The timing of UV mutagenesis in yeast: Continuing mutation in an excision-defective (rad1-1) strain

Summary

Mutation induction by ultraviolet light was studied in excision-defective (rad1-1) strains of Saccharomyces cerevisiae. Information about the timing of mutations in relation to postirradiation DNA replication was obtained. The experimental system involved pedigree analysis of G1 diploid cells and subsequent tetrad analysis of the mitotic segregants to detect mutations.

The mutation pattern of rad1-1 strains differed from that of wild type in two respects: (1) few or none of the mutations affected both strands of the DNA, (2) mutations appeared as frequently in the second postirradiation mitotic generation as in the first.

The data have led to the following conclusions about the mutation process in excision-defective (rad1-1) yeast: (a) Mutations are not fixed prior to the first postirradiation round of DNA replication. (b) Unexcised thymine dimers persist as mutagenic lesions through repeated postirradiation cell divisions. (c) Heteroduplex repair is involved in the mutation process. (d) Overlapping daughter-strand gaps are not a prerequisite for mutation. (e) The results provide no evidence that error-prone repair in this strain is inducible rather than constitutive. The data also show that (f) all 2-strand mutations (whole-colony mutants) induced in yeast by exposure to low levels of ultraviolet light are associated with excision repair, and that (g) essentially all lesions induced in excision-proficient strains have been excised at the time of the second round of postirradiation DNA synthesis.