Phase separation, transcriptional elongation control and human diseases.

Precise regulation of gene transcription is of great importance to development and diseases. Promoter-proximal transcriptional pause is a key and general mechanism to precisely control transcription in metazoans. Subsequent to transcription initiation and synthesis of a short RNA, RNA polymerase II (Pol II) usually pauses at the promoter-proximal regions, standing by for further signals to be released into the productive elongation stage. Fine regulation of Pol II pausing and release is achieved by the concerted action of many negative and positive elongation factors, including the super elongation complex (SEC). Recent studies suggested that phase-separated assemblies of transcription regulatory complexes could provide a general biophysical basis for the dynamic regulation of transcription in response to various cellular needs, though direct evidence at endogenous level in living cells is still largely lacking. Here, we summarize and discuss latest advances in understanding how phase separation contributes to RNA polymerase II-mediated transcription, with a focus on transcriptional pause and release.