Operon Model

The operon model describes a unit of genomic organization in which a cluster of genes, typically encoding proteins that act together in the same metabolic pathway, is transcribed together as a single messenger RNA molecule under the shared control of one promoter region, allowing the whole cluster to be switched on or off as one coordinated unit rather than through separate regulation of each individual gene. French molecular biologists Francois Jacob and Jacques Monod proposed the model in a landmark 1961 paper, based substantially on their detailed study of the lac operon in the bacterium Escherichia coli, the gene cluster that governs the enzymes the bacterium needs to metabolize the sugar lactose, and which is switched on only when lactose is present and a preferred sugar such as glucose is scarce. Jacob and Monod shared the 1965 Nobel Prize in Physiology or Medicine together with Andre Lwoff, largely in recognition of this work on genetic regulatory mechanisms. The operon model was a foundational discovery in the study of gene regulation, demonstrating for the first time that gene expression is an actively controlled process rather than something that occurs constitutively at a fixed rate, though true operons of this kind are now understood to be far more common in bacteria and other prokaryotes than in eukaryotic organisms, whose genes are typically regulated individually.