The effects of three PSO genes on induced mutagenesis : a novel class of mutationally defective yeast.

Reverse and forward mutation, induced by photoaddition of 8-methoxypsoralen (8-MOP) and 3-carbethoxypsoralen (3-CPs) or ultraviolet light (UV), are reduced in three pso mutants of Saccharomyces cerevisiae. The pso1-1 strain exhibits a lower frequency of spontaneous reversion (anti-mutator) and is almost entirely unaffected by the three agents in both the haploid and diploid states. The pso2-1 strain demonstrates very reduced frequencies of 8-MOP and 3-CPs plus 365 nm radiation-induced mutations in haploid and diploid cells. UV-induced mutation are slightly reduced, whereas survival is almost normal. The pso3-1 strain is mutable by 8-MOP and 3-CPs photoaddition only in the low-dose range. After UV treatment, survival of pso3-1 is nearly normal, whereas the frequencies of induced mutants are diminished as compared to the normal PSO+. An analogue of adenine, 6-N-hydroxyaminopurine, is capable of inducing reversions in wild type, as well as in pso and rad6-1 mutant strains, indicating that this drug may act as a direct mutagen in yeast. The comparison of photoaddition of the bifunctional agent (8-MOP) to that of the monofunctional one (3-CPs) confirms that cross-links, as well as monoadditions, are mutagenic in S. cerevisiae. Repair, of the recombinational type, taking place in diploid cells or in haploid cells in G2 phase leads to higher survival, but appears to be error-free.