operons

An operon is a functioning unit of genomic DNA that contains a cluster of adjacent genes that are co-regulated and transcribed together as a single mRNA under the control of a common promoter and regulatory elements. In its simplest form, an operon includes structural genes, a promoter, an operator sequence, and a regulatory gene that encodes a regulator protein. The operator is the DNA site where a repressor or activator can bind to influence transcription of the operon's genes. The regulator protein is typically produced in a separate location (in many bacteria) and acts in trans.

Most operons are found in prokaryotes, notably bacteria and some archaea, and they often enable coordinated expression of genes that participate in a common pathway, such as a metabolic or biosynthetic process. Transcription of an operon yields a single polycistronic mRNA that contains multiple coding sequences, allowing efficient production of several proteins from one transcript.

Regulation of operons can be inducible or repressible. In the lac operon of Escherichia coli, the presence of lactose inactivates the repressor, enabling transcription of the genes required to metabolize lactose. In the trp operon, high levels of tryptophan enable the repressor to block transcription, preventing excess tryptophan synthesis. Some operons use attenuation, a mechanism that modulates transcription termination based on the translation status of a leader sequence.

Historically, the operon model was proposed by François Jacob and Jacques Monod in 1961, establishing a framework for understanding gene regulation. While operons are common in bacteria and archaea, eukaryotic genomes generally employ different regulatory architectures, with operon-like gene clusters being rare and often processed into separate mRNAs.