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In genetics, an enhancer is a short (50–1500 bp) region of DNA that can be bound by proteins to increase the likelihood that transcription of a particular gene will occur. [1] [2] These proteins are usually referred to as transcription factors. Enhancers are cis-acting. They can be located up to 1 Mbp (1,000,000 bp) away from the gene ...
The active enhancer is transcribed on each strand of DNA in opposite directions by bound RNAP IIs. Mediator (a complex consisting of about 26 proteins in an interacting structure) communicates regulatory signals from the enhancer DNA-bound transcription factors to the promoter. NELF, in complex with DSIF and RNAP II, can pause transcription.
Transcription factors can be divided in two main categories: activators and repressors. While activators can interact directly or indirectly with the core machinery of transcription through enhancer binding, repressors predominantly recruit co-repressor complexes leading to transcriptional repression by chromatin condensation of enhancer regions.
An enhancer localized in a DNA region distant from the promoter of a gene can have a very large effect on gene transcription, with some genes undergoing up to 100-fold increased transcription due to an activated enhancer. [10] Enhancers are regions of the genome that are major gene-regulatory elements.
An inactive enhancer may be bound by an inactive transcription factor. Phosphorylation of the transcription factor may activate it and that activated transcription factor may then activate the enhancer to which it is bound (see small red star representing phosphorylation of a transcription factor bound to an enhancer in the illustration). [12]
Enhancers are bound by transcription activator proteins and transcriptional regulation is typically controlled by more than one activator. Enhanceosomes are formed in special cases when these activators cooperatively bind together along the enhancer sequence to create a distinct three-dimensional structure.
Transcription preinitiation complex, represented by the central cluster of proteins, causes RNA polymerase to bind to target DNA site. The PIC is able to bind both the promoter sequence near the gene to be transcribed and an enhancer sequence in a different part of the genome, allowing enhancer sequences to regulate a gene distant from it.
A silencer is a sequence-specific element that induces a negative effect on the transcription of its particular gene. There are many positions in which a silencer element can be located in DNA. The most common position is found upstream of the target gene where it can help repress the transcription of the gene. [4]