Chromatin remodeling by the thyroid hormone receptor in regulation of the thyroid-stimulating hormone alpha-subunit promoter

The chromatin architecture of a promoter is an important determinant of its transcriptional response. For most target genes, the thyroid hormone recep tor (TR) activates gene expression in response to thyroid hormone (T-3). In contrast, the thyroid-stimulating hormone a-subunit (TSH alpha) gene promo ter is down-regulated by TR in the presence of T-3. Here we utilize the cap acity for the Xenopus oocyte to chromatinize exogenous nuclear-injected DNA to analyze the chromatin architecture of the TSHa promoter and how this ch anges upon TR-mediated regulation. Interestingly, in the oocyte, the TSH al pha promoter was positively regulated by T-3. In the inactive state, the pr omoter contained six loosely positioned nucleosomes. The addition of TR/ret inoid X receptor together had no effect on the chromatin structure, but the inclusion of T-3 induced strong positioning of a dinucleosome in the TSHa proximal promoter that was bordered by regions that were hypersensitive to cleavage by methidiumpropyl EDTA. We identified a novel thyroid response el ement that coincided with the proximal hypersensitive region. Furthermore, we examined the consequences of mutations in TR that impaired coactivator r ecruitment. In a comparison with the Xenopus TR betaA promoter, we found th at the effects of these mutations on transactivation and chromatin remodeli ng were significantly more severe on the TSH alpha promoter.