1-Nitropyrene Efficiently Induces Mitotic Recombination in Saccharomyces cerevisiae

Nitropyrene, a mutagenic and carcinogenic component of diesel exhaust, has been shown to be a potent bacterial and mammalian mutagen. There is, however, some controversy regarding the genotoxic effects of 1-nitropyrene towards yeast. To obtain insights into the mechanisms of 1-nitropyrene-induced mutations in Saccharomyces cerevisiae, we have attempted to characterize the genetic alterations that inactivate the endogenous CAN1 gene either in haploid cells or in heterozygous diploid cells. 1-Nitropyrene, without any activation treatment, showed a substantial toxic effect until 500μM. The mutation frequency in haploid cells treated with 500μM of 1-nitropyrene was 1.59×10-5, which is 15-fold higher than the control value. Sequencing of mutants indicated that both frameshifts and base substitutions were increased. In diploid cells treated with 500μM of 1-nitropyrene, the frequency with which can1Δ::LEU2/can1Δ:: LEU2 was converted from CAN1/cantΔ:: LEU2, a phenotypic change from a canavanine-sensitive to canavanine-resistant form, was 8.59×10-4, which is 9.15-fold higher than the spontaneous level. More than 99% of the 1-nitropyrene-induced mutations in canavanine-resistant diploid cells constituted a gene conversion or crossover. Chromosome loss was not increased after treatment with 1-nitropyrene. These results suggest that 1-nitropyrene is an agent that efficiently induces point mutations, gene conversion, and crossover, but not chromosome loss, in S. cerevisiae.