Transcriptional repression of Xenopus TRβ 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 hormone receptor (TR) gene that is regulated by the hormone itself. The receptor gene, TRβA, is one of the two TRβ genes in Xenopus laevis. It has two transcription start sites. The mRNAs derived from one of them are up- regulated by thyroid hormone, whereas those derived from the other are independent of the hormone. We have characterized the hormone-inducible promoter using a transient transfection assay in a Xenopus 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 complexes with extracts of A6 cells that have similar sequence specificities as those found for the complexes between mammalian TRs and TREs. However, unlike TREs found in other thyroid hormone-inducible promoters, this TRE is located at the putative transcription start site and mediates transcriptional repression by unliganded TRs. The addition of thyroid hormone at physiological concentrations overcomes the repression and induces further transcriptional activation at higher concentrations. These results suggest a potential mechanism for the regulation of amphibian metamorphosis, a process that is entirely controlled by thyroid hormone.