MATING-TYPE REGULATION OF METHYL METHANESULFONATE SENSITIVITY IN SACCHAROMYCES CEREVISIAE

George P. Livi ¹ and Vivian L. Mackay ¹

¹ Waksman Institute of Microbiology, Rutgers University, P. O. Box 759, Piscataway, New Jersey 08854

Heterozygosity at the mating-type locus (MAT) in Saccharomyces cerevisiae has been shown previously to enhance X-ray survival in diploid cells. We now show that a/ diploids are also more resistant to the radiomimetic agent methyl methanesulfonate (MMS) than are diploids that are homozygous at MAT (i.e., either a/a or /). Log-phase a/ cultures exhibit biphasic MMS survival curves, in which the more resistant fraction consists of budded cells (those cells in the S and G2 phases of the cell cycle). Survival curves for log-phase cultures of a/a or / diploids have little if any biphasic nature, suggesting that the enhanced S- and G2-phase repair capacity of a/ cells may be associated with heterozygosity at MAT. The survival of cells arrested at the beginning of the S phase with hydroxyurea indicates that MAT-dependent MMS repair is limited to S and G2, whereas MAT-independent repair can occur in G1.