In prokaryotic cells such as bacteria, genes are usually found grouped together in operons. The operon is a cluster of genes that are related (often coding for enzymes in a metabolic pathway), and which are under the control of a single promoter/regulatory region. Perhaps the best known example of this arrangement is the lac operon, which codes for the enzymes responsible for lactose catabolism. Within the operon there are three genes that code for proteins (termed structural genes) and an upstream control region encompassing the promoter and a regulatory site called the operator. In this control region there is also a site which binds a complex of cyclic AMP and CRP (cyclic AMP rcceptor protein), which is important in positive regulation (stimulation) of transcription. Lying outside the operon itself is the repressor gene, which codes for a protein (the Lac repressor) that binds to the operator site and is responsible for negative control of the operon by blocking the binding of RN A polymerase.
The fact that structural genes in prokaryotes are often grouped together means that the transcribed mRNA may contain information for more than one protein. Such a molecule is known as a polycistronic mRNA. Thus much of the genetic information in bacteria is expressed via polycistronic mRNAs whose synthesis is regulated in accordance with the needs of the cell at any given time. This system is flexible and efficient, and enables the cell to adapt quickly to changing environmental conditions.