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The operon consists of a promoter (where RNA polymerase binds to begin transcription), an operator (section of DNA that acts as a switch by being the binding site for repressors), and the coding genes themselves. Located not within the operon is a regulatory gene, which codes for a protein that acts as a repressor. Operons affect transcription by blocking and stopping it or by speeding it up and allowing it.

In bacterial cells, like E. coli, the amino acid tryptophan can be made if necessary. The trp operon is a repressible system. The system will be transcribing and producing mRNA for proteins to make trp until stopped. The regulatory gene, trpR, will make repressor proteins. They are inactive. Tryptophan is its own corepressor. It will bind to the repressor, cause a structural change that makes it active, and the repressor will then bind to the operator. This doesn't allow RNA polymerase to bind to the promoter to begin transcription, so the production of messenger RNA stops.

Another example of negative control is in the inducible lac-operon system. The bacteria use glucose if it's present, but when it isn't then they break down lactose into glucose and galactose. The regulatory gene, lacI, will produce repressors. An isomer of lactose-allolactose-acts as an inducer. It binds to the repressor and causes a structural change that makes it leave the operator. Until the lactose is present, it is bound to the operator, not allowing transcription of genes to make proteins that would help lactose into the cell or to break it down.

A form of positive control can be seen in the cAMP-CRP system. When glucose isn't present, cyclicAMP builds up. This cAMP will bind to the cAMP repressor protein, CRP. This will then bind to the CRP binding site, located just before the promoter. This magnifies transcription of the genes that code for proteins that help lactose enter the cell or be broken down. When the CRP is present, RNA polymerase binds to the promoter more quickly, so transcription occurs more often.

P.S. Yes, this is all me talking...

For a simpler definition:

An operon is a controllable unit of gene transcription that is composed of a number of structural genes that get transcribed together. An operon contains at least two distinct regions: the operator and the promoter. Examples include the lac operon.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

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