LIGAND MODULATES THE INTERACTION OF THYROID-HORMONE RECEPTOR-BETA 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 beta (TR-beta). We developed a cell-free transcription system containing HeLa cell nuclear extracts in which unliganded human TR-beta 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-beta to the thyroid hormone response element at the target promoter is crucial for silencing. Studies employing TR-beta 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-beta inhibits the assembly of a functional transcription preinitiation complex (PIC) at the target promoter. We postulate that interaction with TR-beta 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-beta or a heterodimer of TR-beta 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-beta 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-beta and the initiation factors.