Expression of the radiation-induced depression of the rate of DNA synthesis in HeLa S3 cells can be delayed for at least 6 hr by subjecting cultures to either low temperatures (5°C) or inhibitors of DNA synthesis (1 μM fluorodeoxyuridine or 4 μM cytosine arabino-side) at 38°C immediately after irradiation with 1-krad doses of 220-kV X rays. These findings indicate that full expression of, and recovery from, the damage manifested as depressed synthesis requires not only active metabolism, but DNA replication as well. They imply that the depressed rate does not derive from a decrease in availability of DNA precursors. Confirmatory evidence for this was obtained from measurements of both the rate of total thymidine uptake into the pool under conditions in which the endogenous pathway of thymidylate synthesis is either operating or blocked, and the amounts of thymidine mono-, di-, and triphosphates formed in the pool from thymidine, during the postirradiation period, when incorporation into DNA is depressed. The results are interpreted in terms of the recent identification of replicon initiation as the step in the replication process that is affected by irradiation.

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