enhancer

An enhancer is a short DNA sequence that increases the level of expression of another gene, that is, the enhancer up-regulates transcription of genes within the regulated gene-cluster.

Specific trans-acting, transcription factors bind to the enhancer to bring about the increase in transcription rate – recruiting the initiation complex proteins, or stabilizing the initiation complex.

Because of looping of the DNA strands, there may be a separation of several thousand base pairs between the enhancer and initiator gene (start site). However, the enhancer and its regulated gene are located on the same chromosome (cis).

The enhancer segment may be situated upstream or downstream of the enhanced gene, and its orientation is not fixed – that is, the enhancer’s sequence may be reversed without altering its function. Enhancer segments may be excised and repositioned without interrupting their regulatory function. Enhancers may occur within introns. Enhancers cause the opposite effect to that of silencers which repress transcription.

Regulatory Genes & Enhancers
http://sciencematters.berkeley.edu/archives/volume1/issue7/story3.php: "Regulatory DNA, Levine explains, controls how and where a gene is expressed in a cell. Of the three types of regulatory DNA--enhancer, silencer, and insulator--'enhancers are king, activating gene expression in specific cell types for specific tissues,' he says. Scientists conservatively estimate that while the human genome has less than 30,000 genes, it may contain 100,000 enhancers at the minimum. So far, just 50 or so have been identified."

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