strains, the mata-2 allele can be readily switched to a functional MATα and subsequently to a functional MATa locus. Overall, the results are in accord with the cassette model (HICKS, STRATKERN and HERSKO-WITZ 1977b) proposed to explain MAT interconversions.