A functional unit consisting of a promoter, an operator and a number of structural
genes, found mainly in prokaryotes. The structural genes commonly code for several functionally
related
enzymes, and although they are transcribed as one (polycistronic)
mRNA each is independently translated. In the typical operon, the operator region acts
as a controlling element in switching on or off the synthesis of mRNA.
Source:
PAC, 1992, 64, 143
(Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992))
on page 162