T4-endonuclease V-sensitive Sites in DNA from Ultraviolet-irradiated Human Cells

Human diploid cells (WI38) were pre-labeled with 32Pi, exposed to ultraviolet irradiation and then pulse labeled with [3H]thymidine. The extracted DNA from these cells was subsequently treated with the T4-endonuclease V, an enzyme which specifically nicks DNA strands at positions adjacent to pyrimidine dimers. Sedimentation in alkaline sucrose gradients revealed that the DNA synthesized after irradiation, as well as that made before, contained endonuclease-sensitive sites. Our results suggest that pyrimidine dimers are transferred from parental to daughter DNA strands during post-irradiation incubation. Sedimentation in neutral sucrose gradients showed that the molecular weight of native DNA was not affected by the endonuclease treatment, suggesting that the gaps appearing in daughter strands after irradiation are not opposite dimers or that the enzyme cannot recognize dimers in the gap regions.