Mutations preventing transpositions of yeast mating type alleles

Homothallic strains ofSaccharomyces cerevisiae can switch from one mating type to the other as often as every cell division. The conversion of mating type alleles (fromMAT a toMAT α orvice versa ) depends on other, unexpressed copies of a or α information that can be transposed toMAT . Previously, “inconvertible” mutations withinMAT α andMAT a have been described that block the excision of theMAT allele. In this paper we describe twocis -acting mutations that also impair mating type switching and lie very near, but outside, theMAT locus. Both “stuck” mutations,stk1 andstk2 , diminish the efficiency of convertingMAT a toMAT α to less than 10% of normal. Thestk1 mutation also slightly reduces conversion ofMAT α toMAT a, whereasstk2 has no discernible effect. Unlike the inconvertibleMAT α-inc andMAT a-inc mutations withinMAT , thestk mutations are not replaced by wild-type sequences after the “stuck” cells occasionally switch to the opposite mating type. Because these mutations are not “healed” by mating type conversions, they must lie in sequences outside of the transposable mating type information. These results indicate that the efficient replacement ofMAT alleles depends on sequences both within and adjacent to theMAT locus. Among subclones of homothallicstk MAT a strains, approximately 2% show “illegal” transpositions of mating type genes. In these colonies the silent copy of α information at theHML α locus has been converted to a, without any change ofMAT a or the silent a copy atHMR a. Such conversions of the unexpressed library genes are not found in wild-type homothallic strains that can switch mating type efficiently, but they are found inMAT a-inc andMAT α-inc strains. It appears that all of thecis -acting mutations within or adjacent to mating type result in these unusual switches of mating type information atHML andHMR .