Deletions and single base pair changes in the yeast mating type locus that prevent homothallic mating type conversions.

Several cis-acting mutations that prevent homothallic mating type conversions in Saccharomyces cerevisiae have been examined. Deletions within the mating type (MAT) locus were obtained by selecting for survivors among homothallic MAT alpha cells carrying the rad52 mutation. The survivors were unable to switch mating type, even in RAD+ derivatives. The deletions varied in size from fewer than 50 to more than 750 base pairs. All of the deletions removed a Hha I site at the border between the alpha-specific sequences (Y alpha) and the adjacent Z region. We also examined several spontaneous inc mutations that prevent MAT switching. Two of these mutations were cloned in recombinant DNA plasmids and their sequences were determined. The MAT alpha-inc 3-7 mutation proved to have an altered Hha I site at the Y alpha/Z border, by virtue of a single base pair substitution G . C leads to A . T in the second base pair of the Z region (Z2). Restriction fragment analysis showed that two other independently isolated strains with MAT alpha-inc mutations had altered the same Hha I site. The MAT a-inc 4-28 mutation contains a single base pair substitution C . G leads to T . A at position Z6. A base pair difference at position Z11 in two MATa strains does not affect MATa conversions. We conclude that the region near the Y/Z border is essential for the efficient switching of MAT alleles and constitutes an enzyme recognition site for a specific nucleolytic cleavage of MAT DNA.