A MUTATION ALLOWING EXPRESSION OF NORMALLY SILENT a MATING-TYPE INFORMATION IN SACCHAROMYCES CEREVISIAE
ABSTRACT Mating type in haploid cells of the yeast Saccharomyces cerevisiae is determined by a pair of alleles MAT a and MATα. Under various conditions haploid mating types can be interconverted. It has been proposed that transpositions of silent cassettes of mating-type information from HML or HMR to MAT are the source of mating type conversions. A mutation described in this work, designated AON1, has the following properties. (1) MATα cells carring AON1 are defective in mating. (2) AON1 allows MATα/MATα but not MAT a/MAT a diploids to sporulate; thus, AON1 mimics the MAT a requirement for sporulation. (3) mat a-1 cells that carry AON1 are MAT a phenocopies, i.e., MATα/mat a-1 AON1 diploids behave as standard MATα/MAT a cells; therefore, AON1 suppresses the defect of mat a-1. (4) AON1 maps at or near HMR a. (5) Same-site revertants from AON1 lose the ability to convert mating type to MAT a, indicating that reversion is associated with the loss of a functional HMR a locus. In addition, AON1 is a dominant mutation. We conclude that AON1 is a regulatory mutation, probably cis-acting, that leads to the constitutive expression of silent a mating-type information located at HMR a.