Genomic organization and alternative promoter usage of the two thyroid hormone receptor beta genes in Xenopus laevis.

Each of the two Xenopus laevis thyroid hormone receptor beta genes is at least 70 kilobases in length with similar intron-exon organization. There are up to eight alternatively spliced exons in the 5'-untranslated region. Excluding the extreme amino terminus, each receptor is encoded by six exons spanning about 6 kilobases of the genome, in which each of the two zinc fingers that comprise the DNA-binding domain is encoded by a separate exon and the hormone-binding domain is split into three exons. The last exon of the coding region also contains at least 600 base pairs of the 3'-untranslated region, which is about 8 kilobases. Each of the receptor genes has two promoters and just one of them is up-regulated in tadpoles by the administration of thyroid hormone.