Abstract
Mating-type switching in the yeast Saccharomyces cerevisiae involves the transposition of a copy of a or α information from unexpressed “library” genes, HML or HMR, to replace the sequence at the mating type locus, MAT. In normal homothallic strains, where conversions of MAT may occur as often as every cell division, the switching of MAT alleles does not alter the alleles at HML or HMR. We have discovered that several mutations within or adjacent to MAT that impair the excision of the MAT allele permit conversions of the alleles at HML or HMR in more than 1% of the cells analyzed. The two mutations within the MAT locus (MATa-inc and MATα-inc) can transpose to HML or HMR without being lost at MAT. Thus a MATα-inc HMLα HMRa HO strain can switch to MATα-inc HMLα HMRα-inc HO. Even though the α-inc and a-inc alleles prevent their own replacement at MAT, these sequences are efficiently transposed back from HMLα-inc or HMLa-inc to replace normal MAT alleles. When these alleles reappear at MAT, they are again blocked in excision. Thus the sequences used to remove an allele from MAT must differ from those used to replicate and transpose it. Two cis-acting stk mutations adjacent to MAT that block switching of MATa to MATα also induce the conversion of HMLα to HMLa. However, we have previously shown that these events do not occur in strains carrying a recessive “switch” mutant (swi1) or in strains carrying a defective allele of the HO gene. In stk1 MATa HO strains, HMLα was converted to HMLa in approximately 4% of the subclones examined. In contrast, the HMLα-inc sequence was not converted in similar stk1 MATa HO strains. Thus the excision of the α-inc sequence seems to be prevented at both MAT and HML. These results suggest that the illegal conversions of HML and HMR occur by a mechanism similar to that used for normal conversions of MAT.