A CIS-ACTING MUTATION WITHIN THE MATa LOCUS OF SACCHAROMYCES CEREVISIAE THAT PREVENTS EFFICIENT HOMOTHALLIC MATING-TYPE SWITCHING

Deborah Wygal Mascioli ¹ and James E. Haber ²

¹ Department of Biology, Brandeis University, Waltham, Massachusetts 02254
² Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254

Homothallic strains of Saccharomyces cerevisiae are able to switch from one mating-type to the other as frequently as every cell division. We have identified a cis-dominant mutation of the MATa locus, designated MATa-inc, that can be converted to MAT at only about 5% of the normal efficiency. In homothallic MATa-inc/mata* diploids, the MATa-inc locus switched to MAT in only one of 30 cases, while the mata* locus switched to MAT in all 30 cases. The MATa-inc mutation can be "healed" by a series of switches, first to MAT and then to a normal allele of MATa. These data are consistent with the "cassette" model of Hicks, Strathern and Herskowitz (1977), in which mating conversions involve the transposition of wild-type copies of a or information from silent genes elsewhere in the genome. The MATa-inc mutation appears to alter a DNA sequence necessary for the replacement of MATa by MAT. The MATa-inc mutation has no other effect on MATa functions. In beterothallic backgrounds, the mutation has no effect on the sensitivity to -factor, synthesis of a-factor, expression of barrier phenotype or ability to mate or sporulate.——The MATa-inc allele does, however, exhibit one pleiotropic effect. About 1% of homothallic MATa-inc cells become completely unable to switch mating type because of mutations at HMa, the locus proposed to carry the silent copy of information.——In addition, we have isolated a less efficient allele of the HO gene.