Operon

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An operon is a group of key nucleotide sequences including an operator, a common promoter, and one or more structural genes that are controlled as a unit to produce messenger RNA (mRNA). Operons occur primarily in prokaryotes and nematodes. They were first described by François Jacob and Jacques Monod in 1961.

Operons are related to regulons and stimulons - operons contain a set of genes under regulation by the same operator, regulons contain a set of genes under regulation by a single regulatory protein and stimulons contain a set of genes under regulation by a single cell stimulus.

Contents 1 The operon as a unit of transcription 2 Promoter 3 Operator 4 Operon gene regulation 5 The lac operon 6 Predicting the number and organization of operons 7 See also 8 References

[edit] The operon as a unit of transcription

An operon contains one or more structural genes which are transcribed into one polycistronic mRNA: a single mRNA molecule that codes for more than one protein. Upstream of the structural genes lies a promoter sequence which provides a site for RNA polymerase to bind and initiate transcription. Close to the promoter lies a section of DNA called an operator. The operon may also contain regulatory genes such as a repressor gene which codes for a regulatory protein that binds to the operator and inhibits transcription. Regulatory genes need not be part of the operon itself, but may be located elsewhere in the genome. The repressor molecule will reach the operator to block the transcription of the structural genes.

[edit] Promoter

Main article: promoter.

A promoter is a DNA sequence that enables a gene to be transcribed. The promoter is recognized by RNA polymerase, which then initiates transcription. In RNA synthesis, promoters are a means to demarcate which genes should be used for messenger RNA creation - and, by extension, control which proteins the cell manufactures.

[edit] Operator

An operator is a segment of DNA which regulates the activity of the structural genes of the operon that it is linked to, by interacting with a specific repressor or activator. It is a regulatory sequence for shutting a gene down or turning it "on".

[edit] Operon gene regulation

Control of operon genes is a type of gene regulation that enables organisms to regulate the expression of various genes depending on environmental conditions. Operon regulation can be either negative or positive.

Negative regulation involves the binding of a repressor to the operator to prevent transcription.

• In negative inducible operons, a regulatory repressor protein is normally bound to the operator and it prevents the transcription of the genes on the operon. If an inducer molecule is present, it binds to repressor and changes its conformation so that it is unable to bind to the operator. This allows for the transcription of the genes on the operator.

• In negative repressible operons, transcription of the genes on the operon normally takes place. Repressor proteins are produced by a regulator gene but they are unable to bind to the operator in their normal conformation. However certain molecules called corepressors can bind to the repressor protein and change its conformation so that it can bind to the operator. The activated repressor protein binds to the operator and prevents transcription.

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

• In positive inducible operons, activator proteins are normally unable to bind to the pertinent DNA. However, certain substrate molecules can bind to the activator proteins and change their conformations so that they can bind to the DNA and enable transcription to take place.

• In positive repressible operons, the activator proteins are normally bound to the pertinent DNA segment. However, certain molecules can bind to the activator and prevent it from binding to DNA. This prevents transcription.

[edit] The lac operon

Main article: lac operon.

The lac operon of the model bacterium Escherichia coli was the first operon discovered, and provides a typical example of operon function. It consists of three adjacent structural genes, a promoter, a Terminator (genetics), and an operator. The lac operon is regulated by several factors including the availability of glucose and of lactose.

[edit] Predicting the number and organization of operons

The number and organization of operons has been studied most critically in E. coli. Predictions can be made based on genome sequence.

One method uses the intergenic distance between reading frames as a primary predictor of the number of operons in the genome. The separation merely changes the frame and guarantees that the read through is efficient. Longer stretches exist where operons start and stop, often up to 40-50 bases ^[1].

Operon prediction is even more accurate if the functional class of the molecules is considered. Bacteria have clustered their reading frames into units, sequestered by co-involvement in protein complexes, common pathways, or shared substrates and transporters. Thus, accurate prediction would involve all of these data, a difficult task indeed.

[edit] See also

• gene regulatory network
• L-arabinose operon – another well-studied operon in E. coli
• Protein biosynthesis
• Genetic code
• Prokaryote

[edit] References

1. ^ Salgado, H; Moreno-Hagelsieb, G; Smith, T. F.; Collado-Vides, J. Operons in Escherichia coli: Genomic analyses and predictions. PNAS. 2000, 97, 6652-6657. [1]