lncRNA SLERT controls phase separation of FC/DFCs to facilitate Pol I transcription

Keeping the nucleolus a liquid condensate The nucleolus is a multilayered, membraneless nuclear condensate in which DNA polymerase I (Pol I)–mediated ribosomal DNA (rDNA) transcription and pre-rRNA processing occur in fibrillar center and dense fibrillar component (FC/DFC) units. How its biophysical properties are regulated has remained elusive. Wu et al . found that the RNA helicase DDX21 forms a shell coating each FC/DFC unit in the nucleolus (see the Perspective by Yamazaki and Hirose). The authors found that a long noncoding RNA called SLERT facilitates the transition from the open to the closed configuration of the helicase using a chaperone-like mechanism. DDX21 in the closed conformation forms loose clusters that confer the FC/DFC unit sufficient liquidity and space required for Pol I processivity. In addition, DDX21 within the loose clusters cannot approach and wrap rDNA, thus licensing rDNA for transcription. —DJ