Response of Plateau-phase Mouse Embryo Fibroblasts to Ultra-violet Light

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Abstract
Clonogenic survival was measured in plateau-phase cultures of the 10T½ mouse cell line exposed to 254 nm ultra-violet light. The survival curve was found to be biphasic, D0 for the two components being 37 and 1191 erg/mm2 respectively. This extreme resistance at higher doses can only be partly accounted for by the increased cytoplasmic absorption of U.V.L. due to an increased thickness of plateau-phase cells. When the cultures were held for 24 hours in plateau phase in conditioned medium after irradiation, recovery yielding a 1·4-fold enhancement of survival was found at higher doses. This recovery process was inhibited by neither caffeine nor cycloheximide. When caffeine was given for 48 hours after sub-culture, the effect on survival was also negligible. We propose that this plateau-phase recovery process is associated with excision repair of DNA adducts induced by U.V.L. Delayed sub-culturing favours the excision mode of repair and renders the post-replication mode less necessary.