<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> <P> Operons are related to regulons, stimulons and modulons; whereas operons contain a set of genes regulated by the same operator, regulons contain a set of genes under regulation by a single regulatory protein, and stimulons contain a set of genes under regulation by a single cell stimulus . According to its authors, the term "operon" means "to operate". </P> <P> An operon contains one or more structural genes which are generally transcribed into one polycistronic mRNA (a single mRNA molecule that codes for more than one protein). However, the definition of an operon does not require the mRNA to be polycistronic, though in practice, it usually is . Upstream of the structural genes lies a promoter sequence which provides a site for RNA polymerase to bind and initiate transcription . Close to the promoter lies a section of DNA called an operator . </P>

How many genes are under the control of a single promoter in an operon