Operon

An operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splicing to create monocistronic mRNAs that are translated separately, i.e. several strands of mRNA that each encode a single gene product. The result of this is that the genes contained in the operon are either expressed together or not at all. Several operons are influenced by a repressor protein that inhibits mRNA synthesis.

History

The concept of the operon was first proposed in 1960 by François Jacob and Jacques Monod. Jacob and Monod were the first to discover the operon while studying the E. coli lac operon. They found out that the three structural genes lacZ, lacY, and lacA, are transcribed as a single unit, under the control of a single promoter and operator.

Structure

An operon contains one or more structural genes which are generally transcribed into one polycistronic mRNA (a single mRNA molecule that codes for multiple proteins). However, the definition of an operon does not require the mRNA to be polycistronic, though in practice, it usually is.

Function

The function of the operon is to regulate the efficient expression of genes according to the metabolic needs of the cell. If the products of the genes are required by the cell, the operon is switched on, allowing the genes to be expressed and the proteins to be produced. If the products are not required, the operon is switched off, and the genes are not expressed.

Types of Operons

There are two types of operons: inducible operons and repressible operons. Inducible operons are usually associated with catabolic pathways and are normally off, but can be turned on when the substrate that needs to be catabolized is present. Repressible operons are usually associated with anabolic pathways and are normally on, but can be turned off when the end product of the pathway is present in sufficient amounts.

See also

• Gene regulation
• Lac operon
• Trp operon
• AraC

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