MAR1—A REGULATOR OF THE HMa AND HM LOCI IN SACCHAROMYCES CEREVISIAE

Amar J. S. Klar ¹, Seymour Fogel ¹, and Kathy Macleod ¹

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

A mutation in the MAR1 (mating-type regulator) locus causing sterility in Saccharomyces cerevisiae is reported. The mutation maps on the left arm of linkage group IV between trp1 and cdc2 at a distance of about 27 cM from trp1 and about 31 cM from cdc2. Haploid strains with genotype MAT HM HMa mar1–1 and MATa HM HMa mar1–1 are sterile. However, MAT hm HMa mar1–1 and MATa HM hma mar1–1 strains exhibit and a mating type, respectively. The sterile strains can be "rare mated" with standard strains as a consequence of mutational changes at HMa and HM. It is proposed that the MAR1 locus blocks the expression of MAT and MATa information thought to exist at HMa and HM loci, respectively (Hicks, Strathern and Herskowitz, 1977). In a mar1–1 mutant, the expression of both HM and HMa information leads to a nonmating phenotype similar to that of MATa/MAT diploids. The genetic evidence reported here is consistent with a central feature of the "cassette model", namely that HM and hma carry MATa information and HMa and hm carry MAT information.

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