Thyroid hormone-induced gene expression program for amphibian tail resorption.

The changes in gene expression leading to tail resorption that are initiated by thyroid hormone (TH) were studied in Xenopus laevis. Four of the less than 10 genes that are down-regulated during this period have been isolated; their mRNAs decay with identical kinetics. Twenty of the approximately 35 genes that are up-regulated in the first 48 h have been isolated. The up-regulated genes fall into two kinetic patterns. After a lag of several hours, the direct response genes (including thyroid hormone receptor beta) increase their mRNA level steadily for 24-48 h. The delayed genes respond mainly in the second 24 h after TH addition. The importance of these genes for tail resorption is supported by the fact that they are all regulated developmentally during normal metamorphosis in tail and respond to hormone induction when the tail becomes competent to respond to TH. The relatively simple gene expression program leading to tail resorption is contrasted with the complex and multiple periods of gene expression during limb development. The gene expression screen defines the tail resorption program and has isolated the majority of TH-regulated genes.