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

Amar J. S. Klar ¹, Seymour Fogel ¹, and David N. Radin ¹

¹ Department of Genetics, University of California, Berkeley, California 94720

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 MATa allele, designated mata-2, and MAT ho strains show a high capacity for mating with MATa strains. The MAT/mata-2 diploids do not sporulate. However, zygotic clones obtained by mating MAT homothallic (HO) cells with mata-2 ho cells are unable to mate and can sporulate. Tetrad analysis of such clones revealed two diploid (MAT/MATa):two haploid segregants. Therefore, MAT switches occur in MAT/mata-2 HO/ho cells to produce MAT/MATa cells capable of sporulation. In heterothallic strains, the mata-2 allele can be switched to a functional MAT and subsequently to a functional MATa. Among 32 MAT to MATa switches tested, where the MAT 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 mat ste1–5 mutant, suggesting that these alleles are not identical and that the recovered allele presumably arose as a mutation of the MAT locus. No mata-2 was recovered by HO-mediated switching of MAT (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 MAT and subsequently to a functional MATa locus. Overall, the results are in accord with the cassette model (Hicks, Strathern and Herskowitz 1977b) proposed to explain MAT interconversions.