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

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Homothallic yeasts switch cell types (mating types a and α) at high frequency by changing the alleles of the mating type locus, MATa and MATα. We have proposed in the cassette model that yeast cells contain silent MATa and MATα 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 MATα locus (matα) to a functional MATα 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 matα mutations studied earlier: a defective MATa (mata1) switches readily to MATa and various matα loci switch readily to MATα. These observations satisfy the prediction of the cassette model that all mutations within MATa and MATα 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/α cells also inhibit the switching system in a/α cells. Finally, we present additional characterization of a natural variant of MATα, MATα-inc [Takano, I., Kusumi, T. & Oshima, Y. (1973) Mol. Gen. Genet. 126, 19-28] that is insensitive to switching. Our observation that MATα-inc acts in cis suggests that it may be altered in a site concerned with excision of MATα-inc or its replacement by another cassette.

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