Abstract
The thyroid hormone receptor (TR) beta genes in Xenopus laevis are regulated by thyroid hormone in all organs of an animal during metamorphosis. This autoregulation appears to be critical for systematic transformations of different organs as a tadpole is transformed into a frog. To understand this autoregulation, we have previously identified a thyroid hormone response element in the hormone-dependent promoter of the X. laevis TRbetaA gene. We report here the detailed characterization of the promoter. We have now mapped the transcription start site and demonstrated the existence of an initiator element at the start site critical for promoter function. More important, our deletion and mutational experiments revealed a novel upstream DNA element that is located 125 base pairs upstream of the start site and that is essential for active transcription from the promoter. Promoter reconstitution experiments showed that this novel element does not function as an enhancer, but acts as a core promoter element, which, together with the initiator, directs accurate transcription from the promoter. Finally, we provide evidence for the existence of a protein(s) that specifically recognizes this element. Our studies thus demonstrate that the TRbetaA promoter has a unique organization consisting of an initiator and a novel upstream promoter element. Such an organization may be important for the ubiquitous but tissue-dependent temporal regulation of the gene by thyroid hormone during amphibian metamorphosis.
Highlights
From the §Laboratory of Molecular Embryology, NICHD, National Institutes of Health, Bethesda, Maryland 20892-5431 and the ‡Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030
Thyroid hormone receptors mediate the biological effects of thyroid hormone
Perhaps the most dramatic T3-dependent process is amphibian metamorphosis, during which an aquatic tadpole is transformed into a terrestrial frog
Summary
From the §Laboratory of Molecular Embryology, NICHD, National Institutes of Health, Bethesda, Maryland 20892-5431 and the ‡Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030. The thyroid hormone receptor (TR)  genes in Xenopus laevis are regulated by thyroid hormone in all organs of an animal during metamorphosis This autoregulation appears to be critical for systematic transformations of different organs as a tadpole is transformed into a frog. To understand this autoregulation, we have previously identified a thyroid hormone response element in the hormone-dependent promoter of the X. laevis TRA gene. Our studies demonstrate that the TRA promoter has a unique organization consisting of an initiator and a novel upstream promoter element Such an organization may be important for the ubiquitous but tissue-dependent temporal regulation of the gene by thyroid hormone during amphibian metamorphosis. This specific positioning of the basal transcription machinery at a promoter by a TATA and/or initiator element allows basal transcription, which can be enhanced by transcription activators
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