HEPATOCYTE NUCLEAR FACTOR-3 DETERMINES THE AMPLITUDE OF THE GLUCOCORTICOID RESPONSE OF THE RAT TYROSINE AMINOTRANSFERASE GENE

Hepatocyte nuclear factor 3 (HNF3) recognizes two apparently distinct classes of sequence. However, a detailed mutational analysis of a repr esentative binding site of each class reveals that these sequences dis play common features. We propose a unified consensus sequence for HNF3 -binding sites. The basis of the sequence specificity of the interacti on of HNF3 with DNA is analyzed in light of the recently determined st ructure of an HNF3-DNA complex (Clark et al., Nature 364, 412-420, 199 3). Particularly, our study reveals that the DNA site used for this st ructural analysis is too short to account for all HNF3-DNA interaction s. The better knowledge of the sequence determinant recognized by HNF3 has allowed us to analyze its function in the glucocorticoid response of the rat tyrosine aminotransferase (TAT) gene. This response is med iated through a complex array of neighboring and overlapping transcrip tion factor binding sites. Selective inactivation of the HNF3-binding sites in this glucocorticoid response unit (GRU) allows us to demonstr ate unambiguously that they play a major role in the amplitude of the glucocorticoid response. Furthermore, HNF3 beta overexpression results in a stimulation of the glucocorticoid response that is dependent on the integrity of its binding sites. We also show that the relative lev el of HNF3 determines the extent of the contribution of one of the glu cocorticoid receptor binding sites. Our results indicate that HNF3 acc ounts for most of the liver-specific activity of this GRU.