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

Abstract

ABSTRACT 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 MATα HMα HMa mar1-1 and MATa HMα HMa mar1-1 are sterile. However, MATα hmα HMa mar1-1 and MATa HMα hma mar1-1 strains exhibit a and a mating type, respectively. The sterile strains can be "rare mated" with standard strains as a consequence of mutational changes at HMa and HMα. It is proposed that the MAR1 locus blocks the expression of MATα and MATa information thought to exist at HMa and HMα loci, respectively (HICKS, STRATHERN and HERSKOWITZ, 1977). In a mar1-1 mutant, the expression of both HMα and HMa information leads to a nonmating phenotype similar to that of MATa/MATα diploids. The genetic evidence reported here is consistent with a central feature of the "cassette model", namely that HMα and hma carry MATa information and HMa and hmα carry MATα information.