Mutations leading to expression of the cryptic HMRa locus in the yeast Saccharomyces cerevisiae

Mutations leading to expression of the silent HMRa information in Saccharomyces cerevisiae result in sporulation proficiency in mata1/MAT alpha 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 alpha, resulting in the nonmating phenotype of an a/alpha diploid. However, sir5-2/sir5-2 mata1/MAT alpha diploids mate as alpha 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 alpha 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.