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Home >>Regulation of Gene Expression - in Prokaryotes >> CIS Acting Elements
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CIS Acting Elements - Eukaryotic genes are regulated by promoter elements located just upstream (5 ') from the transcription initiation sites in a manner quite similar to the regulation of prokaryotic genes. In addition to the nearby promoters, many eukaryotic genes are also regulated by more distant cis acting elements called enhancers and silencers.

Enhancers increase the transcription and are independent of orientation. Enhancers can act over relatively large distances. They are present several thousand bp from the regulated genes and are relatively large elements up to several hundred nucleotide pairs in length.

They sometimes contain repeated sequences that have partial enhancer activity by themselves. Most enhancer elements function in a complete or partially tissue specific manner, that is, they will only enhance the transcription of genes in specific target tissues frequently.

Many enhancers have now been characterized that play key roles in the regulation of gene expression. A striking feature of the characterized enhancers that they exhibit tissue specificity. The tissue specificity of the enhancers is fascinating, however, we have not understood the molecular basis of this specificity.

Presumably, the tissue specificity may result from interactions of enhancer sequences with transcriptional activators present only in cells in which a given set of genes is expressed. The ability of enhancers and silencers to act at a distance of 1000 bp or more is intriguing.

At present this question cannot be answered in detail. But one point is clear, the factor bound at the enhancer sequence and the promoter sequence can act in a cooperative manner, either positively or negatively.

 
 

 
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