Pattern of Switching and Fate of the Replaced Cassette in Yeast Mating-type Interconversion

The a and α mating types of the yeast Saccharomyces cerevisiae are controlled by the mating-type locus (MAT), which can harbor one of two genetic blocks (cassettes), a or α. Homothallic strains of yeast, those carrying the HO allele and appropriate accessory genes, are able to interconvert mating types by replacing one MAT allele with the other MAT allele (Fig. 1) (Hicks et al. 1977a; Herskowitz et al. 1980; Herskowitz and Oshima 1981). Cryptic a and α cassettes reside at HMR and HML; standard laboratory strains have a information at HMR (=HMRa) and α information at HML (=HMLα). In addition, variants have been isolated that have an a cassette at HML (=HMLa) and an α cassette at HMR (=HMRα). Mating-type interconversion occurs by transposition of a copy of the mating-type information at HMR or HML to the mating-type locus, where the transposed cassette is expressed and determines the corresponding mating...