THE THYROID HORMONE-INDUCED TAIL RESORPTION PROGRAM DURING XENOPUS-LAEVIS METAMORPHOSIS

Genes that are up- and down-regulated by thyroid hormone in the tail r esorption program of Xenopus laevis have been isolated by a gene expre ssion screen, sequenced, and identified in the GenBank data base, The entire program is estimated to consist of fewer than 35 up-regulated a nd fewer than 10 down-regulated genes; 17 and 4 of them, respectively, have been isolated and characterized, Up-regulated genes whose functi on can be predicted on the basis of their sequence include four transc ription factors (including one of the thyroid hormone receptors), an e xtracellular matrix component (fibronectin) and membrane receptor (int egrin), four proteinases, a deiodinase that degrades thyroid hormone, and a protein that binds the hypothalamic corticotropin-releasing fact or, which has been Implicated in controlling thyroid hormone synthesis in Xenopus tadpoles, All four down-regulated genes encode extracellul ar proteins that are expressed in tadpole epidermis, This survey of th e program provides insights Into the biology of metamorphosis.