Recovery of RNA synthesis from the DHFR gene following UV-irradiation precedes the removal of photolesions from the transcribed strand

It is thought that recovery of RNA synthesis following UV-irradiation is cl osely related to the removal of UV-induced lesions from the transcribed str and of active genes. To test this hypothesis, nascent RNA synthesis from th ree different locations within the DHFR gene in CHO cells was assessed foll owing exposure to UV light (254 nm), Using both in vivo RNA labeling as wel l 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 similar t o 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 slowe r 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 D HFR gene in these cells, it is concluded that RNA synthesis resumed signifi cantly 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 RN A polymerase II was able to bypass these lesions prior to their removal.