Operon
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Operon
A group of distinct genes that are expressed and regulated as a unit. Each operon is a deoxyribonucleic acid (DNA) sequence that contains at least two regulatory sites, the promoter and the operator, and the structural genes that code for specific proteins (see illustration). The promoter (p) site is the location at which ribonucleic acid (RNA) polymerase binds to the operon. RNA polymerase moves down the operon catalyzing the synthesis of a messenger RNA (mRNA) molecule with a sequence that is complementary to DNA. This process is called transcription. The mRNA is used as a template by ribosomes to synthesize the proteins coded for by the structural genes (in the original DNA) in a process called translation. This mRNA is referred to as polycistronic because its sequence directs the synthesis of more than one protein. The operator (o) site is located between the p site and the beginning of the coding region for the first structural gene. It is at this site that molecules called repressors can bind to the DNA and block RNA polymerase from transcribing the DNA, thus shutting off the operon. Some systems can be derepressed by the addition of small molecules called effectors, which bind to the repressor protein and cause a conformational (shape) change that makes it no longer able to bind to the DNA at the operator site. See Deoxyribonucleic acid (DNA), Ribonucleic acid (RNA)
Activation is believed to arise from the binding of a protein immediately adjacent to the promoter. The protein provides additional locations with which RNA polymerase can interact; the extra interactions result in an increased amount of polymerase binding to the promoter. Activators are more frequently involved in the regulation of genes in eukaryotes than in prokaryotes.
Once RNA polymerase begins transcribing a gene, it continues making RNA until a termination site is reached. Antiterminators are proteins that prevent termination at certain sites. In the presence of these antiterminators, RNA polymerase continues along the genome and transcribes the genes following the termination site until a different class of termination site is encountered.
Attenuation is the premature termination of the mRNA translation. Although the exact mechanism of attenuation has not been determined, it is thought that attenuation is due to the formation of a translation termination site in mRNA. See Gene, Gene action
operon
[′äp·ə‚rän]Operon
a group of functionally related genes that controls the synthesis of enzyme proteins that are associated with the successive stages of any biochemical process (seeGENE). The concept of an operon as part of the theory of genetic organization and regulation was advanced in 1961 by the French scientists F. Jacob and J. Monod, on the basis of experiments with the synthesis of inducible enzymes in Escherichia coli mutants. An operon exerts its regulatory effect during the transcription phase of protein synthesis, that is, when genetic information is being transcribed from deoxyribonucleic acid (DNA) to the corresponding portion of messenger ribonucleic acid (mRNA) (seeDEOXYRIBONUCLEIC ACID).
The promoter site is usually located at the beginning of the operon. This is where the enzyme RNA polymerase binds in order to bring about the transcription of the DNA in that operon. Behind the promoter site is the operator—the portion of DNA with which a regulatory protein, or repressor, interacts. The rest of the operon consists of structural genes that contain information about the amino-acid sequence of the polypeptide chains in proteins. The synthesis of the repressor proteins is controlled by regulator genes, which are not necessarily associated with the operon whose effect they repress. By interacting with an operator, a repressor influences the rate of transcription of structural genes.
A repressor is capable of both recognizing the sequence of bases in operator DNA and interacting with substances of low molecular weight called effectors. The effectors are generally substrates or products of the enzymes that are formed by the particular operon in question. Effectors greatly alter a repressor’s affinity for an operator; some effectors diminish this affinity, while others enhance it. The binding of a repressor to an operator prevents RNA polymerase from moving along the operon. As a result, the synthesis of mRNA is inhibited. Removal of the repressor from the operator activates the operon. Thus, the operator controls the activity of the operon as a whole.
The process that has just been described is an example of the negative control of gene expression. Gene expression is also under positive control, in which case an activator protein becomes attached to the initial part of the operon—in front of the promoter; from this site, the protein initiates the transcription of the operon. The end of the operon is a series of nucleotides to which a specific protein is bound. This protein, called a terminator, halts the synthesis of RNA. Although these mechanisms have been studied only in prokaryotes, the cells of higher organisms are believed to exhibit similar mechanisms.
The concept of the operon has proven to be very fruitful in molecular genetics. The theory has been confirmed by many researchers, who used both genetic and biochemical approaches. The discovery of operons proves that the activity of a gene in a cell is ordered and dependent on both external conditions and the activity of other genes. The concept of the operon also helps explain how the genetic apparatus of a cell is able to react adequately to changes in external conditions.
REFERENCES
Jacob, F., and J. Monod. “Reguliatsiia aktivnosti genov.” In the collection Reguliatornye mekhanizmy kletki. Moscow, 1964. (Translated from English.)Hartman, P., and S. Suskind. Deistvie gena. Moscow, 1966. (Translated from English.)
Georgiev, G. P. “Reguliatsiia sinteza RNK v kletkakh zhivotnykh.” Uspekhi sovremennoi biologii, 1970, vol. 69, issue 3.
Khesin, R. B. “Sostoianie voprosa o mekhanizmakh reguliatsii sinteza RNK u nizshikh i vysshikh organizmov.” Uspekhi sovremennoi biologii, 1972, vol. 74, issue 2 (5).
Hartman, P., and S. Suskind. Gene Action, 2nd ed. Englewood Cliffs, N.J., 1969.
IU. S. DEMIN