Does ultraviolet light enhance postreplication repair in mammalian cells?

Abstract

D'AMBROSIO AND SETLOW reported an apparent enhancement of postreplication repair in Chinese hamster V79 cells¹. A first exposure of cells to ultraviolet (UV) light was said to make it possible, after re-exposure to UV, for the cells to join the DNA together to form high molecular weight DNA more rapidly than if the cells had not been previously exposed. They irradiated cells with 2.5 J m⁻² of UV 2 h before a second exposure to 5.0 J m⁻², and then pulse-labelled the cells with ³H-thymidine before chasing for 1 h. When analysed by alkaline sucrose gradient velocity sedimentation, the DNA of these cells showed a profile of radioactivity very different from that of cells irradiated only once with 5.0 J m⁻² before pulse-labelling and chasing. For cells irradiated twice the peak of labelled DNA was at a position corresponding to a higher molecular weight than the peak for cells irradiated once. I report here results confirming the data of D'Ambrosio and Setlow, but use of an alternative method of data presentation suggests that interpretations other than the one assumed by D'Ambrosio and Setlow are equally possible or maybe more likely.