Effect of UV light on sister-chromatid exchanges, activation of alternative sites of replicon initiation and thymidine incorporation in CHO AA8, UV61 and UV5 cells.

The relative importance of the UV-induced pyrimidine(5-6)pyrimidine and the pyrimidine(6-4)-pyrimidone lesions in sister-chromatid exchanges (SCEs), activation of alternative sites of replicon initiation and thymidine incorporation were examined using wild-type Chinese hamster ovary (CHO) AA8 cells which remove both lesions, mutant CHO UV61 cells which remove only the (6-4) lesion and mutant CHO UV5 cells which remove neither lesion. Our data suggest that both lesions play a role in each end point examined. The relative importance of these lesions is dependent on the end point studied as well as the fluence used. For SCE induction and the activation of alternative sites of replicon initiation, the (6-4) lesion appears to play a predominant role, while for the thymidine incorporation studies the (6-4) lesion appears to play the predominant role at low fluences while the role of the (5-6) lesion increases at higher fluences.