Identification of a negative regulatory function for steroid receptors.

This report describes the identification of a negative regulator of estrogen and progesterone receptor function. Using a reconstituted estrogen-responsive transcription system in Saccharomyces cerevisiae, we have identified a "repressor function," which when mutated, increases the transcriptional activity of the estrogen and progesterone receptors. In the case of the estrogen receptor this mutation increases the sensitivity of estrogen-mediated activation by at least four orders of magnitude. Analysis of derivatives of the estrogen receptor indicated that this repressor specifically affects the transcription activity of the TAF1 activation domain of the estrogen receptor. The repressor was cloned by complementation and identified as SSN6, a previously described mediator of glucose repression in yeast. Our results indicate that SSN6 is likely to be involved also in the repression of other cellular activators. Interestingly, deletion of the SSN6 protein allows the antiestrogens ICI 164384 and nafoxidine to behave as more potent agonists of estrogen receptor function, while RU486 also becomes a more potent activator of progesterone receptor function. These data suggest that in wild-type cells the role of hormone is twofold: it promotes DNA binding of the receptor and it also induces a conformational change in the receptor which overcomes the effects of this repressor function.