Pathways of ultraviolet mutability in Saccharomyces cerevisiae. III. Genetic analysis and properties of mutants resistant to ultraviolet-induced forward mutation

Abstract

Non-allelic mutants of Saccharomyces cerevisiae with reduced capacity for ultraviolet light (UV)-induced forward mutation from CAN1 to can1 were assigned to seven distinct genetic loci, each with allele designations umr1-1, umr2-1, …, umr7-1 to indicate UV mutation resistance. Each allele complemented rev1-1, rev2-1, and rev3-1. None conferred a great deal of UV sensitivity. When assayed on yeast extract-peptone-dextrose complex growth agar, umr1, umr3, and umr7 ( a mating type) were the most UV-sensitive, with a dose-reduction factor of approximately 1.2 at 10% survival. When assayed on synthetic agar lacking arginine, however, umr3 was the most UV-sensitive (dose-reduction factor of 1.5 at 10% survival). UV revertability of his5-2, lys1-1, and ura4-1 was normal in strains carrying the single genes umr4, umr5, umr6 and umr7; umr1 reduced revertibility of his5-2 and ura4-1 but not lys1-1; umr2 reduced only ura4-1 revertibility; umr3 reduced UV reversion of all three test alleles. Five a/α homozygous umr diploids (except umr1 and umr4) failed to sporulate. One of these, umr7, blocked normal secretion of alpha hormone in α segregants and could not conjugate with a strains. The phenotypes of umr mutants are consistent with the existence of branched UV mutation pathways of different specificity, some of which may function in the single RAD6-dependent error-prone pathway for repair of UV damage. Other possible pathways of action are discussed. It is also suggested that regulatory functions interacting with the mating-type locus or its gene products may play some role in UV mutagenesis or error-prone repair.