FIG. 4.

Transgenic expression of dnTR inhibits T₃-induced changes in the expression of both ubiquitous and tissue-specific direct or indirect-T₃-response genes. Wild-type (lanes 1 and 2) or transgenic (lane 3) tadpoles at stage 54 were treated with 0 or 5 nM T₃ for 3 days, a treatment known to be sufficient to induce changes in the expression of both direct- and late, indirect-T₃-response genes (53). RT-PCR analysis was carried out as in Fig. 3. TRβ and TH/bZip are induced by T₃ ubiquitously, whereas ST3 is a fibroblast-specific direct-response gene. As expected, all three were induced both in the tails and intestines of wild-type tadpoles (compare lanes 1 and 2) but not in the transgenic tadpoles expressing dnTR (lane 3). In addition, the intestine-specific T₃-induced direct-response gene Sonic hedgehog (xhh), the late upregulated gene for BMP-4, and the late downregulated gene for IFABP were regulated as expected in the wild-type animals (compare lanes 1 and 2). Again, T₃ regulation of all of these genes was prevented or greatly inhibited in the transgenic tadpoles (lane 3). There are some variations in the degrees of inhibition on T₃-induced changes in gene expression among different genes and/or in different organs. This is likely due to differences between the tail and intestine and/or among the promoters of different genes.