Determinants of chromatin disruption and transcriptional regulation instigated by the thyroid hormone receptor: hormone-regulated chromatin disruption is not sufficient for transcriptional activation.

Abstract

Chromatin disruption and transcriptional activation are both thyroid hormone-dependent processes regulated by the heterodimer of thyroid hormone receptor and 9-cis retinoic acid receptor (TR-RXR). In the absence of hormone, TR-RXR binds to nucleosomal DNA, locally disrupts histone-DNA contacts and generates a DNase I-hypersensitive site. Chromatin-bound unliganded TR-RXR silences transcription of the Xenopus TRbetaA gene within a canonical nucleosomal array. On addition of hormone, the receptor directs the extensive further disruption of chromatin structure over several hundred base pairs of DNA and activates transcription. We define a domain of the TR protein necessary for directing this extensive hormone-dependent chromatin disruption. Particular TR-RXR heterodimers containing mutations in this domain are able to bind both hormone and their thyroid hormone receptor recognition element (TRE) within chromatin, yet are unable to direct the extensive hormone-dependent disruption of chromatin or to activate transcription. We distinguish the hormone-dependent disruption of chromatin and transcriptional activation as independently regulated events through the mutagenesis of basal promoter elements and by altering the position and number of TREs within the TRbetaA promoter. Chromatin disruption alone on a minichromosome is shown to be insufficient for transcriptional activation of the TRbetaA gene.