Protein-DNA interactions at a dioxin-responsive enhancer. Evidence that the transformed Ah receptor is heteromeric

The Ah receptor in rat hepatic cytosol was transformed to a DNA-binding form by incubation in vitro with the ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin. The transformed receptor was covalently cross-linked to a bromodeoxyuridine-substituted DNA recognition motif by exposure to ultraviolet irradiation. Analyses of the cross-linked protein-DNA complexes by gel electrophoresis and autoradiography imply that the DNA-binding form of the liganded Ah receptor is composed of two protein components, whose molecular masses are about 110 and 100 kDa. Protease digestion studies suggest that the two components have different primary structures. Photoaffinity labeling studies imply that the smaller protein is the ligand-binding component of the receptor. These findings constitute biochemical evidence that the DNA-binding form of the Ah receptor is a heterodimer.