Mediator Tail Subunits Hierarchically Couple Transcriptional Condensates to Gene Activation and Genome Organization
Cells respond to acute environmental stress by rapidly reorganizing transcriptional machinery and genome architecture, yet how these processes are mechanistically integrated remain poorly understood. We find that Mediator Tail subunits function in a hierarchical fashion to coordinate transcription factor condensate assembly, three-dimensional genome organization and transcriptional output during the heat shock response (HSR) in Saccharomyces cerevisiae. We identify the Mediator Tail triad--Med2, Med3, and Med15--as exceptionally enriched in intrinsically disordered regions and possessing strong intrinsic liquid-liquid phase separation potential, in contrast to the more structured Tail subunits Med5 and Med16. Live-cell imaging reveals that this IDR-rich triad is critically required for thermal stress-induced Heat Shock Factor 1 (Hsf1) condensate formation, HSR gene coalescence and robust transcriptional induction. Mechanistically, Med15 executes these functions through its activator-binding domains, with the IDR-rich ABD2 playing a dominant role in stabilizing Hsf1, Mediator, and RNA polymerase II (Pol II) occupancy at HSR loci, while the C-terminal IDRs of Med2 and Med3 provide critical interaction platforms that couple condensate formation to genome organization. Strikingly, Med16 defines a parallel regulatory axis: although dispensable for Hsf1 condensate nucleation, Med16 is required for Mediator and Pol II condensate formation and for HSR gene coalescence, revealing that transcription factor clustering and productive transcriptional condensates are mechanistically separable. Finally, Med5 plays a minor yet detectable role in HSR transcription and gene coalescence. Together, our findings establish a modular and hierarchical organization of the Mediator Tail that integrates phase separation, transcriptional condensate composition, and 3D genome architecture to drive rapid stress-induced gene activation.