THE RATE OF REMOVAL OF SUNLIGHT-INDUCED PYRIMIDINE DIMERS FROM THE DNA OF CULTURED HUMAN DIPLOID FIBROBLASTS

Abstract The rate of excision of sunlight-induced pyrimidine dimers in DNA of exposed human cells was determined. Two normal excision repair-proficient human diploid fibroblast strains (WS-1 and KD) and a repair-deficient strain (XP12BE, group A) maintained in a nondividing state were exposed to summer noon-time sunlight for times (5 and 20 min) that induced numbers of dimers equivalent to far UV (254 nm) exposures of 1 and 4 J/m². Pyrimidine dimers were quantified in extracted DNA using a U V-endonuclease-alkaline sedimentation assay. The excision rates of these dimers were similar to those observed for the excision of UV-induced pyrimidine dimers. No sunlight-induced inhibition or stimulation of DNA repair was observed in either strain at these low exposures.