Inducible nature of tolerance to lesions produced by ultraviolet light in DNA of Chinese hamster V79 cells.

Chinese hamster V79 cells irradiated by split doses of ultraviolet light (2.5 + 5.0 J m-2) synthesize higher molecular weight DNA molecules than those irradiated by a single dose (5.0 or 7.5 J m-2). A second dose applied 8 h following the first one results in a higher portion of higher molecular weight molecules both in relative and absolute terms. Molecular weight of DNA pulse-labeled 8 h after a dose of 2.5 J m-2 is moderately higher than that of pulse-labeled DNA in unirradiated cells, even after the DNA synthesis in preirradiated cells has been fully recovered. Since the amount of DNA synthesized during the 8 h time interval between the two doses is not lower than that of DNA synthesized during an equal period in unirradiated cells, the synthesis of higher molecular weight molecules is not likely to represent elongation of part of the replicons, the other part being blocked. It is supposed that the induction produced by the first dose is involved in the replication of the damaged template.