Inhibition of RNA and DNA synthesis in UV-irradiated normal human fibroblasts is correlated with pyrimidine (6-4) pyrimidone photoproduct formation.

UV-irradiation of living cells results in an inhibition of RNA and DNA synthesis. The purpose of this study was to determine whether specific photoproducts or the total combined yield of lesions were responsible for these effects. Asynchronously dividing human fibroblasts from normal donors were irradiated with UVC (254 nm), broad spectrum UVB (290-320 + nm, Westinghouse FS20 lamp) or narrow spectrum UVB (310-315 nm, Philips TL01 lamp) at fluences which induce known yields of cyclobutane pyrimidine dimers, pyrimidine (6-4) pyrimidone photoproducts or Dewar isomers. DNA synthesis was approximately 3-4 times more sensitive to both UVC and UVB irradiation than RNA synthesis. The immediate inhibition of RNA and DNA synthesis was correlated with (6-4) rather than overall photoproduct formation suggesting that the (6-4) photoproduct is the mediator of these inhibitory effects. In support of this suggestion we found that photoreactivation of cells cultured from the marsupial, mouse Sminthopsis crassicaudata, resulted in removal of 70% of pyrimidine dimers from the overall genome, but had only a slight effect on the recovery of RNA synthesis.