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

The chromatin architecture of a promoter is an important determinant of its transcriptional response. For most target genes, the thyroid hormone receptor (TR) activates gene expression in response to thyroid hormone (T-3). In contrast, the thyroid-stimulating hormone a-subunit (TSH alpha) gene promoter is down-regulated by TR in the presence of T-3. Here we utilize the capacity for the Xenopus oocyte to chromatinize exogenous nuclear-injected DNA to analyze the chromatin architecture of the TSHa promoter and how this changes upon TR-mediated regulation. Interestingly, in the oocyte, the TSH alpha promoter was positively regulated by T-3. In the inactive state, the promoter contained six loosely positioned nucleosomes. The addition of TR/retinoid 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 element that coincided with the proximal hypersensitive region. Furthermore, we examined the consequences of mutations in TR that impaired coactivator recruitment. In a comparison with the Xenopus TR betaA promoter, we found that the effects of these mutations on transactivation and chromatin remodeling were significantly more severe on the TSH alpha promoter.