XENOPUS-LAEVIS ALPHA-THYROID AND BETA-THYROID HORMONE RECEPTORS

The Xenopus laevis genome encodes two genes for the α(TRα) and two genes for the β(TRβ) thyroid hormone receptors. The two TRα genes closely resemble their rat, human, and chicken counterparts. No alternatively spliced TRα cDNA clones were found in the 5' untranslated region (5' UTR). In contrast, complex alternative splicing of TRβ mRNA occurs within the 5' UTR as well as possible alternative transcriptional start sites. As many as eight exons encoding mainly the 5' UTR are alternatively spliced, giving rise to at least two amino termini for each of the two TRβ proteins. The 5' UTR of transcripts from both TRα and TRβ genes contain multiple AUG sequences with short open reading frames suggesting translational control mechanisms might play a role in expression of TR genes.