Abstract
ROR alpha 1 and ROR alpha 2 are two isoforms of a novel member of the steroid-thyroid-retinoid receptor superfamily and are considered orphan receptors since their cognate ligand has yet to be identified. These putative receptors have previously been shown to bind as monomers to a DNA recognition sequence composed of two distinct moieties, a 3' nuclear receptor core half-site AGGTCA preceded by a 5' AT-rich sequence. Recognition of this bipartite hormone response element (RORE) requires both the zinc-binding motifs and a group of amino acid residues located at the carboxy-terminal end of the DNA-binding domain (DBD) which is referred to here as the carboxy-terminal extension. In this report, we show that binding of ROR alpha 1 and ROR alpha 2 to the RORE induces a large DNA bend of approximately 130 degrees which may be important for receptor function. The overall direction of the DNA bend is towards the major groove at the center of the 3' AGGTCA half-site. The presence of the nonconserved hinge region which is located between the DBD and the putative ligand-binding domain (LBD) or ROR alpha is required for maximal DNA bending. Deletion of a large portion of the amino-terminal domain (NTD) of the ROR alpha protein does not alter the DNA bend angle but shifts the DNA bend center 5' relative to the bend induced by intact ROR alpha. Methylation interference studies using the NTD-deleted ROR alpha 1 mutant indicate that some DNA contacts in the 5' AT-rich half of the RORE are also shifted 5', while those in the 3' AGGTCA half-site are unaffected. These results are consistent with a model in which the ROR alpha NTD and the nonconserved hinge region orient the zinc-binding motifs and the carboxy-terminal extension of the ROR alpha DBD relative to each other to achieve proper interactions with the two halves of its recognition site. Transactivation studies suggest that both protein-induced DNA bending and protein-protein interactions are important for receptor function.