ANALYSIS OF GENE-SPECIFIC AND STRAND-SPECIFIC REPAIR IN THE MODERATELY UV-SENSITIVE SACCHAROMYCES-CEREVISIAE RAD23 MUTANT

The RAD23 gene of Saccharomyces cerevisiae is involved in nucleotide e xcision repair (NER) and mutations in this gene confer a moderate sens itivity to UV irradiation. However, no repair of either cyclobutane py rimidine dimers (CPDs) and 6-4 photoproducts, the major types of lesio ns formed upon UV ii-radiation, was detectable during the first 4 h po st UV irradiation in a rad23 mutant. rad23, like the rad7 and rad16 mu tants, is not as UV sensitive as completely NER-deficient mutants. The rad7 and rad16 mutants are only partly defective in NER: non-transcri bed strands are completely refractory to repair while transcription-co upled repair is not affected. To investigate whether the rad23 mutant has similar strand-specific repair characteristics we analyzed gene-sp ecific CPD removal from several loci using strand-specific probes but did not detect any repair. The moderate UV sensitivity of rad23 mutant s as compared to completely NER-deficient mutants is therefore not due to gene- or strand-specific removal of lesions, indicating that rad23 mutants do not have a similar repair defect as rad7 or rad16 mutants, but are presumably defective in general NER. The rad23 mutation does not suppress the high UV sensitivity of completely NER-deficient rad1 or rad14 strains. This demonstrates that the relatively high survival of rad23 mutants is not due to an increased tolerance for the lesions that seem to persist in the genome but rather requires some NER functi on.