ISOLATION AND CHARACTERIZATION OF A CDNA-ENCODING A CHICKEN-BETA THYROID-HORMONE RECEPTOR

We have isolated and characterized a cDNA encoding a chicken beta-homolog of c-erbA, or thyroid hormone receptor (TR). Chicken liver cDNA libraries were screened with a rat TR-beta-1 cDNA probe, and several cDNA inserts were isolated and characterized. The sequence of one cDNA predicts a 369-amino-acid open reading frame (ORF), with a protein sequence that possesses 96% identity with that of rat TR-beta-1, but only 88% identity with chicken TR-alpha. These data indicate that the cDNA likely encodes a beta-form of TR that has the expected putative DNA and T3 binding domains. The chicken TR-beta (chTR-beta) in vitro translated protein binds T3 with high affinity, and binds both the thyroid hormone response element (TRE) from the rat growth hormone gene and the Xenopus vitellogenin A2 gene estrogen response element (ERE), similarly to that of the rat TR-beta-1. Northern blot analysis revealed the expression of a 7.0-kb RNA in several tissues including cerebellum, pituitary, kidney, and liver. This chicken liver TR-beta cDNA sequence varies in both the 5' and 3' untranslated regions from the chicken kidney TR-beta cDNA sequence recently reported (Forrest et al., 1990). The 5' untranslated cDNA sequence divergence occurs near a potential splice site junction of the human TR-beta gene, suggesting that this chicken liver cDNA may represent an alternatively spliced RNA product of the chicken TR-beta gene.