Operon

An operon is a DNA unit where a cluster of genes is controlled by a single promoter. These genes get transcribed together into one mRNA molecule. That mRNA may then be translated as a single piece in the cytoplasm, or it might be spliced into separate monocistronic mRNAs, each coding for one protein. The key result is that all the genes in the operon are either turned on or off as a group. For an operon to exist, several genes must be co-transcribed.

Operons were originally believed to occur only in prokaryotes—including bacteria-derived organelles like plastids. However, they were found in eukaryotes in the early 1990s, though they remain rare there. In prokaryotes, operons usually produce polycistronic mRNAs, while eukaryotic operons typically yield monocistronic mRNAs. Viruses, such as bacteriophages, also have operons. For instance, T7 phages contain two operons: the first codes for several products, including a T7 RNA polymerase that binds to and transcribes the second operon, which includes a lysis gene that makes the host cell burst.

The term "operon" was first proposed in a short 1960 paper in the Proceedings of the French Academy of Sciences. That paper introduced the general theory of the operon, which held that genes within an operon are always negatively controlled by a repressor acting at a single operator located before the first gene. Later research showed that genes could be positively regulated and also controlled after transcription initiation. Because different operons use different mechanisms, there is no single general regulatory mechanism. Today, an operon is simply defined as a cluster of genes transcribed into a single mRNA molecule. Still, the concept’s development is considered a landmark in molecular biology. The first operon described was the *lac* operon in *E. coli*, and the 1965 Nobel Prize in Physiology or Medicine was awarded to François Jacob, André Michel Lwoff, and Jacques Monod for their discoveries concerning genetic control of enzyme and virus synthesis.

Operons are found mainly in prokaryotes, but also rarely in some eukaryotes, such as nematodes like *C. elegans* and the fruit fly *Drosophila melanogaster*. rRNA genes often exist in operons, which have been found in a range of eukaryotes, including chordates. An operon consists of several structural genes arranged under a common promoter and regulated by a common o