T

Gene A

Promoter

Transcription start

Enhancer I

Insulator binding protein

Insulator

^ Enhancer II can stimulate translation of gene B but its effect on gene A is blocked by the insulator.

^ Enhancer II can stimulate translation of gene B but its effect on gene A is blocked by the insulator.

Enhancer II

Transcription start

Enhancer II

Transcription start stream of the gene's promoter. Furthermore, the exact position and orientation of an enhancer relative to the promoter can vary. How can an enhancer affect the initiation of transcription taking place at a promoter that is tens of thousands of base pairs away? The mechanism of action of many enhancers is not known, but evidence suggest that, in some cases, activator proteins bind to the enhancer and cause the DNA between the enhancer and the promoter to loop out, bringing the promoter and enhancer close to one another, so that the transcriptional activator proteins are able to directly interact with the basal transcription apparatus at the core promoter.

Most enhancers are capable of stimulating any promoter in their vicinities. Their effects are limited, however, by insulators (also called boundary elements), which are DNA sequences that block or insulate the effect of enhancers in a position-dependent manner. If the insulator lies between the enhancer and the promoter, it blocks the action of the enhancer; but, if the insulator lies outside the region between the two, it has no effect (< Figure 16.23). Specific proteins bind to insulators and play a role in their blocking activity, but exactly how this takes place is poorly understood. Some insulators also limit the spread of changes in chromatin structure that affect transcription. __

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