p300 and estrogen receptor cooperatively activate transcription via differential enhancement of initiation and reinitiation.

Estrogen- and antiestrogen-regulated, AF-2-dependent transcriptional activation by purified full-length human estrogen receptor (ER) was carried out with chromatin templates in vitro. With this system, the ability of purified human p300 to function as a transcriptional coactivator was examined. In the absence of ligand-activated ER, p300 was found to have little effect (less than twofold increase) on transcription, whereas, in contrast, p300 was observed to act synergistically with ligand-activated ER to enhance transcription. When transcription was limited to a single round, p300 and ER were found to enhance the efficiency of transcription initiation in a cooperative manner. On the other hand, when transcription reinitiation was allowed to occur, ER, but not p300, was able to increase the number of rounds of transcription. These results suggest a two-stroke mechanism for transcriptional activation by ligand-activated ER and p300. In the first stroke, ER and p300 function cooperatively to increase the efficiency of productive transcription initiation. In the second stroke, ER promotes the reassembly of the transcription preinitiation complex. Therefore, ER exhibits distinct, dual functions in transcription initiation and reinitiation.