Inhibition of semiconservative DNA synthesis by dimer and non-dimer UV-induced photoproducts

/sup 14/C-labeled ICR/sub 2/2A frog cells were treated with either 10 J/m/sup 2/ of 254 nm UV, 40 kJ/m/sup 2/ of UV produced by a FS40 fluorescent SunLamp filtered through 48A Mylar plus 30 kJ/m/sup 2/ of photoreactivating light (PRL) from a F40B Blue lamp or 1000 rad of /sup 60/Co γ-rays. The cells were incubated for 2 hr and then pulse labeled for 30 min with /sup 3/H-thymidine. These treatments reduced the level of total DNA synthesis to 20-50% of the level measured in unirradiated cells. The size distribution of the DNA synthesized during the pulse labeling was then estimated using both sucrose gradient sedimentation and alkaline step elution. Using either of these techniques it was found that 254 nm UV, which produces mainly pyrimidine dimers, caused a reduction principally in the synthesis of high molecular weight (MW) DNA. In contrast, exposure to the SunLamp plus PRL, which produces a relatively pure population of non-dimer photoproducts, resulted in a strong inhibition in the synthesis of small MW DNA. Hence, the induction of pyrimidine dimers in DNA mainly affected the elongation of replicons whereas non-dimer photoproducts primarily caused an inhibition of replicon initiation