MUTATIONS LEADING TO EXPRESSION OF THE CRYPTIC HMR a LOCUS IN THE YEAST SACCHAROMYCES CEREVISIAE

Yona Kassir ¹ and Giora Simchen ¹

¹ Department of Genetics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

Mutations leading to expression of the silent HMR a information in Saccharomyces cerevisiae result in sporulation proficiency in mata1/MAT diploids. An example of such a mutation is sir5-2, a recessive mutation in the gene SIR5. As expected, haploids carrying the sir5-2 mutation are nonmaters due to the simultaneous expression of HMRa and HML, resulting in the nonmating phenotype of an a/ diploid. However, sir5-2/sir5-2 mata1/MAT diploids mate as yet are capable of sporulation. The sir5-2 mutation is unlinked to sir1-1, yet the two mutations do not complement each other: mata1/MAT sir5-2/SIR5 SIR1/sir1-1 diploids are capable of sporulation. In this case, recessive mutations in two unlinked genes form a mutant phenotype, in spite of the presence of the normal wild-type alleles.—The PAS1-1 mutation, Provider of a Sporulation function, is a dominant mutation tightly linked to HMRa. PAS1-1 does not affect the mating ability of a strain, yet it allows diploids lacking a functional MATa locus to sporulate. It is proposed that PAS1-1 leads to partial expression of the otherwise cryptic a1 information at HMRa.