No abstract

Transcriptional activation of target genes by the human progesterone receptor is thought to involve direct or indirect protein-protein interactions between the progesterone receptor and general transcription factors. A key role in transcription plays the general transcription factor TFIID, a multiprotein complex consisting of the TATA-binding protein and several tightly associated factors (TAFs). TAFs have been shown to be required for activated transcription and are, thus, potential targets of activator proteins. Using in vitro interaction assays, we could identify specific interactions between the progesterone receptor and the TATA-binding protein-associated factor dTAF II 110. The dTAF II 110 domain responsible for the interaction is distinct from that reported to suffice for binding to Sp1. Somewhat surprisingly, deletion analysis indicated that the previously identified activation functions 1 and 2 of the progesterone receptor are not required for this interaction but pointed to an important role of the DNA binding domain. In cotransfection experiments and an in vitro transcription assay, the DNA binding domain of the progesterone receptor displayed significant activation potential. These findings, taken together, suggest that an interaction between the progesterone receptor and TAF II 110 may represent an important step in the mechanism of activation.In eukaryotes, at least seven basal transcription factors (TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and TFIIJ) are required for basal levels of transcription at promoters by RNA polymerase II in vitro (reviewed in Ref. 1). In mammalian systems, these factors assemble into functional initiation complexes in a highly ordered, stepwise fashion beginning with the binding of TFIID to the TATA box (1-3), while in yeast a number of factors seem to be brought in as a preassembled RNA polymerase II holoenzyme complex (4). Sequence-specific activators that bind to specific regulatory elements within distal promoter regions or enhancers are capable of stimulating the rate of transcription initiation. Although the precise mechanism is presently unknown, there is increasing support for models proposing that protein-protein interactions between activators and basal transcription factors play a decisive role in this process (5). Such interactions might help to recruit these basal transcription factors or the preassembled RNA polymerase II holoenzyme complex to the TATA box, stabilize intermediates in the assembly of the initiation complex, or activate certain components by inducing conformational changes.Biochemical and molecular characterization of TFIID has shown that it is a multi-subunit complex consisting of the TATA-binding protein (TBP) 1 and a number of TBP-associated factors (TAFs) (6 -8), ranging from 20 to 250 kDa in size. In cell-free transcription systems reconstituted from partially purified factors, recombinant TBP is able to support basal transcription of TATA-only promoters (9, 10). However, unlike TFIID, TBP fails to mediate transcriptional stimulation b...

Antihormones are by definition antagonists of steroid hormone action. They interact with the ligand binding domains of steroid hormone receptors and competitively inhibit the action of the receptors by mechanisms that are not quite understood. In certain cases antihormones also exhibit agonistic activity especially in connection with certain naturally occurring receptor mutants. These observations together with findings of indiscriminate interaction of antihormones with several classes of steroid receptors have necessitated a search of more effective and reliable antihormones. Recent advances in the resolution of the crystal structure of the ligand binding domains of certain members of the steroid receptor family and identification of non-liganded activation of steroid receptors have produced considerable information that can be harnessed into a fruitful search for a new generation of antihormones.

No abstract

Hormone-induced progesterone receptors (PR) bound to response elements stimulate transcription initiation at target promoters through a mechanism that presumably involves cofactors or coactivators. To allow identification of such cofactors of transcriptional activation in a functional assay, we have established a reconstituted transcription system that is characterized by a specific loss of responsiveness to purified baculovirus-expressed wild type PR. In contrast to wild type PR, a C-terminally truncated PR mutant displayed strong activation potential in this system. As the purified recombinant full-length PR is capable of DNA binding, our results suggest that C-terminal sequences of PR mediate a cis-repression of N-terminal activation functions. Moreover, using this PR-nonresponsive transcription system, we identified and partially purified an activity from rat liver, termed COPRA (cofactor of PR activation), that restores transactivation by full-length PR. Characterization of COPRA revealed that this cofactor exhibits activator specificity and is not involved in basal transcription. We postulate that COPRA acts by relieving the repression of activation functions mediated by C-terminal sequences.