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. Although located independent of the operon gene, a "Regulator" gene is present which codes for the production of a repressor or corepressor protein. The location and condition of the regulator, promoter, operator and structural DNA sequences can determine the effects of common mutations. They were first described by François Jacob and Jacques Monod in 1961. An operator is a segment of DNA that regulates the activity of the structural genes of an operon it is linked to, by interacting with a specific repressor or activator. ... A promoter is a DNA sequence that contains the information, in the form of DNA sequences, that permits the proper activation or repression of the gene which it controls, i. ... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... The interaction of mRNA in a eukaryote cell. ... Prokaryotes (pro-KAR-ee-oht) (from Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -otos, pl. ... Classes Adenophorea    Subclass Enoplia    Subclass Chromadoria Secernentea    Subclass Rhabditia    Subclass Spiruria    Subclass Diplogasteria The nematodes or roundworms (Phylum Nematoda from Greek (nema): thread + -ode like) are one of the most common phyla of animals, with over 20,000 different described species (over 15,000 are parasitic). ... François Jacob (June 17, 1920 Nancy, France -- ) is a French biologist, who together with Jacques Monod, originated the idea that control of enzyme levels in all cells happens through feedback on transcription. ... Jacques Lucien Monod (February 9, 1910 – May 31, 1976) was a French biologist and a Nobel Prize Winner in Physiology or Medicine in 1965. ... 1961 (MCMLXI) was a common year starting on Sunday (the link is to a full 1961 calendar). ...

Image File history File links Operon. ...

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. In cell biology a regulon is a collection of genes (which may be in operons) under regulation by the same regulatory protein. ... In cell biology a stimulon is a collection of genes (which may be in operons and regulons) under regulation by the same stimulus. ...

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. A structural gene is a gene that codes for any RNA or protein product other than a regulatory element (i. ... Messenger RNA (mRNA) is said to be polycistronic when it contains the genetic information to translate more than one protein. ... The interaction of mRNA in a eukaryote cell. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... A promoter is a DNA sequence that contains the information, in the form of DNA sequences, that permits the proper activation or repression of the gene which it controls, i. ... RNAP from pictured during elongation. ... A regulator gene or regulatory gene is a gene involved in controlling the expression of one or more other genes. ... A repressor is a DNA-binding protein that regulates the expression of one or more genes by decreasing the rate of transcription. ...

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. A promoter is a DNA sequence that contains the information, in the form of DNA sequences, that permits the proper activation or repression of the gene which it controls, i. ... The structure of part of a DNA double helix Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the development and function of living organisms. ... For a non-technical introduction to the topic, see Introduction to Genetics. ... A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. ... RNAP from pictured during elongation. ...

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". The structure of part of a DNA double helix Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the development and function of living organisms. ... For a non-technical introduction to the topic, see Introduction to Genetics. ... A repressor is a DNA-binding protein that regulates the expression of one or more genes by decreasing the rate of transcription. ... An activator is a DNA-binding protein that regulates one or more genes by increasing the rate of transcription. ...

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. Gene regulation is the general term for cellular control of protein synthesis at the DNA-RNA transcription step. ...

Negative regulation involves the binding of a repressor to the operator to prevent transcription. A repressor is a DNA-binding protein that regulates the expression of one or more genes by decreasing the rate of 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). An inducer is a molecule in biology that starts gene expression. ... A regulator gene is a gene that codes for a protein or RNA involved in regulating the expression of other genes. ... An activator is a DNA-binding protein that regulates one or more genes by increasing the rate of transcription. ...

• 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.

The lac operon

Main article: lac operon.

The lac operon of the model bacterium Escherichia coli was the first operon to be discovered and provides a typical example of operon function. It consists of three adjacent structural genes, a promoter, a terminator, and an operator. The lac operon is regulated by several factors including the availability of glucose and lactose. The lac operon is an operon required for the transport and metabolism of lactose in Escherichia coli and some other enteric bacteria. ... A model organism is a species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. ... E. coli redirects here. ... A structural gene is a gene that codes for any RNA or protein product other than a regulatory element (i. ... A promoter is a DNA sequence that contains the information, in the form of DNA sequences, that permits the proper activation or repression of the gene which it controls, i. ... In genetics, a terminator marks the end of a gene on the DNA for transcription. ... An operator is a segment of DNA that regulates the activity of the structural genes of an operon it is linked to, by interacting with a specific repressor or activator. ... Glucose (Glc), a monosaccharide (or simple sugar), is the most important carbohydrate in biology. ... Lactose is a disaccharide that consists of β-D-galactose and β-D-glucose molecules bonded through a β1-4 glycosidic linkage. ...

Trp operon

Main article: trp operon.

Discovered in 1953 by Jacques Monod and colleagues, the trp operon in E. coli was the first repressible operon to be discovered. While the lac operon can be activated by a chemical (lactose), the tryptophan (Trp) operon is inhibited by a chemical (tryptophan). This operon contains five structural genes: trp E, trp D, trp C, trp B, and trp A, which encodes tryptophan synthetase. It also contains a promoter which binds to RNA polymerase and an operator which blocks transcription when bound to the protein synthesized by the repressor gene and a repressor gene (trp R) that binds to 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. Also different between the trp operon and the lac operon, the trp operon contains a leader peptide and an attenuator sequence which allows for graded regulation.^[1] Trp operon is an anabolic operon which promotes the production of tryptophan in the absence of tryptophan in the environment. ... Jacques Lucien Monod (February 9, 1910 – May 31, 1976) was a French biologist and a Nobel Prize Winner in Physiology or Medicine in 1965. ... Tryptophan synthestase(EC 4. ... The attenuator plays an important regulatory role in prokaryotic cells because of the absence of the nucleus in prokaryotic organisms. ...

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. Binomial name Escherichia coli T. Escherich, 1885 Escherichia coli (usually abbreviated to E. coli) is one of the main species of bacteria that live in the lower intestines of warm-blooded animals (including birds and mammals) and are necessary for the proper digestion of food. ...

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 ^[2].

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.

See also

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

A gene regulatory network (also called a GRN or genetic regulatory network) is a collection of DNA segments in a cell which interact with each other (indirectly through their RNA and protein expression products) and with other substances in the cell, thereby governing the rates at which genes in the... The L-arabinose operon of the model bacterium Escherichia coli has been a focus for research in molecular biology for over 40 years, and has been investigated extensively at the genetic, biochemical, physiological and biophysical levels. ... An overview of protein synthesis. ... A series of codons in a short RNA molecule. ... Prokaryotes (pro-KAR-ee-oht) (from Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -otos, pl. ...

References

1. ^ William Klug, Cummings, and Spencer. "Concepts of Genetics." 8th Ed. Pearson Education Inc, New Jersey: 2006. pg. 394-402
2. ^ 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]