Mating-Type Regulation of Methyl Methanesulfonate Sensitivity in SACCHAROMYCES CEREVISIAE.

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/alpha 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 alpha/alpha). Log-phase a/alpha 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 alpha/alpha diploids have little if any biphasic nature, suggesting that the enhanced S- and G2-phase repair capacity of a/alpha 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.