Transcription-dependent degradation of topoisomerase I-DNA covalent complexes.

Topoisomerase I (Top I)-DNA covalent complexes represent a unique type of DNA lesion whose repair and processing remain unclear. In this study, we show that Top I-DNA covalent complexes transiently arrest RNA transcription in normal nontransformed cells. Arrest of RNA transcription is coupled to activation of proteasomal degradation of Top I and the large subunit of RNA polymerase II. Recovery of transcription occurs gradually and depends on both proteasomal degradation of Top I and functional transcription-coupled repair (TCR). These results suggest that arrest of the RNA polymerase elongation complex by the Top I-DNA covalent complex triggers a 26S proteasome-mediated signaling pathway(s) leading to degradation of both Top I and the large subunit of RNA polymerase II. We propose that proteasomal degradation of Top I and RNA polymerase II precedes repair of the exposed single-strand breaks by TCR.