Killing and mutation induction in quiescent V79 cells as influenced by nalidixic acid

Abstract Involvement of topoisomerase II in the repair of damage by N -methyl- N ′-nitro- N -nitrosoguanidine, UV light and γ-rays has been studied using quiescent V79 cells. In each case, the presence of nalidixic acid, the inhibitor of topoisomerase II, decreased the survival through suppression of potentially lethal damage repair. There was also an increase in the yield of mutants because of such suppression. The observations were in contrast with those made with exponential cells in the following aspects: (a) density-inhibited cells showed a positive response to suppression of topoisomerase II activity after treatment with UV light and γ-rays and (b) for MNNG exposure, mutational yield increased instead of decreasing as in exponential cells. The results showed that topoisomerase II played an important part in the repair of damage of density-inhibited cells.