ENHANCEMENT OF THYROID-HORMONE RECEPTOR ISOFORM SPECIFICITY BY INSERTION OF A DISTANT HALF-SITE INTO A THYROID-HORMONE RESPONSE ELEMENT

The two isoforms of thyroid hormone receptor (TR), alpha and beta, are highly homologous, except in the amino-terminal domain. Specific phys iological roles for the receptor isoforms have not yet been defined. I n transient transfection assays, TR alpha is twice as potent a thyroid hormone (T-3)-dependent transactivator as TR beta on a number of thyr oid hormone response elements (TREs). Using chimeras of TR alpha and - beta, we have determined that the higher transactivation by TR alpha r equires the entire ligand-binding domain. The amino-terminal and DNA-b inding domains of the two isoforms are interchangeable. These studies were facilitated by the use of a synthetic TRE composed of a direct re peat separated by 4 bp which also included a third half-site 19 bp 3' to this on the opposite strand. In the presence of T-3, this TRE confe rs a 5-fold higher response to TR alpha than to TR beta, but there is no difference in expression without T-3. Functional studies indicate t hat all three half-sites are needed for the increased responsiveness t o TR alpha, but gel shift analyses show no striking differences in the ratios of TR alpha to TR beta binding compared to other wild-type TRE s. These results suggest that important functional differences are pre sent in the ligand-binding domain of TR alpha and -beta despite their high homology.