Decreased u.v. mutagenesis in an excision-deficient mutant of yeast

The u.v.-sensitive rad4 mutant of yeast was found to decrease u.v.-induced reversion in the cdc8 and lys2 loci. Survival curves obtained following u.v. irradiation and u.v.-induced reversion in rad4, cdc8 and cdc8rad4 double mutants show that cdc8 is epistatic to rad4 and belongs to the 'rad3' epistasis group. A study of u.v.-induced reversion in the thermosensitive cdc8 mutant at the restrictive temperature (prevents DNA synthesis) and at the permissive temperature (DNA synthesis takes place) indicates the essential role of DNA replication in u.v.-induced mutagenesis.