Identification of a novel DNA binding site for nuclear orphan receptor OR1.

The nuclear orphan receptor OR1 has been shown to bind as a heterodimer with retinoid X receptor (RXR) to direct repeat 4 (DR4) response elements. It remained unclear, however, whether this represents the only or the optimal binding site for this receptor. Therefore, we performed a DNA binding site selection assay that allows the identification of novel DNA binding sites for OR1 in an unbiased manner. While OR1 alone was not able to select a specific sequence from the pool of oligonucleotides, the OR1/RXR heterodimer selected a highly conserved DR1 element, termed DR1s, with two AGGTCA motifs spaced by one adenosine. The functional activity of the consensus binding site was verified in transient transfection assays and corroborated by in vitro studies. Based on the sequence of the consensus DR1s, we located putative natural binding sites in the 5'-promoter flanking regions of the rat S14 gene and the rat cholecystokinin type A receptor gene. Furthermore, we could show that although the OR1/RXR heterodimer has a distinct binding orientation on a DR4 element, it is able to bind in both orientations to the DR1s element. The OR1 paralog LXRalpha does not bind as a heterodimer with RXR to the DR1s element, indicating that these receptors, despite their homology, are involved in the regulation of different sets of genes.