A MUTATION ALLOWING EXPRESSION OF NORMALLY SILENT a MATING-TYPE INFORMATION IN SACCHAROMYCES CEREVISIAE

Harry Gruenspan ¹ and Norman R. Eaton ¹

¹ Biology Department, Brooklyn College of the City University of New York, Brooklyn, New York 11210

Mating type in haploid cells of the yeast Saccharomyces cerevisiae is determined by a pair of alleles MATa and MAT . 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) MAT cells carring AON1 are defective in mating. (2) AON1 allows MAT/MAT but not MATa/MATa diploids to sporulate; thus, AON1 mimics the MATa requirement for sporulation. (3) mata-1 cells that carry AON1 are MATa phenocopies, i.e., MAT/mata- 1 AON1 diploids behave as standard MAT/MATa cells; therefore, AON1 suppresses the defect of mata- 1. (4) AON1 maps at or near HMRa. (5) Same-site revertants from AON1 lose the ability to convert mating type to MATa, indicating that reversion is associated with the loss of a functional HMRa 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 HMRa.