Mating type interconversion in Saccharomyces cerevisiae occurs by transposition of copies of the a or α mating type cassettes from inactive loci, HML and HMR, to an active locus, MAT. The lack of expression of the a and α genes at the silent loci results from repression by trans-acting regulators encoded by SIR (Silent Information Regulator) genes. In this paper we present evidence for the existence of four SIRgenes. Inactivation of any of these genes leads to expression of cassettes at both HML and HMR. Unusual complementation properties are observed for a number of sir mutations. Specifically, some recessive mutations in different genes fail to complement. The correspondence between SIR1, SIR2, SIR3, SIR4 and other genes with similar roles (MAR, CMT, STE8 and STE9) is presented.
- Received September 16, 1986.
- Accepted January 30, 1987.