THE TYPE-III 5-DEIODINASE IN RANA-CATESBEIANA TADPOLES IS ENCODED BY A THYROID HORMONE-RESPONSIVE GENE

We have recently reported that the Xenopus laevis complementary DNA (cDNA), XL-15, encodes a selenoprotein that is a 5-deiodinase (5D). XL-15 represents a gene that is up-regulated by thyroid hormone in this species. We now report the isolation from a Rana catesbeiana (RC) cDNA library of a cDNA for the RC 5D (RC5D). RC5D a 1534-base pair cDNA that exhibits 78% identity to XL-15 in the coding region, including the TGA codon, which in XL-15 encodes selenocysteine, and a putative selenocysteine insertion sequence (SECIS) located in the 3'-untranslated region. Transcripts of RC5D, synthesized in vitro, induce 5D activity after their injection into Xenopus laevis oocytes. RC5D hybridizes to a 2.2-kilobase messenger RNA (mRNA) species in RC tissues. The expression of RC5D was studied in 10 tissues from untreated and T-3-treated premetamorphic tadpoles. RC5D mRNA transcripts and 5D activity were detected in most tissues examined, and the levels of both were greatly increased in T-3-stimulated tadpoles. Furthermore, the levels of RC5D mRNA transcripts correlated closely with 5D, but not with 5'-deiodinase (5'D), activity. 5'D was not enhanced by a 4-day exposure of the tadpoles to T-3. The up-regulation of RC5D gene expression by T-3 was demonstrable in tadpoles by stage V, but was found to be transient. The levels of RC5D transcripts and 5D activity were highest after 2-3 days and 5 days of T-3 exposure, respectively, but had fallen by 10 days to levels comparable to those in unexposed tadpoles. It is concluded that RC5D encodes a selenoprotein that is a 5D, and it represents a gene that is up-regulated by T-3. On the basis of the findings presented herein, it is suggested that the 5D system plays a major role in regulating intracellular T-3 levels in developing tadpoles.