RESUMO
Orphan nuclear receptors belong to the nuclear receptor superfamily of liganded transcription factors, whose ligands either do not exist or remain to be identified. We report here the cloning and characterization of the chicken orphan nuclear receptor, cTR2 (chicken testicular receptor 2). The cTR2 gene encodes a protein of 569 amino acids which shows approximately 72% overall identity with TR2 (NR2C1) and 95% identity in the DNA-binding domain (DBD). The cTR2 gene is expressed in almost all adult tissues and embryonic stages examined unlike its mammalian relative TR2, which is specifically expressed in testis. Electrophoretic mobility shift assays demonstrate that cTR2 binds the canonical direct repeat DNA recognition sequences spaced by one, four, and five nucleotides (DR1, DR4, and DR5), and in consistence with the results with canonical DNA-binding sequences, cTR2 forms specific DNA-protein complex with chicken phenobarbital response elements containing DR4 motifs. Both in vitro and in vivo interaction studies demonstrate that cTR2 forms homodimer. Moreover, transient transfection studies reveal its capability to transactivate canonical DR1, DR4, and DR5 sequences and the constitutive activity of cTR2 is mapped to the N-terminal region of this orphan receptor. Finally, cTR2 represses transactivation of estrogen receptor in a dose-dependent manner.
