In the yeast Saccharomyces cerevisiae, the mating-type locus (mat) exerts a regulatory effect on conjugation in haploids and, in diploids, on meiosis and sporulation, mitotic recombination, and X-ray survival. We have recently determined that the repair of MMS-induced damaged is also mediated in part through the genetic configuration at mat. The mating-type effect is particularly pronounced in diploid cells that are undergoing or have completed DNA synthesis (S and G2 phase cells). A mutant has been isolated that is altered in both sporulation and MMS sensitivity; experiments are in progress to determine its capacity to repair X-ray damage and to undergo mitotic recombination. In haploids, the synthesis of and response to diffusible mating hormones is also controlled by mat. We have shown by fluorescence staining that the cell surface is altered when cells are incubated with the appropriate hormone and that these alterations probably are involved in intercellular recognition and agglutination leading to fusion. The synthesis and/or activity of a-factor (the mating hormone secreted by mating type a cells) may be at least partially inducible. Its production is being examined in a variety of a strains and a mutants. BIBLIOGRAPHIC REFERENCES: Crandall, M., R. Egel and V. L. MacKay (1977). Physiology of mating in three yeasts. Adv. in Micro. Physiol., vol. 15 (ed., A. H. Rose), in press. MacKay, V. L. (1977). "Mating-type specific pheromones as mediators of sexual conjugation in yeast" in Molecular control of proliferation and differentiation. Proc. of the 35th Annual Symp. of the Soc. for Devel. Biol. (ed., J. Papaconstantinou), Academic Press, N. Y., in press.