Mitotic recombination events and single-base mutations induced by ultraviolet light in G1-arrested yeast cells

Ultraviolet light (UV) is a potent inducer of both single-base mutations and mitotic recombination. Although these genomic alterations are often attributed to the action of error-prone DNA polymerases on UV-induced DNA lesions during replicative DNA synthesis, UV damage can also result in mutagenic and recombinogenic DNA damage in nondividing cells. We examined the effects of UV on cells of the yeast Saccharomyces cerevisiae arrested in G₁ of the cell cycle. By mating an irradiated haploid with an unirradiated haploid, we found that recombination was initiated only on the irradiated chromosome. This result indicates that trans effects of UV on recombination (for example, induction of recombinogenic proteins stimulating DNA breaks on the unirradiated homolog) are small or negligible. In addition, we show that most of the UV-induced mutations produced in G₁-irradiated cells result in mutations at identical positions in both strands of the duplex. As observed for recombination events, mutations are almost exclusively on the irradiated chromosome, indicating the near absence of a trans effect on mutations.