Sequential recruitment of steroid receptor coactivator-1 (SRC-1) and p300 enhances progesterone receptor-dependent initiation and reinitiation of transcription from chromatin

Employing a cell-free chromatin transcription system that recapitulates pro gesterone receptor (PR)-mediated transcription in vivo, we have investigate d further the coactivator functions of steroid receptor coactivator-1 (SRC- 1) in terms of its functional domains as well as cooperation with other coa ctivators in PR transactivation. By analyzing wild-type and mutant SRC-1 wi th liganded PR in the chromatin transcription system in vitro, the basic he lix-loop-helix/ Per-Arnt-Sim domain, the p300-binding domain, and the carbo xyl-terminal region (containing the PR-binding site) of SRC-1 were shown to be important for FIR transactivation. Although in context of a synthetic p romoter its histone acetyltransferase activity was nonessential for PR-medi ated transcription, SRC-1 was observed to act synergistically with p300 to enhance PR transactivation from chromatin. Moreover, SRC-1 and p300 were fo und to function cooperatively to increase the efficiency of productive tran scription initiation and reinitiation. Further analysis of synergism betwee n SRC-1 and p300 revealed an obligatory "sequential" recruitment of SRC-1 a nd p300 to liganded PR. Efficient recruitment of p300 required the presence of SRC-1. In addition, functional analysis of SRC-2 and SRC-3 coactivators indicated that the SRC family modulated PR transactivation from chromatin by a similar mechanism.