Deoxyribonucleic acid excision repair in chromatin after ultraviolet irradiation of human fibroblasts in culture

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Abstract
Confluent normal human fibroblasts were exposed to UV fluences of 5, 14 or 40 J/m2 and monitored the specific activity of post-UV repair synthesis in chromatin with [3H]thymidine pulses. Under conditions where no semiconservative DNA synthesis is detectable, the specific activity of repair label in micrococcal nuclease resistant (core particle) DNA is about 1/5 that in bulk DNA at all 3 UV fluences. The distribution of thymine-containing pyrimidine dimers in bulk and nuclease-resistant regions measured immediately after irradiation or at later times showed no significant differences; preferential labeling of linker (nuclease-sensitive) DNA during repair synthesis is thus apparently not due to a predominance of UV-induced photoproducts in linker relative to core particle DNA in the nucleosome. Pulse and pulse-chase experiments at 14 or 40 J/m2 with normal human or repair-deficient xeroderma pigmentosum cells showed that at most 30% of repair label in all these cells shifts from nuclease-sensitive (linker) DNA to nuclease-resistant (core particle) DNA.