The induction of lethal events and cytoplasmic "petite" mutations in random and synchronously dividing cultures of haploid and diploid cells of Saccharomyces cerevisiae was determined after treatment with 8-Methoxypsoralen plus 365 nm light.
Diploid cells were always more resistant than haploid cells in stationary phase whereas in exponential phase haploid and diploid cells demonstrated the same response. For both degrees of ploidy exponential phase cells were more resistant than stationary phase.
Mitotic recombination is induced with the same efficiency in both phases. Dark holding reduces the frequency of recombinants.
In diploid cells the recovery of survival after dark holding was more pronounced than in haploid cells.
The "petite" mutation is induced more efficiently in stationary phase cells than in exponential phase cells. The p-frequency observed on dark holding depends upon the growth phases and ploidy.
As after UV irradiation in a synchronized population, Gt cells were the most sensitive as compared to S and G2 phase cells after 8-Methoxypsoralen plus light treatment. The most sensitive stages are the ones which show the highest recovery after dark holding.
The cyclic pattern of p-induction does not appear to be strictly correlated with that of the lethal effect. In S-G2 phase cells the p-frequency decreased after delayed plating.
The results are interpreted assuming that the excision repair acts mainly in the G1 phase whereas an efficient recombinational type of repair operates in