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For example, the E. coli tryptophan repressor (TrpR) is only able to bind to DNA and repress transcription of the trp operon when its corepressor tryptophan is bound to it. TrpR in the absence of tryptophan is known as an aporepressor and is inactive in repressing gene transcription. [ 2 ]
A co-repressor is a molecule that can bind to the repressor and make it bind to the operator tightly, which decreases transcription. A repressor that binds with a co-repressor is termed an aporepressor or inactive repressor. One type of aporepressor is the trp repressor, an important metabolic protein in bacteria.
In addition, compressors bind preferentially to the apo (ligand free) form of the nuclear receptor (or possibly antagonist bound receptor). CtBP 602618 SIN3A (associates with class II histone deacetylases) LCoR (ligand-dependent corepressor) Nuclear receptor CO-Repressor (NCOR) NCOR1
If the repressor has a higher affinity for its motif than the activator, transcription would be effectively blocked in the presence of the repressor. Tight regulatory control is achieved by the highly dynamic nature of transcription factors. Again, many different mechanisms exist to control whether a transcription factor is active.
The nuclear receptor co-repressor 1 also known as thyroid-hormone- and retinoic-acid-receptor-associated co-repressor 1 (TRAC-1) is a protein that in humans is encoded by the NCOR1 gene. [ 5 ] [ 6 ] NCOR1 is a transcriptional coregulatory protein which contains several nuclear receptor interacting domains.
In the reverse reaction, histone deacetylase (HDAC) removes the acetyl group from the histone tails and binds it to coenzyme A to form acetyl-CoA. Some coactivators indirectly regulate gene expression by binding to an activator and inducing a conformational change that then allows the activator to bind to the DNA enhancer or promoter sequence.
The nuclear receptor co-repressor 2 is a transcriptional coregulatory protein that contains several nuclear receptor-interacting domains. In addition, NCOR2 appears to recruit histone deacetylases to DNA promoter regions. Hence NCOR2 assists nuclear receptors in the down regulation of target gene expression.
The ability of nuclear receptors to alternate between activation and repression in response to specific molecular cues, is now known to be attributable in large part to a diverse group of cellular factors, collectively termed coregulators and including coactivators and corepressors.