TRANSCRIPTIONAL REPRESSION OF XENOPUS TR-BETA GENE IS MEDIATED BY A THYROID-HORMONE RESPONSE ELEMENT LOCATED NEAR THE START SITE

We report here the detailed analysis of the promoter of a thyroid horm one receptor (TR) gene that is regulated by the hormone itself. The re ceptor gene, TR beta A, is one of the two TR beta genes in Xenopus lae vis. It has two transcription start sites. The mRNAs derived from one of them are up-regulated by thyroid hormone, whereas those derived fro m the other are independent of the hormone. We have characterized the hormone inducible promoter using a transient transfection assay in a X enopus tissue culture cell line (A6). Deletion and mutational analysis identifies the first amphibian thyroid hormone response element (TRE) . This TRE consists of near perfect direct repeats of AGGTCA with a 4- base pair spacing similar to mammalian TREs. The TRE forms specific co mplexes with extracts of A6 cells that have similar sequence specifici ties as those found for the complexes between mammalian TRs and TREs. However, unlike TREs found in other thyroid hormone-inducible promoter s, this TRE is located at the putative transcription start site and me diates transcriptional repression by unliganded TRs. The addition of t hyroid hormone at physiological concentrations overcomes the repressio n and induces further transcriptional activation at higher concentrati ons. These results suggest a potential mechanism for the regulation of amphibian metamorphosis, a process that is entirely controlled by thy roid hormone.