The genetic defect in the Chinese hamster ovary cell mutant UV61 permits moderate selective repair of cyclobutane pyrimidine dimers in an expressed gene.

We examined removal of cyclobutane pyrimidine dimers (CPDs) from the dihydrofolate reductase (DHFR) gene in ultraviolet-irradiated Chinese hamster ovary (CHO) UV61 and UV5 cells. The sensitivity of UV61 cells to UV-irradiation is intermediate between that of the parental CHO cells and that of mutants such as UV5 that are highly defective in excision repair. UV61 cells have been characterized as having normal repair of pyrimidine(6-4)pyrimidone photoproducts (6-4 PPs) but no detectable removal of CPDs from the genome overall. We find that the extent of removal of CPDs from the DHFR gene in UV61 cells is intermediate between that of the parental CHO cells and that of the UV5 mutant, and the observed repair appears to be confined to the transcribed strand. We detected no removal of CPDs from the DHFR gene in UV5 cells. Our findings in UV61 cells demonstrate a correlation between survival after UV-irradiation and CPD repair in an expressed gene in a cell line with moderate UV-sensitivity and yet no apparent removal of CPDs from the genome as a whole. We have thus demonstrated that overall repair measurements can be misleading. Our results have implications for the determination of the relative biological importance of the CPD and the 6-4 PP, and they further support the hypothesis that removal of CPDs from transcriptionally active DNA is crucial for UV-resistance.