Effects of UV light on DNA chain growth and replicon initiation in human cells.

Exposure of mammalian cells to 254 nm UV light produces lesions that block DNA polymerases at least on the leading strand. For rodent cells the extent and duration of this blockage is both cell line- and fluence-dependent. Using DNA fiber autoradiography we report here similar findings for human cells. Wild-type human cells did not exhibit significant blockage following exposure to 2.5 J/m2. After exposure to 5.0 J/m2, there was significant blockage immediately after exposure, but by 5 h segment lengths returned to control values. Excision-deficient human cells, on the other hand, exhibited significant blockage both immediately and 5.0 h after exposure to 2.5 J/m2. Exposure of rodent cells to UV light is also known to activate alternative sites of replication. Such activation would enable cells to replicate areas of DNA which do not contain a 'normal' site of initiation, yet contain blocking lesions both upstream and downstream. We have previously shown (Griffiths and Ling, 1987) that this activation is more pronounced and long-lived in excision-deficient Chinese hamster ovary (CHO) cells than it is in wild-type CHO cells. We report here that excision-deficient human cells also exhibited a marked and prolonged activation of alternative sites of replicon initiation. Wild-type human cells, on the other hand, exhibited little if any activation.