RECOVERY OF MAMMALIAN-CELLS TREATED WITH METHYL METHANESULFONATE, NITROGEN-MUSTARD OR UV LIGHT .2. IMPORTANCE OF DNA-REPAIR PRIOR TO THE INITIATION OF S-PHASE

CHO cells were synchronized in G1 phase and treated with UV light or HN2. These treatments resulted in a dose-dependent reduction in teh rate of DNA replication and cell survival. Holding UV-irradiated cells in G1 phase (in HU medium) for an additional 10 h prior to their release into S phase did not assist recovery as measured by either of these criteria. The survival of cells treated with HN2 was also not enhanced by this recovery period. However, following 2 × 10-5 M HN2 the rate of DNA replication increased from 30% to 70% of the control level when the period in HU medium was extended to 14 h. The induction of cross-links following HN2 treatment of asynchronous cells was shown to be dose dependent. Subsequent incubation in fresh medium resulted in complete recovery within 20 h at concentrations of HN2 up to 10-5 M, and at 2 × 10-5 M HN2, 75% of the cross-links were removed at 14 h post treatment. © 1979.