CDNA CLONING OF THE BETA-SUBUNIT OF TELEOST THYROTROPIN

cDNA clones encoding the beta subunit of thyrotropin (thyroid-stimulat ing hormone; TSH) were isolated from a cDNA library made from the pitu itaries of immature rainbow trout and sequenced. The precursor of rain bow trout TSHbeta consists of 147 aa, which can be cleaved into a sign al peptide (20 aa) and a mature protein (127 aa) containing one potent ial N-glycosylation site and 12 cysteine residues. The protein showed highest homology with human TSHbeta (51%) and lesser homology with hum an follitropin (42%), human lutropin (32%), and salmon gonadotropin (3 1-33%) beta subunits. The identification of TSH in addition to two gon adotropins (gonadotropins I and II) in the teleost fish suggests that the divergence of three kinds of glycoprotein hormones from an ancestr al molecule took place earlier than the time of divergence of teleosts from the main line of evolution leading to tetrapods. Northern blot a nalysis showed that the expression of the rainbow trout TSHbeta gene i s specific to the pituitary gland and is significantly higher in immat ure fish than in mature fish, suggesting that TSH plays some role in t he biological processes of immature fish.