Single-stranded breaks in DNA but not oxidative DNA base damages block transcriptional elongation by RNA polymerase II in HeLa cell nuclear extracts.

Transcription and repair of many DNA helix-distorting lesions such as cyclobutane pyrimidine dimers have been shown to be coupled in cells across phyla from bacteria to humans. The signal for transcription-coupled repair appears to be a stalled transcription complex at the lesion site. To determine whether oxidative DNA lesions can block correctly initiated human RNA polymerase II, we examined the effect of site-specifically introduced oxidative damages on transcription in HeLa cell nuclear extracts. We found that transcription was blocked by single-stranded breaks, common oxidative DNA lesions, when present in the transcribed strand of the transcription template. Cyclobutane pyrimidine dimers, which have been previously shown to block transcription both in vitro and in vivo, also blocked transcription in the HeLa cell nuclear transcription assay. In contrast, the oxidative DNA base lesions, 8-oxoguanine, 5-hydroxycytosine, and thymine glycol did not inhibit transcription, although pausing was observed with the thymine glycol lesion. Thus, DNA strand breaks but not oxidative DNA base damages blocked transcription by RNA polymerase II.