Chromosomal Rearrangements Accompanying Yeast Mating-type Switching: Evidence for a Gene-conversion Model

Mating type of the yeast Saccharomyces cerevisiae is controlled by transposable alleles of the mating-type (MAT) locus. According to the controlling-element model postulated by Oshima and Takano (1971) and the more explicit cassette model of Hicks et al. (1977), each haploid yeast strain contains two unexpressed copies of mating-type alleles at two loci, HML and HMR, which are located far from the MAT locus, where mating-type alleles are expressed (Fig. 1). The mating type of a cell can be changed from MATa to MATα or vice versa by the replacement of the MAT allele with a copy of one of the library genes. This model has been confirmed both genetically and biochemically. In particular, a mutant mating-type allele such as matαl can be healed by replacing it with an a allele from HMRa and then converting the new MATa allele to a wild-type MATα allele, transposed from HMLα (Hicks and...