Carcinogenesis, Vol. 20, No. 3, 395-399,
March 1999
© 1999 Oxford University Press
Recovery of RNA synthesis from the DHFR gene following UV-irradiation precedes the removal of photolesions from the transcribed strand
Department of Radiation Oncology, Division of Cancer Biology, University of Michigan Comprehensive Cancer Center, 4306 CCGC, 1500 East Medical Center Drive, Ann Arbor, MI 48109-0936, USA
It is thought that recovery of RNA synthesis following UV-irradiation is closely related to the removal of UV-induced lesions from the transcribed strand of active genes. To test this hypothesis, nascent RNA synthesis from three different locations within the DHFR gene in CHO cells was assessed following exposure to UV light (254 nm). Using both in vivo RNA labeling as well as the nuclear run-on technique, it was found that RNA synthesis from the middle and the 3'-end of the gene was inhibited within 20 min by ~30 and 70%, respectively, while RNA synthesis from the 5'-end of the DHFR gene was enhanced. RNA synthesis from the middle portion of the gene fully recovered within 30–45 min of post-UV incubation, while recovery was slower from the 3'-end of the gene. Compared with previously published data for the kinetics of removal of UV-induced DNA lesions from the 5'-half of the DHFR gene in these cells, it is concluded that RNA synthesis resumed significantly faster in this region than could be accounted for by the removal of photolesions from the transcribed strand. Thus, although RNA synthesis was initially inhibited by UV-induced photolesions, the results suggest that RNA polymerase II was able to bypass these lesions prior to their removal.
Abbreviations: CPD, cyclobutane pyrimidine dimer; DHFR, dihydrofolate reductase gene; hMT, human metallothionein gene; TCR, transcription-coupled repair.
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