HSPA1A and DNAJB1 regulate NELF condensate dynamics to safeguard transcriptional recovery under heat stress.

Promoter-proximal pausing by negative elongation factor (NELF) establishes a critical checkpoint for RNA polymerase II (RNA Pol II) transcription. Heat shock (HS) induces NELF to form nuclear condensates, yet how their dynamics are regulated and coupled to transcriptional adaptation remains unclear. Using a nanobody-based proximity labeling strategy (NbPro), we identify the molecular chaperones HSPA1A and DNAJB1 as key regulators of NELF condensate dynamics. Although dispensable for initial HS-induced transcriptional repression, chaperone-mediated regulation is required for efficient transcriptional reactivation during recovery. Mechanistically, DNAJB1 recognizes NELFA's tentacle domain and facilitates HSPA1A recruitment, thereby preventing aberrant aggregation and enabling timely condensate disassembly. Disruption of NELF condensate dynamics leads to persistent NELFA phosphorylation, impaired chromatin association, destabilized RNA Pol II pausing, and premature release of non-productive RNA Pol II complexes. Together, these findings reveal a chaperone-dependent mechanism that governs NELF condensate dynamics and highlight promoter-proximal pausing as a checkpoint to prevent immature RNA Pol II escape, rather than merely a means of transcriptional repression.