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

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Abstract
Homothallic yeasts switch cell types (mating types a and .alpha.) at high frequency by changing the alleles of the mating type locus, Mata and MAT.alpha.. In the cassette model, yeast cells contain silent MATa and MAT.alpha. 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 MAT.alpha. locus (mat.alpha.) to a functional MAT.alpha. locus. This healing of mat mutations is a general property of the mating type interconversion system and is not specific to the class of mat.alpha. mutations studied earlier: a defective MATa (mata 1) switches readily to MATa and various mat.alpha. loci switch readily to MAT.alpha.. These observations satisfy the prediction of the cassette model that all mutations within MATa and MAT.alpha. be healed. These studies also identify Mat functions that control the switching process: the same functions which promote sporulation and prevent mating in a/.alpha. cells also inhibit the switching system in a/.alpha. cells. Additional characterization of a natural variant of MAT.alpha., MAT.alpha.-inc [Takano, I., Kusumi, T. and Oshima, Y.] that is insensitive to switching is presented. Since MAT.alpha.-inc acts in cis, it may be altered in a site concerned with excision of MAT.alpha.-inc or its replacement by another cassette.