Inhibition of DNA chain elongation in Chinese hamster cells by damage localized behind the replication fork

Chinese hamster fibroblasts were pulse labelled with 5-bromodeoxyuridine and exposed at time intervals (Ti) to near-ultraviolet (U.V.A.) light in the presence of a bisbenzimidazole derivative (Hoechst 33342). The sensitivity of the cells in terms of colony forming ability fluctuated depending on Ti. Inhibition of DNA synthesis also depended on Ti and was maximal when Ti = 0. Using the alkaline elution technique it was shown that the effect of a large dose of light was to inhibit both initiation and elongation of DNA chains. These effects were most pronounced for Ti = 0. It is concluded that DNA damage in an active replicon can inhibit initiation of new replicons and that damage localized behind the replication fork can retard elongation of nascent DNA chains. This effect on chain elongation decreases with increased distance of the damage from the replication fork.