FIG. 4.

UVC-induced inhibition of replicon initiation is sensitive to caffeine. NHF1 and ATM^−/− cells were uniformly labeled as described above and incubated for 30 min in medium alone (A and C) or medium containing 3 mM caffeine (B and D) for 30 min. Cells were sham treated or irradiated with 1 J/m² UVC, incubated in reserved medium for 30 min, and then pulse-labeled with [³H]thymidine for 15 min. Cells treated with caffeine remained in the presence of this inhibitor throughout the treatment period. Cells were harvested, and nascent DNA was separated by velocity sedimentation as described in the legend to Fig. 1C. ○, sham-treated cells; •, UVC-irradiated cells.