Transcription factor
A transcription factor is a protein of molecular biology, which is the initiation of the RNA polymerase in transcription of importance. It may also be involved in the regulation of the elongation, and termination. Transcription factors can bind to DNA and activate or repress the promoter. There are also transcription factors which do not bind directly to DNA, but for example, other DNA-binding proteins. We distinguish general ( basal ) and gene-specific transcription factors.
General transcription factors
General transcription factors as sub-units of required for transcription of protein complexes assume different roles and bind either directly to the DNA, for example, on general subjects such as promoter elements (such as the TATA box ) to the RNA polymerase or other proteins of the pre-initiation complex.
This " basal " transcription factors always appear as complexes with other proteins. By binding to the DNA, they provide a " platform " for the RNA polymerase forth the polymerase binds to the platform, and transcription is initiated. Transcription factors are diverse in their structure and have different tasks. Some have binding sites for important regulators (eg for anti- terminators), others have protein kinase functions or show helicase activity (eg TAF250 - TFIID ). They are ubiquitous, that is equally present in all cells of an organism, and have at the specific gene regulation usually no share.
Specific transcription factors
Specific transcription factors mediate the polymerase, which gene to be activated. They are therefore only present in the cells in which the gene which they regulate, activated ( or repressed, depending on the well ) is to be. The DNA regions to which they bind, have a specific sequences ( cis-elements such as enhancers or silencers) which is recognized and bound by the transcription factor. Specific transcription factors are often activated by protein kinases. Activation is the end of a long signal transmission chain, which is triggered by a receptor.
Activators function according to two principles:
Repressors work on a reverse principle, histone deacetylases lead to denser packaging of DNA, and by blocking polymerase binding sites follows the lowering of the binding affinity. A complex regulation comes about through the network-like interplay of many transcription factors.
The activity of transcription factors is defined by their regulation.
Is regulated by
- Binding of ligand ( steroid hormones)
- Phosphorylation ( kinase activity )
- Maturation (domains anchored in the membrane receptors)
- Concentration ( activate low concentrations, inhibit the reaction high )
- DNA-binding
- Binding of co-factors ( binding of co-factors inhibit transcription )
- Formation of heterodimers ( only complexes activate promoters)
- Blockade of the DNA binding site ( the ligand -bound TF prevents transcription)
- Repression of the DNA - binding site ( repressors prevent binding of TF)
- Conditioning (sequence of interactions )
Types of transcription factors
- Helix - turn-helix transcription factors
- Homeodomain transcription factors
- Helix-loop -helix transcription factors
- Zinc - finger transcription factors
- Leucine zipper transcription factors