Histone acetylation-dependent clustering of BRD2 instructs transcription dynamics.

Bromodomain (BD) and extra-terminal domain (BET) proteins are key regulators of RNA polymerase II (Pol II)-mediated transcription and their BDs represent promising drug targets. Yet, the interplay between histone acetylation and the chromatin dynamics of individual BET proteins with respect to transcriptional regulation is not fully understood. Here in mouse embryonic stem cells, we uncover an essential role of BRD2 in maintaining Pol II recruitment at promoters through its interaction with TFIID, which becomes particularly critical under the conditions of impaired pause release. Combining rapid protein degradation, chemogenomics and super-resolution microscopy, we show that MOF-mediated histone H4 acetylation promotes BRD2 chromatin association, which in turn enables BRD2 clustering. Accordingly, MOF depletion or deletion of the BRD2's intrinsically disordered region largely recapitulates defects in promoter enrichment and clustering of the transcription machinery observed upon BRD2 loss. Thus, these findings support a model in which histone acetylation-dependent spatiotemporal dynamics of BRD2 coordinate the transcription machinery to regulate transcription initiation.