Steroid receptor transcriptional synergy is potentiated by disruption of the DNA-binding domain dimer interface

The dimer interface within the DNA-binding domain (DBD) of the steroid receptors stabilizes receptor binding to a palindromic DNA site termed a hormone response element (HRE) and is essential for receptor transcriptional activity when a single HRE drives transcription of a target gene. However, most steroid-responsive genes are driven by multiple HREs, and synergy between HREs is an important determinant of receptor activity. We have examined the effects of mutations within the DBD dimer interface on synergistic transcriptional activation by the mineralocorticoid and glucocorticoid receptors. As expected, mutations in either the mineralocorticoid receptor or glucocorticoid receptor that destabilized the DBD dimer interface disrupted receptor binding and activity at a single HRE. However, in striking contrast, these same mutations markedly increased receptor synergistic activity on a reporter gene containing multiple HREs and modestly increased DNA binding. Reestablishing intersubunit contacts by compensatory mutation or by coexpression of complementary mutants returned activity to near-wild type levels. These observations strongly suggest that the DBD dimer interface restrains steroid receptor transcriptional synergy and may play an unexpected role in the regulation of receptor activity.