A new orphan member of the nuclear hormone receptor superfamily closely related to Rev-Erb.

We have isolated complementary DNA clones encoding a novel orphan member of the nuclear receptor superfamily, termed BD73. This protein shows strong amino acid sequence similarity to the previously described Rev-ErbA alpha. Unlike Rev-Erb, in which the opposite strand of the C-terminal coding region encodes the C-terminal portion of a variant thyroid hormone receptor isoform, the opposite strand of the C-terminal coding region of BD73 does not have any extensive open reading frames. BD73 messenger RNA is expressed in a wide variety of tissues and cell lines. In quiescent HepG2 cells, BD73 messenger RNA levels are strongly induced by planar aromatic antioxidants. Like Rev-Erb, BD73 binds as a monomer to a DNA sequence which consists of a specific A/T-rich sequence upstream of the consensus hexameric half-site specified by the P box of the DNA-binding domain. Amino acid sequence comparisons suggest that the A box sequence, which has been suggested to mediate monomer binding by other superfamily members, lies closer to the DNA-binding domain in BD73 and Rev-Erb than in other receptors. Under the conditions examined, neither BD73 nor Rev-Erb activated reporters containing multiple copies of their common binding site. Thus, these two orphans may require an as yet unidentified ligand or other signal for such activation. Together, BD73 and Rev-Erb define a subgroup of orphan receptors that bind as monomers to a half-site flanked by a specific and extended A/T-rich sequence.