Repair replication and unscheduled DNA synthesis in mammalian cell lines showing differential sensitivity to alkylating agents

Repair replication has been measured by CsCl density gradient centrifugation in cell lines showing differential sensitivity to mono- and bifunctional alkylating agents. A correlation between cellular sensitivity as measured by the D₀ value and amount of repair replication was demonstrated after exposure of Yoshida cells to nitrogen mustard (HN2) and methylene dimethanesulphonate (MDMS). No differences in the amount of repair replication after methyl methanesulphonate (MMS) were observed in two L5178Y cell lines which differed in sensitivity by virtue of the shoulder size only. The Yoshida cell lines showed no difference in sensitivity to MMS and no difference in amount of repair replication. Incorporation of tritiated thymidine 9³H]TdR) after drug treament was also measured by autoradiography. The qualitative differences observed between the two cell lines were similar to those obtained in density gradient experiments. The temporal pattern of ³H]TdR uptake indicated that the reduced repair replication observed in the sensitive line after HN2 and MDMS is not due to slower synthesis. The kinetics of ^3H]TdR incorporation differed for all three mutagens suggesting that different enzymes may be involved in each case.