enhancer element

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Function

a short region of DNA that can bind transcription factors to enhance transcription of genes within in a gene cluster
enhancers are usually cis-acting
an enhancer need not be close to the genes it activates, (it may be several thousand base pairs distant) but must be at least 100 bp from the transcription start site
enhancers need not be located on the same chromosome
enhancers may be found within introns
an enhancer may be located upstream or downstream of the gene that it regulates
orientation of enhancers is often unimportant
this is so because chromatin is folded in a way that may allow the enhancer to be geometrically close to the promoter element & transcription start site despite its distance on the DNA strand
goemetric proximity allows the enhancer element to interact with general transcription factors & RNA polymerase 2
enhancers do not act on the promoter region itself, but are bound by activator proteins that interact with the mediator complex, which recruits polymerase 2 and general transcription factors
many enhancer elements is that they contain several transcription factor binding sites clustered within 100 nucleotides

↑ Molecular Cell Biology (2nd ed) Darnell J; Lodish H & Baltimore D (eds),Scientific American Books, WH Freeman, NY 1990, pg 296
↑ Wikipedia: Enhancer (genetics) http://en.wikipedia.org/wiki/Enhancer_(genetics)