The earliest changes in gene expression in tadpole intestine induced by thyroid hormone

Genes induced by thyroid hormone (TH) to change their expression in the Xenopus laevis gastrointestinal (GI) tract have been isolated using a subtractive hybridization method. An exhaustive search for down-regulated genes identified a single gene. Thirty-two different cDNA fragments derived from the up-regulated mRNA of tadpole intestine 18 h after addition of TH were cloned. They map to no more than 22 distinct genes. The isolation of multiple cDNA fragments derived from a single mRNA indicates that the complexity of up-regulated genes is limited. Both ubiquitous and intestine-specific up-regulated genes were found in this screen. The majority of these genes respond directly to TH induction as judged by the resistance of up-regulation to inhibitors of protein synthesis. The biological significance of these genes is supported by their dramatic regulation in the GI tract during spontaneous metamorphosis.