Postreplication repair of ultraviolet damage to DNA, DNA-chain elongation, and effects of metabolic inhibitors in mouse L cells.
Alkaline sucrose sedimentation studies of DNA from mouse L cells have demonstrated the following effects of several inhibitors of nucleic acid and protein synthesis on postreplication repair of ultraviolet (UV) damage to their DNA. The DNA newly synthesized by a 2 h [(3)H]thymidine (dThd) label following 254 nm UV irradiation of 20 J/m(2) is made in smaller segments of the number average mol wt (Mn) of approximately 10 x 10(6) than the control of approximately 40 x 10(6). The presence of caffeine at a concentration of 2 mM during the labeling of the irradiated cells reduces the Mn value to 5.8 x 10(6), which is nearly comparable to, but somewhat larger than the expected distance between dimers in parental DNA. Afterwards, such an interrupted DNA made in the irradiated cells is completely repaired to the present maximum Mn value of 40 x 10(6) in the consecutive 4 h chase in unlabeled dThd. The presence of the nucleic acid inhibitor, either 2 mM hydroxyurea, 50 microM arabinofuranosyl cytosine, 2 mM excess dThd or 5 microg/ml of actinomycin D (AMD) during 2- to 24-h chase periods after a 2 h postirradiation label prevents the repair to various extents, while 2 mM caffeine completely inhibits it. In the unirradiated cells, these agents except excess dThd and caffeine also interfere severely with normal elongation of nascent DNA made by a 3 min pulse label, but do not appreciably induce single chain breaks of either newly synthesized or parental DNA. The inhibition of the repair by AMD suggests that de novo elongation of DNA to close the gaps in new DNA made in the irradiated cells requires at least a template-dependent DNA polymerase. In contrast, 100 microg/ml of cycloheximide allows to complete the gap-filling repair, while it simply reduces the rates of chain growth for the repair and normal replication. Therefore, the similar sensitivity of gap-filling repair and normal replication towards the above inhibitors indicates that a preexisting DNA polymerizing system appears to be responsible and to play a common role without new protein synthesis, as far as the repair at early time after UV is concerned.
Fujiwara
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Department ofRadiation Biophysics, Kobe University School ofMedicine, Kobe 650, Japan.