Characterization of a Sea Bream (Sparus aurata) Thyroid Hormone Receptor-β Clone Expressed during Embryonic and Larval Development

A clone encoding thyroid hormone receptor-β (TR-β) was isolated from a sea bream (Sparus aurata) ovary cDNA library. Sea bream (sb)TR-β is closely related to its counterparts from other vertebrates and, like them, preferentially binds T3 rather than T4. However, the putative sbTR-β protein contains a nine-amino-acid insert that is also present in the corresponding proteins from flounder and salmon but absent in TR-βs from zebrafish and terrestrial vertebrates. Semiquantitative RT–PCR analysis showed that sbTR-β transcripts begin to accumulate during gastrulation and increase markedly in quantity up to the period around hatch (ca. 40 h postfertilization) before declining slightly. In adult tissues, TR-β mRNA was present in approximately equal quantities in heart, intestine, brain, kidney, skeletal muscle, liver, and gill. The significance of the relatively strong expression of TR-β during sea bream embryogenesis is discussed.