Abstract：Thyroid hormone plays an important role in the metamorphosis of flatfish and its biological effects are mediated by thyroid hormone receptors (TRs). In this study, the full length of TRβ had been sequenced in Cynoglossus semilaevis and its biological activities were analyzed. The nucleotide sequence of TRβ was 2 345 bp, including 5' terminal UTR of 454 bp, 3' terminal UTR of 703 bp and complete open reading frame of 1 188 bp which encoded 396 amino acid residues. Compared to the amino acid sequences of TRs in different species, we found a insertion of 9 amino acids in the hinge region which was unique to TRβ of teleosts. We found that the expression level of TRβ in liver was significantly higher than that in other tissues using Real-time PCR. We speculated that TRβ had diverse functions in tissues of adult Cynoglossus semilaevis, especially might play an important role in liver metabolism. We also found that TRβ gene transcript increased rapidly in the beginning of metamorphosis, reached peak in metamorphic climax. Moreover, larvae were exposed to the thyroid hormone(T4) and goitrogen thiourea(TU) and the expression levels of TRβ were detected. The expression levels of TRβ in control and T4-treated group rapidly increased as metamorphosis began. On the contrary, the expression levels of TRβ in TU-treated group still remained in a low level. In conclusion, TRβ gene played an important role in the metamorphosis of Cynoglossus semilaevis. The research provides a theoretical basis and reference for molecular mechanism research of metamorphosis of Cynoglossus semilaevis.
