A chromosomal gene required for killer plasmid expression, mating, and spore maturation in Saccharomyces cerevisiae.

Abstract

Killer strains of S. cerevisiae are those that harbor a double-stranded RNA plasmid and secrete a toxin that kills only strains not carrying this plasmid (sensitives). Two chromosomal genes (kex1 and kex2) are required for the secretion of toxin by plasmid-carrying strains. The kex2 gene, which maps at a site distinct from the mating-type locus, is also required for normal mating by .alpha. strains and meiotic sporulation in all strains. Strains that are .alpha. mating-type and kex2 fail to secrete the pheromone .alpha.-factor or to respond to the a-factor II pheromone which causes a morphological change, but they do respond to a-factor I which causes G1 arrest in .alpha. cells. Strains that are a mating-type and kex2 show no defect in mating, pheromone secretion or response to .alpha.-factor. Diploids that are homozygous for the kex2 mutation, unlike wild-type or heterozygous diploids, fail to undergo sporulation, with the defect occurring in the final spore maturation stage. These same defects in the sexual cycle are present in all kex2 mutants independent of the presence of the killer plasmid.