HETERODUPLEX REPAIR AS AN INTERMEDIATE STEP OF UV MUTAGENESIS IN YEAST

Friederike Eckardt ¹, Soo-Jeet Teh ¹, and R. H. Haynes ¹

¹ Department of Biology, York University, Toronto, Canada M3J 1P3

We have measured UV-induced mutation frequencies in yeast in a forward, nonselective assay system by scoring white adex ade2 double auxotrophs among parental red-pigmented ade2 clones. The frequencies of sectored and pure mutant clones were determined separately. In excision-defective strains carrying the genes rad1–1, rad3–2 and rad4–4, as well as in the double mutants, rad 1–1 rad 3–2 and rad 1–1 rad 4–4, considerably more sectored than pure clones are induced in the low-dose range; in repair-competent strains, pure mutant clones substantially outnumber the sectored clones. These results can be explained on the basis of known differences in the timing of error-prone repair during the cell division cycle; that is, we assume that error-prone repair occurs primarily before replication in RAD wild-type strains but after replication in excision-deficient mutants. It has been suggested that excision deficiency has a pleiotropic effect on heteroduplex repair and nucleotide excision repair; however, the high percentage (36.6%) of half-sectored clones found in the rad1–1 strain is hard to reconcile with this hypothesis. We propose that heteroduplex repair occurs subsequent to error-prone repair in both excision-proficient and excision-deficient strains.

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