A mechanism for coordinating chromatin modification and preinitiation complex assembly.

Transcription of eukaryotic genes within a chromatin environment requires the sequential recruitment of histone modification enzymes and the general transcription factors (GTFs) by activators. However, it is unknown how preinitiation complex assembly is coordinated with chromatin modification. Here, we show that the model activator GAL4-VP16 directs the ordered assembly of Mediator, histone acetyltransferases (HATs), and GTFs onto immobilized chromatin and naked DNA templates in vitro. Using purified proteins, we found that the Mediator regulates this assembly process by binding to p300 and TFIID. An acetyl-CoA-dependent catalytic switch causes p300 to acetylate chromatin and then dissociate. Dissociation of p300 enhances TFIID binding and active transcription. The dissociation is caused by an autoacetylation-induced conformational change in the catalytic domain of p300. We conclude that autoacetylation-induced dissociation of p300 acts as a catalytic switch, which allows TFIID binding and subsequent preinitiation complex assembly.