Sequence of DNA, in eukaryotes, to which specific transcription factors can bind, but which is not necessarily located in close proximity to the genes whose transcription is being impacted.
Enhancers are similar to promoters in that both are bound by transcription factors, contributing to 'enhancement' of rates of transcription from specific genes. Distinguishing enhancers and promotors – in addition to what sequences are involved along with what proteins bind to them – are primarily their locations, with enhancer sequences potentially quite distant (thousands of nucleotides) from the gene or genes affected. Note that eukaryotes nonetheless also possess promoters that act in addition to the functioning of enhancers.
The standard explanation for how this 'action at a distance' is possible – a phrase which, yes, I employ 'tongue in cheek' – involves DNA topography such that while enhance and gene might be quite distance linearly along the chromosome, with the chromosome folded upon itself the two instead can be quite close.
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