Switching ofSaccharomyces cerevisiaeMating‐Type Genes

Homothallic switching of the budding yeast mating-type (MAT) genes has provided one of the most intensively studied examples of a programmed genetic rearrangement. A site-specific double-strand break (DSB) at the MAT locus, induced by HO endonuclease, provokes the replacement of mating-type specific sequences through homologous recombination. Homothallic organisms have the capacity to self-diploidize by converting some offspring of a haploid cell of one mating type to cells of the opposite mating type. This chapter briefly looks at the determination of cell lineage and at the mechanism of silencing the donor sequences. The conversion of one mating type to the other involves the replacement at the MAT locus of Ya or Yα by a gene conversion induced by a DSB. Additional information has been gleaned from the analysis of DSB-induced recombination in meiotic cells. Saccharomyces cerevisiae has evolved an elaborate mechanism that gives it the ability to choose between its two donors. It makes sense that MATa should seek out and recombine with HMLα rather than HMRa , so that the recombinational repair of the DSB will lead to a switch to the opposite mating type. By comparing the recombination enhancer (RE) sequences of S. cerevisiae and S. carlsbergensis (which is functional in S. cerevisiae ), it was possible to narrow down the RE to 270 contiguous base pairs in S. cerevisiae or 244 in S. carlsbergensis , within which are four well-conserved subdomains.