Wong J, Shi Y B, Wolffe A P
Laboratory of Molecular Embryology, National Institute of Child Health and Human Development, NIH, Bethesda, MD 20892-5431, USA.
EMBO J. 1997 Jun 2;16(11):3158-71. doi: 10.1093/emboj/16.11.3158.
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.
染色质破坏和转录激活都是甲状腺激素依赖性过程，由甲状腺激素受体和9-顺式视黄酸受体（TR-RXR）的异二聚体调节。在没有激素的情况下，TR-RXR与核小体DNA结合，局部破坏组蛋白-DNA接触并产生一个DNase I超敏位点。结合在染色质上的未结合配体的TR-RXR使非洲爪蟾TRβA基因在典型核小体阵列中的转录沉默。加入激素后，受体引导在数百个碱基对的DNA上进一步广泛破坏染色质结构并激活转录。我们定义了TR蛋白中一个指导这种广泛的激素依赖性染色质破坏所必需的结构域。在该结构域中含有突变的特定TR-RXR异二聚体能够在染色质内结合激素及其甲状腺激素受体识别元件（TRE），但无法指导广泛的激素依赖性染色质破坏或激活转录。通过对基础启动子元件进行诱变以及改变TRβA启动子内TRE的位置和数量，我们将染色质的激素依赖性破坏和转录激活区分为独立调节的事件。结果表明，仅在微型染色体上的染色质破坏不足以激活TRβA基因的转录。
