Transcriptional elongation control of hypoxic response

The release of paused RNA polymerase II (RNAPII) from promoter- proximal regions is tightly controlled to ensure proper regulation of gene expression. The elongation tor PTEF-b is known to release paused RNAPII via phosphorylation of the RNAPII C - terminal domain by its cyclin- dependent kinase component, CDK9. However, the nal and stress- specific roles of the various RNAPII- associated macromolecular complexes containing PTEF- b/CDK9 are not yet clear. Here, we identify and characterize the CDK9 complex required for transcriptional response to hypoxia. Contrary to previous reports, our data indicate that a CDK9 complex containing BRD4 but not AFF1/4 is essential for this hypoxic stress response. We demonstrate that BRD4 bromodomains (BET) dispensable for the release of paused RNAPII at hypoxia- activated genes and that inhibition by JQ1 is insufficient to impair hypoxic gene response. Mechanistically, demonstrate that the C - terminal region of BRD4 is required for Polymerase- Associated Factor - 1 Complex (PAF1C) recruitment to establish an elongation- competent RNAPII complex at hypoxia- responsive genes. PAF1C disruption using a small- molecule inhibitor (iPAF1C) impairs hypoxia- induced, BRD4- mediated RNAPII release. Together, our results provide insight into potentially targetable mechanisms that control the hypoxia- responsive transcriptional elongation.