Ligand modulates the interaction of thyroid hormone receptor β with the basal transcription machinery

We investigated the molecular mechanisms underlying the transcriptional silencing and the hormone-induced activation of target genes by thyroid hormone receptor β (TR-β). We developed a cell-free transcription system containing HeLa cell nuclear extracts in which unliganded human TR-β represses basal transcription from a promoter bearing thyroid hormone response elements. Binding of hormonal ligand to the receptor reverses this transcriptional silencing. Specific binding of TR-β to the thyroid hormone response element at the target promoter is crucial for silencing. Studies employing TR-β mutants indicate that the silencing activity is located within the C-terminal rather than the N-terminal domain of the receptor. Our studies reveal further that unliganded TR-β inhibits the assembly of a functional transcription preinitiation complex (PIC) at the target promoter. We postulate that interaction with TR-β impairs the function(s) of one or more assembling transcriptional complexes during the multistep assembly of a PIC. Consistent with this hypothesis, we observe that, in the absence of thyroid hormone, TR-β or a heterodimer of TR-β and retinoid-X-receptor undergoes direct protein-protein interactions with the transcription factor IIB-TATA binding protein complex, an early intermediate during PIC assembly. Binding of hormone to TR-β dramatically reduces the interaction between the receptor and the transcription factor IIB-TATA binding protein complex. We propose that the role of ligand is to facilitate the assembly of functional PICs at the target promoter by reducing nonproductive interactions between TR-β and the initiation factors.