Hyperphosphorylation of RNA Polymerase II in Response to Topoisomerase I Cleavage Complexes and Its Association with Transcription- and BRCA1-dependent Degradation of Topoisomerase I

Abstract

The progression of RNA polymerase II can be blocked by lesions on the DNA template. In this study, we focused on the modifications of the largest subunit of RNA polymerase II, Rpb1, in response to stabilized topoisomerase I (Top1)–DNA cleavage complexes. In addition to DNA modifications (base damages and strand breaks), Top1 cleavage complexes can be trapped by camptothecin (CPT) and its derivatives used in cancer treatment. We found that, within a few minutes, CPT produces the complete hyperphosphorylation of Rpb1 in both primary and transformed cancer cells. Hyperphosphorylation is rapidly reversible following CPT removal. Hyperphosphorylation occurs selectively on the serine 5 residue of the conserved heptapeptide repeats in the Rpb1 carboxy-terminal domain and is mediated principally by the transcription factor IIH-associated cyclin-dependent kinase Cdk7. Hyperphosphorylated Rpb1 is not primarily targeted for proteosomal degradation and instead is subjected to cycles of phosphorylation and dephosphorylation as long as Top1 cleavage complexes are trapped by CPT. Finally, we show that transcription-induced degradation of Top1 is Brca1 dependent, suggesting a role for Brca1 in the repair or removal of transcription-blocking Top1–DNA cleavage complexes.