How does transcription inhibition differ between prokaryotes and eukaryotes? Describe the two mechanisms used by eukaryotes to direct the enhancers toward certain promoters and away from others
What will be an ideal response?
Prokaryotes typically use a repression mechanism in which repressor proteins inhibit transcription by binding to operator sequences that overlap promoters. This blocks the binding of RNA polymerase, thus inhibiting the gene or operon.
In eukaryotes, this mechanism of transcription inhibition is not seen. Instead, eukaryotic repressors inhibit transcription through other mechanisms.
1. Eukaryotic repressors can bind to silencer sequences, cis-acting regulatory sequences that block transcription by directly preventing enhancer-mediated transcription. For example, yeast use the proteins Mig1 and Tup1 to bind to the silencer site when glucose is present, thus inhibiting transcription of the galactose utilization pathway.
2. Eukaryotic insulator sequences can block the action of the enhancer. Insulators are cis-acting sequences located between enhancers and promoters of genes that are to be insulated from the effects of the enhancer. Insulators direct enhancers to interact with the intended promoter and block communication between enhancers and other promoters. This mechanism likely involves allowing the formation of DNA loops containing enhancers and their intended promoter targets while preventing the formation of DNA loops containing an enhancer and a promoter that is not its intended target.
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