Several genes have been identified that are activated or repressed by thyroid hormone in tadpole tissues during metamorphosis of the frog Xenopus laevis. One rapidly and strongly induced gene encodes the Xenopus homolog of basic transcription element-binding protein 1 (xBTEB1), an SP1-related transcription factor. xBTEB1 has similar DNA-binding activity and transcriptional activation properties as mammalian BTEB1. The thyroid hormone-dependent regulation of xBTEB1 was investigated using a modified electrophoretic mobility shift assay to scan genomic DNA for receptor-binding sites. Due to the tetraploid X. laevis genome, xBTEB1 is duplicated, and thyroid hormone regulates both copies. A consensus thyroid hormone response element (TRE) lies far up-stream of the transcriptional start site of both genes. The TRE is nested within a 200-bp region of high sequence conservation between these two genes that duplicated millions of years ago. The TRE acts as a strong response element in transfection assays using a heterologous promoter or its native context. Thus, one of the earliest thyroid hormone-induced genes in tadpoles is a transcription factor regulated through an evolutionarily conserved TRE. xBTEB1 is predicted to play an important role in downstream gene regulation leading to the growth and remodeling of tissues at metamorphosis.
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