Figure 7. The model for transcriptional regulation by a distinct layer of histone acetylation-unbound BRD4-PTEFb complex.

Under normal cellular conditions, the majority of BRD4 molecules are associated with chromatin through binding to acetylated histone tails, and the PTEFb complex is recruited by its interaction with the C terminal of BRD4 to form the acetylation-bound “layer” of BRD4-PTEFb. A small portion of the remaining BRD4-PTEFb complex interacts with the Pol II CTD regardless of histone acetylation, forming a critical layer of BRD4-PTEFb that phosphorylates the Ser2 and Ser5 positions of the CTD heptapeptide repeats (Panel 1). BRD4 depletion achieved either by auxin in the BRD4 degron cells or dBET6 treatment results in genome-wide Pol II pausing (Panel 2). The acetylation-bound BRD4-PTEFb layer is displaced from the chromatin when cells are treated with the bromodomain inhibitor JQ1, but the acetylation-independent layer of BRD4-PTEFb can still function to release Pol II without histone recognition-mediated chromatin association (Panel 3). In the absence of acetylation (e.g., H3K27ac, H3K9ac, and H3K18ac deposited by CBP/GCN5²⁸), the acetylation-independent, Pol II-bound BRD4-PTEFb complex retains the ability in releasing Pol II (Panel 4).