Steroid receptor coactivator-1 (SRC-1) enhances ligand-dependent and receptor-dependent cell-free transcription of chromatin

Progesterone receptor (PR) functions as a transcription factor that modulat es the transcription of target genes in response to progesterone and other signals. The transcriptional activity of PR requires the involvement of coa ctivators such as steroid receptor coactivator-l (SRC-1). To dissect the ro le of SRC-1 in PR transactivation, we established an in vitro transcription system with chromatin templates, in which PR induced transcription in a li gand-dependent and PRE-dependent manner. In the presence of ligand, purifie d PR bound to chromatin templates, resulting in chromatin remodeling. With this system, the ability of purified SRC-1 to act as a coactivator of PR wa s examined. SRC-1 potentiated transcription by ligand-activated PR, whereas it had no effect on transcription in the absence of ligands, As SRC-1 poss esses intrinsic histone acetyltransferase activity, we tested the role of a cetylation in PR-mediated transcription by using a histone deacetylase inhi bitor, trichostatin A (TSA). We found that addition of TSA strongly enhance d PR-dependent transcription on chromatin but not on naked DNA template, an d the effects of SRC-1 and TSA on PR transactivation were partially redunda nt. In addition, SRC-1 was able to potentiate PR transactivation with nonch romatin templates. Thus, our results substantiate a two-step mechanism wher eby recruitment of coactivator SRC-1 by the ligand-activated PR in vivo lea ds to (i) chromatin remodeling through histone acetylation and (ii) recruit ment/stabilization of the preinitiation complex.