An operon is a functioning unit of key nucleotide sequences including an operator, a common promoter, and one or more structural genes, which is controlled as a unit to produce messenger RNA (mRNA), in the process of protein transcription. Overview Operons occur primarily in prokaryotes but also in some eukaryotes, including nematodes. Although it may not be located in the operon gene, a "Regulator" gene is present which codes for the production of a repressor or corepressor protein. The locat… (More on Operons)