Effect of UV light on DNA replication and chain elongation in Chinese hamster UV61 cells

Exposure of eukaryotic cells to ultraviolet light results in a temporary inhibition of DNA replication as well as a temporary blockage of DNA fork progression. Recently there has been considerable debate as to whether the (5-6)cyclobutane pyrimidine dimer, the pyrimidine(6-4)pyrimidone lesion or both are responsible for these effects. Using cell lines that repair both of these lesions (CHO AA8), only (6-4) lesions (CHO UV61) or neither (CHO UV5), we have shown that in rodent cells both lesions appear to play a role in both the inhibition of thymidine incorporation and the blockage of DNA fork progression. Specifically, after exposure to 2.5 J/m2, AA8 cells recover normal rates of DNA replication within 5 h after exposure, while UV5 cells exhibit a greater depression in thymidine incorporation for at least 10 h. UV61 cells, on the other hand, show an intermediate response, both with respect to the extent of the initial depression and the rate of recovery of thymidine incorporation. UV61 cells also exhibit an intermediate response with respect to blockage of DNA fork progression. In previous publications we have shown that UV5 cells exhibit extensive blockage of DNA fork progression and only limited recovery of this effect within the first 5 h after exposure to UV. In this report we show that UV61 cells exhibit a more extensive blockage of fork progression than is observed in AA8 cells. These blocks also appear to be removed (or overcome) more slowly than in the AA8 cells, but more rapidly than in UV5 cells. Taken together we conclude that both lesions appear to be involved in the initial depression in thymidine incorporation and the initial blockage of DNA fork progression in rodent cells. These data also indicate that (6-4) lesions may be responsible for the prolonged depression in thymidine incorporation and the prolonged blockage of DNA fork progression observed in UV5 cells.