The binding site for Xenopus glucocorticoid receptor accessory factor and a single adjacent half-GRE form an independent glucocorticoid response unit

In Xenopus laevis, transcription of the gamma-fibrinogen subunit gene is ac tivated by glucocorticoids. Hormone induction is regulated by three glucoco rticoid response element (GRE) half-sites and an additional DNA sequence wh ich binds a novel hepatocyte nuclear protein, Xenopus glucocorticoid recept or accessory factor (XGRAF). The XGRAF binding site (GAGTTAA) is located di rectly upstream of the most distal half-GRE. The proximity of the binding s ites for XGRAF and the glucocorticoid receptor (GR) led to the hypothesis t hat these two sites form a glucocorticoid response unit (GRU). By transfect ing DNA into primary hepatocytes, we showed that this GRU confers hormone r esponsiveness in the absence of other half-GREs. The XGRAF binding site enh ances function of the half-GRE without itself responding to glucocorticoids . The GRU retains efficacy in other locations relative to the gamma-fibrino gen gene promoter, further increases transcription when present in multiple copies, and activates a heterologous promoter. Despite the contiguity of t he XGRAF binding site and half-GRE, the two sites can be occupied simultane ously in vitro. The binding characteristics correlate with function since m utations that disrupt concurrent XGRAF and GR binding also impair transcrip tion. This novel GRU represents a new regulatory mechanism that may be appl icable to other glucocorticoid responsive genes that lack a full GRE.