SEC Mediates Phase Transition In Transcriptional Pause Release

In metazoans,after RNA polymerase II(Pol II)initiates transcription and synthesizes a short RNA,Pol II pauses at the promoter-proximal region,standing by for further cues to enter the productive elongation stage.Promoter-proximal Pol II pausing is one of the rate-limiting steps during transcription,functioning as an early transcriptional elongation check point.The release of paused Pol II can be triggered by the positive transcription elongation factor b(P-TEFb).SEC,one of the most active P-TEFb containing complexes,is required for rapid gene expression in response to stress or developmental signals.However,the biophysical mechanism underlying transcriptional pause release and dynamic transition of P-TEFb from inactive to active state remains unclear.Here we demonstrate that the paused Pol II associated factor SPT5 undergoes phase transition during pause release.SPT5 per se is prone to form gel-like condensates and proper establishment of paused Pol II.We also found that the SEC components are able to compartmentalize and concentrate P-TEFb from its inactive partners through phase separation,triggering rapid transcription induction.SEC also induces multivalent phase transition of SPT5 into the elongation droplets by dynamic exchange the pausing and elongation factors at the proximal promoter region.Furthermore,disease-associated SEC mutations can result in aberrant properties of the elongation droplets and transcriptional misregulation.In summary,our results suggest a critical role of multivalent phase separation of SEC in controlling a step-wise transcriptional pause release,and that aberrant phase properties could contribute to transcription abnormality in diseases.