A group of adjacent genes coding for a set of enzymes in a particular biochemical pathway. They act as a single unit in that they are either all transcribed together or none are transcribed at all. In order for transcription to occur, mRNA polymerase, which catalyses mRNA synthesis, must first bind with the DNA at a site called the promoter. mRNA polymerase activity is prevented (negative control) or promoted (positive control) by the binding of a regulator protein between the promoter and the structural genes (the genes that actually code for the enzymes) at a site called the operator. The regulator protein is itself the product of a gene, the regulator, which may be some distance from the operon. A classic case of negative control is the lac (lactose) operon in the bacterium Escherichia coli. Here the regulator protein prevents transcription in the absence of an inducer (a substrate molecule, i.e. lactose, or some derivative). However, if an inducer is present, it complexes with the regulator, the protein formed by the latter no longer binds to the operator, mRNA polymerase activity is permitted, and the enzymes are synthesized. See also Jacob-Monod model. |
