Novel Accessory Factor-Binding Site Required for Glucocorticoid Regulation of the γ-Fibrinogen Subunit Gene fromXenopus laevis

Abstract

Glucocorticoids induce gene expression by binding to an intracellular receptor that interacts with genomic DNA and stimulates transcription of specific genes. The consensus DNA-binding site for the glucocorticoid receptor, called a glucocorticoid response element (GRE), is GGTACAnnnTGTTCT. In the classical model, binding of the receptor as a dimer to the two halves of the GRE is required for activation of transcription. For some glucocorticoid-regulated genes, additional DNA-binding proteins called accessory factors are necessary for hormonal responsiveness. We have identified a new factor required for glucocorticoid-induced expression of the gamma-fibrinogen subunit gene from the frog Xenopus laevis. Transfection of cloned DNA fragments into primary Xenopus hepatocytes showed that the DNA between 163 and 187 bp upstream of the transcription initiation site is essential for hormonal activation. A single complex forms when this small region of DNA is incubated in vitro with Xenopus liver nuclear proteins. The protein recognition site has been narrowed to AAGAGTTAA, a sequence not previously described as a transcription factor-binding site. We have named the protein(s) bound to this sequence Xenopus glucocorticoid receptor accessory factor (XGRAF). In addition to the XGRAF-binding site, glucocorticoid regulation of the gamma-fibrinogen gene requires at least three nearby GREs, each of which is a poor match to the consensus GRE. The position of the binding site for XGRAF overlaps the putative upstream half of the most important GRE. Models are presented to show possible ways that the novel accessory factor and the glucocorticoid receptor could act through closely juxtaposed sites on the DNA.