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

Mutations leading to expression of the silent HMR  a information in Saccharomyces cerevisiae result in sporulation proficiency in mat  a  1/MATα 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 HMR  a and HMLα, resulting in the nonmating phenotype of an a/α diploid. However, sir5-2/sir5-2 mat  a  1/MATα diploids mate as α yet are capable of sporulation. The sir5-2 mutation is unlinked to sir1-1, yet the two mutations do not complement each other: mat  a  1/MATα 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 HMR  a. PAS1-1 does not affect the mating ability of a strain, yet it allows diploids lacking a functional MAT  a locus to sporulate. It is proposed that PAS1-1 leads to partial expression of the otherwise cryptic a1 information at HMR  a.