operon

An operon is a unit of DNA in prokaryotes that consists of a cluster of functionally related genes under the control of a single promoter and regulated together as a single transcriptional unit. The operon enables coordinated expression of genes involved in a common metabolic pathway. Its basic structure includes a promoter, an operator, and the structural genes; regulation is mediated by a regulator gene that encodes a repressor or activator, which binds to the operator or other regulatory elements. The entire operon is typically transcribed into a single polycistronic mRNA.

Components and regulation function: The promoter is the site where RNA polymerase binds to initiate transcription. The operator is a DNA sequence that can bind a repressor to block transcription. The structural genes encode enzymes or proteins that carry out a shared function. The regulatory gene, often located outside the operon, produces the repressor or activator protein. Operons can be under negative control, where binding of a repressor inhibits transcription, or positive control, where an activator enhances transcription.

Classic examples and regulatory modes: The lac operon in Escherichia coli is a well-known inducible system for lactose metabolism, activated when lactose or its derivative allolactose is present and glucose is scarce, with CAP–cAMP providing additional positive regulation. The trp operon is a repressible system that downregulates tryptophan biosynthesis when tryptophan levels are high, and it may employ attenuation as an additional regulatory mechanism in some organisms.

Historical significance: The operon model, proposed by Francois Jacob and Jacques Monod in 1961, established a foundational framework for understanding prokaryotic gene regulation and the coordination of gene expression. It highlighted how groups of genes can be controlled as a unit in response to cellular and environmental signals.