The directed and controlled switching of mating type which occurs in homothallic forms of the yeast Saccharomyces cerevisiaee is dependent upon the presence of the wild type RAD52 gene. The RAD52 gene is also required for spontaneous mitotic and meiotic recombination. It has been observed that the two haploid mating types of yeast respond differently to the presence of the rad52-1 mutation and the gene conferring the ability to switch mating type (the HO allele). Cells of genotype MATa rad52-1 HO remain as stable haploids instead of switching; cells with genotype MATα rad52-1 HO are inviable. However, some laboratory strains of yeast harbor a MATa allele which, like MATα, is inviable. Both allelic forms of a switch normally in wild type (RAD52) strains. This suggests that the difference between the two MATa alleles may help define the interaction which occurs between the mating type locus and the RAD52 gene product. The difference between the two MATa alleles is not due to major sequence rearrangements, but probably reflects a change of relatively few base pairs. Normally wild type haploid cells which contain the HO gene switch mating type, and then opposite mating types fuse to form MATa/MATα diploids. In such diploids the HO gene is not expressed, and switching does not occur. Strains which have only a (or only α) information of both MAT and the silent copies switch repeatedly. Digestion of DNA from such strains with appropriate restriction enzymes generates two fragments of DNA resulting from a spontaneous double stranded break in the MAT locus (Strathern et al. 1982). Viable MATa HO rad52-1 strains do not have these fragments.

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Malone RE, Hyman D

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