P300 AND ESTROGEN-RECEPTOR COOPERATIVELY ACTIVATE TRANSCRIPTION VIA DIFFERENTIAL ENHANCEMENT OF INITIATION AND REINITIATION

Estrogen- and antiestrogen-regulated, AF-2-dependent transcriptional a ctivation by purified full-length human estrogen receptor (ER) was car ried out with chromatin templates in vitro. With this system, the abil ity of purified human p300 to function as a transcriptional coactivato r was examined. In the absence of ligand-activated ER, p300 was found to have little effect (less than twofold increase) on transcription, w hereas, in contrast, p300 was observed to act synergistically with Lig and-activated ER to enhance transcription. When transcription was limi ted to a single round, p300 and ER were found to enhance the efficienc y of transcription initiation in a cooperative manner. On the other ha nd, when transcription reinitiation was allowed to occur, ER, but not p300, was able to increase the number of rounds of transcription. Thes e results suggest a two-stroke mechanism for transcriptional activatio n by ligand-activated ER and p300. In the first stroke, ER and p300 fu nction cooperatively to increase the efficiency of productive transcri ption initiation. In the second stroke, ER promotes the reassembly of the transcription preinitiation complex. Therefore, ER exhibits distin ct, dual functions in transcription initiation and reinitiation.