A hypothesis for the transcriptional control of amphibian metamorphosis bythe thyroid hormone receptor

Amphibian metamorphosis is controlled by thyroid hormone and the thyroid ho rmone receptor. The thyroid hormone receptor functions as a master control factor that can both activate and repress genes in controlling the transfor mation of the larval tadpole to the adult frog. We have made use of the mic roinjection of both Xenopus laevis oocytes and embryos to investigate the m olecular mechanisms involved in this regulation of transcription. We define three steps in the regulation of transcription by thyroid hormone receptor : the establishment of a repressive chromatin structure, disruption of chro matin and transcriptional activation. Our results also suggest that there a re four major stages in the developmental control of gene expression during metamorphosis 1) Tissue and stage specific expression during embryonic dev elopment prior to tadpole hatching independent of thyroid hormone receptor, 2) The general repression of transcription of these genes by thyroid hormo ne receptor after tadpole hatching (stage 36), but prior to the production of thyroid hormone, which is first detectible in the circulating plasma aro und stage 55. 3) The activation of transcription dependent on thyroid hormo ne and receptor at metamorphosis (stages 55-66), 4) The acquisition of tiss ue specific and possibly thyroid hormone-independent patterns of gene activ ity.