Wong J, Shi Y B, Wolffe A P
Unit on Molecular Morphogenesis, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-2710, USA.
Genes Dev. 1995 Nov 1;9(21):2696-711. doi: 10.1101/gad.9.21.2696.
We have assembled the thyroid hormone-inducible promoter of the Xenopus thyroid hormone receptor (TR)beta A gene into chromatin using replication-coupled and -independent assembly pathways in vivo. We establish that heterodimers of TR and 9-cis retinoic acid receptors (RXR) can bind to their recognition sites within chromatin both in vivo and in vitro and alternately repress or activate transcription dependent on the absence or presence of thyroid hormone. Maximal transcriptional repression requires the presence of unliganded TR/RXR heterodimers during replication-coupled chromatin assembly. We demonstrate an increase in transcription directed by the TR beta A promoter of over two orders of magnitude in vivo, following the addition of thyroid hormone. This increase in transcription involves the relief of the repressed state that is established by the unliganded TR/RXR heterodimer during replication-coupled chromatin assembly. The association of thyroid hormone with the chromatin-bound TR/RXR heterodimer leads to the disruption of local chromatin structure in a transcription-independent process. Thus, chromatin structure has multiple roles in the regulation of TR beta A gene expression in vivo: The TR/RXR heterodimer recognizes the response element within chromatin, TR/RXR makes use of the chromatin assembly process to silence transcription more efficiently, and TR/RXR directs the disruption of local chromatin structure in response to thyroid hormone.
我们利用体内复制偶联和非复制偶联的组装途径，将非洲爪蟾甲状腺激素受体（TR）βA基因的甲状腺激素诱导型启动子组装到染色质中。我们证实，TR和9-顺式维甲酸受体（RXR）的异二聚体在体内和体外均可与染色质内的识别位点结合，并根据甲状腺激素的缺失或存在交替抑制或激活转录。最大转录抑制需要在复制偶联染色质组装过程中存在未结合配体的TR/RXR异二聚体。我们证明，在添加甲状腺激素后，TRβA启动子在体内指导的转录增加了两个数量级以上。这种转录增加涉及到在复制偶联染色质组装过程中由未结合配体的TR/RXR异二聚体建立的抑制状态的解除。甲状腺激素与染色质结合的TR/RXR异二聚体的结合导致在一个与转录无关的过程中局部染色质结构的破坏。因此，染色质结构在体内TRβA基因表达的调控中具有多种作用：TR/RXR异二聚体识别染色质内的反应元件，TR/RXR利用染色质组装过程更有效地使转录沉默，并且TR/RXR响应甲状腺激素指导局部染色质结构的破坏。
