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

[摘要] 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 MAT a. Among 32 MATα to MAT a 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 MAT a locus. Overall, the results are in accord with the cassette model (HICKS, STRATKERN and HERSKO-WITZ 1977b) proposed to explain MAT interconversions.

[学科分类] 医学（综合）