Operon

In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter.

[2] Originally, operons were thought to exist solely in prokaryotes (which includes organelles like plastids that are derived from bacteria), but their discovery in eukaryotes was shown in the early 1990s, and are considered to be rare.

Today, the operon is simply defined as a cluster of genes transcribed into a single mRNA molecule.

[10] The 1965 Nobel Prize in Physiology and Medicine was awarded to François Jacob, André Michel Lwoff and Jacques Monod for their discoveries concerning the operon and virus synthesis.

Operons occur primarily in prokaryotes but also rarely in some eukaryotes, including nematodes such as C. elegans and the fruit fly, Drosophila melanogaster.

The location and condition of the regulators, promoter, operator and structural DNA sequences can determine the effects of common mutations.

[6] Upstream of the structural genes lies a promoter sequence which provides a site for RNA polymerase to bind and initiate transcription.

Gene clustering helps a prokaryotic cell to produce metabolic enzymes in a correct order.

[17] An inducer (small molecule) can displace a repressor (protein) from the operator site (DNA), resulting in an uninhibited operon.

With positive control, an activator protein stimulates transcription by binding to DNA (usually at a site other than the operator).

In the lac operon, lactose binds to the repressor protein and prevents it from repressing gene transcription, while in the trp operon, tryptophan binds to the repressor protein and enables it to repress gene transcription.

One prediction method uses the intergenic distance between reading frames as a primary predictor of the number of operons in the genome.

Bacteria have clustered their reading frames into units, sequestered by co-involvement in protein complexes, common pathways, or shared substrates and transporters.

The 517 polycistronic operons are listed in a 2009 study describing the global changes in transcription that occur in L. monocytogenes under different conditions.

A typical operon
1 : RNA Polymerase, 2 : Repressor, 3 : Promoter, 4 : Operator, 5 : Lactose, 6 : lacZ, 7 : lacY, 8 : lacA. Top : The gene is essentially turned off. There is no lactose to inhibit the repressor, so the repressor binds to the operator, which obstructs the RNA polymerase from binding to the promoter and making lactase. Bottom : The gene is turned on. Lactose is inhibiting the repressor, allowing the RNA polymerase to bind with the promoter, and express the genes, which synthesize lactase. Eventually, the lactase will digest all of the lactose, until there is none to bind to the repressor. The repressor will then bind to the operator, stopping the manufacture of lactase.