Recombination of parent and daughter strand DNA after UV-irradiation in mammalian cells.

The mechanism by which mammalian cells replicate DNA containing pyrimidine dimers is poorly understood. When DNA synthesis is initiated after UV-irradiation in bacteria, parental DNA containing pyrimidine dimers has been shown to 'exchange' into the daughter strand DNA by a recA-dependent mechanism. In earlier experiments, when growing mammalian cells were UV-irradiated and then incubated with labelled thymidine, pyrimidine dimers were initially interpreted to be in the newly synthesized DNA, but later were found to be only adjacent to newly synthesized DNA in daughter strand present before UV treatment. A way to avoid these problems of interpretation would be to use cells in G0 or G1 which are not synthesizing DNA at the time of irradiation. UV damage could then be detected in a very sensitive quantitative assay such as that recently described using alkaline elution and an endonuclease preparation from Micrococcus luteus. I have now used this approach and report that up to 3 dimers per 10(9) daltons of daughter strand DNA could be detected 34-45 h after UV-irradiation in human peripheral blood lymphocytes (PBL), normal human fibroblasts, group A xeroderma pigmentosum (XP) fibroblasts and mouse 3T3 cells. This represents approximately 1-3% of the dimers present in the parent strand at this time after UV.