IDENTIFICATION OF A NOVEL DOMAIN IN THE ARYL-HYDROCARBON RECEPTOR REQUIRED FOR DNA-BINDING

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcriptio n factor that binds DNA in the form of a heterodimer with the AHR nucl ear translocator protein (ARNT). Both proteins possess basic helix-loo p-helix motifs. ARNT binds to the side of the xenobiotic responsive el ement (XRE) that resembles an E-box (the sequence recognized by the ma jority of other basic helix-loop-helix proteins), whereas ABR binds to the side of the XRE that does not conform to the E-box sequence, The basic region of ARNT closely resembles those of other E-box-binding pr oteins, whereas the ''nominal basic region'' of AHR (amino acids 27-39 ), although required for XRE binding, deviates from this consensus. By extensive mutational analysis it is shown here that an additional blo ck of amino acids of AHR (from tyrosine 9 to lysine 20) that contains a highly basic segment is required for XRE binding and transcriptional activation, Deletion of the first nine amino acids negates XRE bindin g. Substitution of either tyrosine 9 or arginine 14 with alanine elimi nates XRE binding, whereas alanine substitutions at certain other site s within the block reduce but do not eliminate binding. The reported a bsence of the first nine amino acids in the purified protein may there fore be artifactual. These results suggest that the amino acids of AHR involved in binding to the XRE constitute a novel DNA-binding domain, comprising amino acids located within and amino-terminal to the nomin al basic region.