<P> The term "operon" was first proposed in a short paper in the Proceedings of the French Academy of Science in 1960 . From this paper, the so - called general theory of the operon was developed . This theory suggested that in all cases, genes within an operon are negatively controlled by a repressor acting at a single operator located before the first gene . Later, it was discovered that genes could be positively regulated and also regulated at steps that follow transcription initiation . Therefore, it is not possible to talk of a general regulatory mechanism, because different operons have different mechanisms . Today, the operon is simply defined as a cluster of genes transcribed into a single mRNA molecule . Nevertheless, the development of the concept is considered a landmark event in the history of molecular biology . The first operon to be described was the lac operon in E. coli . The 1965 Nobel Prize in Physiology and Medicine was awarded to François Jacob, André Michel Lwoff and Jacques Monod for their discoveries concerning the operon and virus synthesis . </P> <Table> <Tr> <Td> Polycistronic operon Regulatory sequence Regulatory sequence Enhancer Enhancer / silencer / silencer Operator Promoter 5'UTR ORF ORF UTR 3'UTR Start Start Stop Stop Terminator Transcription DNA RBS RBS Protein coding region Protein coding region mRNA Translation Protein The structure of a prokaryotic operon of protein - coding genes . Regulatory sequence controls when expression occurs for the multiple protein coding regions (red). Promoter, operator and enhancer regions (yellow) regulate the transcription of the gene into an mRNA . The mRNA untranslated regions (blue) regulate translation into the final protein products . </Td> </Tr> </Table> <Tr> <Td> Polycistronic operon Regulatory sequence Regulatory sequence Enhancer Enhancer / silencer / silencer Operator Promoter 5'UTR ORF ORF UTR 3'UTR Start Start Stop Stop Terminator Transcription DNA RBS RBS Protein coding region Protein coding region mRNA Translation Protein The structure of a prokaryotic operon of protein - coding genes . Regulatory sequence controls when expression occurs for the multiple protein coding regions (red). Promoter, operator and enhancer regions (yellow) regulate the transcription of the gene into an mRNA . The mRNA untranslated regions (blue) regulate translation into the final protein products . </Td> </Tr> <P> Operons occur primarily in prokaryotes but also in some eukaryotes, including nematodes such as C. elegans and the fruit fly, Drosophila melanogaster . rRNA genes often exist in operons that have been found in a range of eukaryotes including chordates . An operon is made up of several structural genes arranged under a common promoter and regulated by a common operator . It is defined as a set of adjacent structural genes, plus the adjacent regulatory signals that affect transcription of the structural genes . The regulators of a given operon, including repressors, corepressors, and activators, are not necessarily coded for by that operon . The location and condition of the regulators, promoter, operator and structural DNA sequences can determine the effects of common mutations . </P>

The operon model of the regulation of gene