A role for coactivators and histone acetylation in estrogen receptor alpha-mediated transcription initiation

Transcriptional regulation by estrogen receptor alpha (ER alpha) involves p rotein-protein interactions among the receptor, its associated coactivators and the RNA polymerase II transcriptional machinery. We have used an in vi tro chromatin assembly and transcription system to examine the biochemistry of interactions among ER alpha, the SRC proteins and p300/CBP. Using polyp eptides designed to block specific receptor- cofactor or cofactor-cofactor interactions, we show that interactions among ER alpha, its coactivators an d the RNA pol II machinery are all required for ER alpha- mediated transcri ption. Furthermore, we show that ER alpha -SRC-p300/CBP interactions are ne cessary and sufficient for the targeted acetylation of nucleosomal histones on estrogen-responsive promoters in the absence of transcription. The prot ein-protein interactions required for histone acetylation constitute a subs et of the interactions required for transcriptional activation. Finally, we show that the major role of SRC-p300/CBP interactions is to enhance ER alp ha- mediated transcription initiation, and they have little or no role in s timulating subsequent rounds of transcription. Together, our results indica te a specific role for the SRC and p300/CBP coactivators, as well as target ed histone acetylation, in ER alpha -mediated transcription.