Interconversion of Yeast Mating Types II. Restoration of Mating Ability to Sterile Mutants in Homothallic and Heterothallic Strains.

The two mating types of the yeast Saccharomyces cerevisiae can be interconverted in both homothallic and heterothallic strains. Previous work indicates that all yeast cells contain the information to be both a and alpha and that the HO gene (in homothallic strains) promotes a change in mating type by causing a change at the mating type locus itself. In both heterothallic and homothallic strains, a defective alpha mating type locus can be converted to a functional a locus and subsequently to a functional alpha locus. In contrast, action of the HO gene does not restore mating ability to a strain defective in another gene for mating which is not at the mating type locus. These observations indicate that a yeast cell contains an additional copy (or copies) of alpha information, and lead to the "cassette" model for mating type interconversion. In this model, HMa and hmalpha loci are blocs of unexpressed alpha regulatory information, and HMalpha and hma loci are blocs of unexpressed a regulatory information. These blocs are silent because they lack an essential site for expression, and become active upon insertion of this information (or a copy of the information) into the mating type locus by action of the HO gene.