Healing of mat mutations and control of mating type interconversion by the mating type locus in Saccharomyces cerevisiae.

Homothallic yeasts switch cell types (mating types a and alpha) at high frequency by changing the alleles of the mating type locus, MATa and MATalpha. We have proposed in the cassette model that yeast cells contain silent MATa and MATalpha blocs ("cassettes"), copies of which can be substituted at the mating type locus for the resident information. The existence of silent cassettes was originally proposed to explain efficient switching of a defective MATalpha locus (matalpha) to a functional MATalpha locus. We report here that this "healing" of mat mutations is a general property of the mating type interconversion system and is not specific to the class of matalpha mutations studied earlier: a defective MATa (mata1) switches readily to MATa and various matalpha loci switch readily to MATalpha. These observations satisfy the prediction of the cassette model that all mutations within MATa and MATalpha be healed. These studies also identify MAT functions that control the switching process: the same functions known to promote sporulation and prevent mating in a/alpha cells also inhibit the switching system in a/alpha cells. Finally, we present additional characterization of a natural variant of MATalpha, MATalpha-inc [Takano, I., Kusumi, T. & Oshima, Y. (1973) Mol. Gen. Genet. 126, 19-28] that is insensitive to switching. Our observation that MATalpha-inc acts in cis suggests that it may be altered in a site concerned with excision of MATalpha-inc or its replacement by another cassette.