Enhancers and Silencers
Several DNA sequences of note have been detected in eukaryotic genes. The first that was described was an enhancer sequence. Enhancers have the ability to greatly increase the expression of genes in their vicinity. More recently, elements have been identified that decrease transcription of neighboring genes, and these elements have been called silencers.
Extensive analysis of enhancers have detected several features. First, these elements are functional over a large distance. For example, an enhancer has been placed 3000 nt from the gene, and it can still increase expression. Second, these elements are orientation independent. This means that the element can been be inverted, and it will still affect gene expression.
Another feature of enhancers is that they are often associated with a gene that is abundantly expressed. For example, immunoglobulin genes, genes that encode antibodies, often have associated enhancers. This enhancer is located in the intron of the gene. Remember, this feature is important because Burkitt's lymphoma results from a translocation that puts the oncogene c-myc in the vicinity of the enhancer. This possibly leads to the over expression of the gene and causes the resulting cancer.
Other examples of genes with enhancers are the ß -hemoglobin gene in humans and storage proteins in soybean. One important feature of these enhancers is their tissue specificity. Storage proteins are only expressed in the seed of the soybean seed. How this specificity is controlled is not understood, but may involve the interaction of these elements with tissue-specific, trans-acting factors. In other words, a factor that only is expressed during the stage of seed development when the seed is producing the storage protein interacts with the enhancer and increases expression.
Copyright © 1997. Phillip McClean