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 reverse 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.
Tong
G X
GX
Population Council, New York, New York 10021, USA.
Tanen
M R
MR
Bagchi
M K
MK
D001483:Base Sequence; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006801:Humans; D008024:Ligands; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D009838:Oligodeoxyribonucleotides / Q000737:chemistry / Q000378:metabolism; D011988:Receptors, Thyroid Hormone / Q000502:physiology*; D015398:Signal Transduction; D016385:TATA Box; D035181:TATA-Box Binding Protein; D035581:Transcription Factor TFIIB; D014157:Transcription Factors / Q000378:metabolism*; D014158:Transcription, Genetic; D014284:Triiodothyronine / Q000031:analogs & derivatives / Q000378:metabolism
D004268:DNA-Binding Proteins; D008024:Ligands; D046911:Macromolecular Substances; D009838:Oligodeoxyribonucleotides; D011988:Receptors, Thyroid Hormone; D035181:TATA-Box Binding Protein; D035581:Transcription Factor TFIIB; D014157:Transcription Factors; D014284:Triiodothyronine; C010642:3,3',5-triiodothyroacetic acid