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 ␣-subunit (TSH␣) 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 nuclearinjected DNA to analyze the chromatin architecture of the TSH␣ promoter and how this changes upon TR-mediated regulation. Interestingly, in the oocyte, the TSH␣ 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 TSH␣ 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 TRA promoter, we found that the effects of these mutations on transactivation and chromatin remodeling were significantly more severe on the TSH␣ promoter.The molecular mechanism of nuclear hormone receptor-mediated gene regulation and the importance of chromatin structure to this process have been intensively investigated over the past decade. Although the packaging of DNA into dense chromatin is a barrier to transcription factor access, multiple mechanisms exist to overcome this obstacle and thereby facilitate regulation of gene expression (1-3). Regulation of the acetylation state of core nucleosomal histone proteins influences their interaction with DNA and, subsequently, the nucleosomal packing density and transcription factor accessibility to chromatin. Transcriptional repression by DNA-bound thyroid hormone receptor (TR) 1 in the absence of hormone (T 3 ) involves the recruitment of histone deacetylase-containing complexes that facilitate the formation of repressive chromatin structure. The addition of T 3 causes the release of the deacetylase complexes and stimulates transcriptional activation by the recruitment of coactivators that include acetyltransferase components (1-3). The acetyltransferases and deacetylases are numerous, but occur in discrete subcomplexes that may exhibit cell type and promoter context dependence (4). The role of ATP-dependent mechanisms such as SWI/SNF, Mi2/NURD, and ISWI (5) in nuclear receptor-mediated regulation has also been demonstrated. Studies using the glucocorticoid receptor on the mouse mammary tumor virus promoter, which has been shown to have positioned nucleosomes (6), have demonstrated a requirement for SWI-SNF complexes and their ligand-induced targeting to the promoter to activate gene expression (7-11). More recently, it has been demonstrated tha...