A functionally integrated genetic unit for the control of gene expression in bacteria, as proposed in the Jacob-Monod hypothesis. Typically it comprises a closely linked group of structural genes, coding for protein, and adjacent loci controlling their expression—an operator site and a promoter site. The structural genes tend to encode enzymes concerned with a particular biochemical pathway. Transcription of the structural genes is prevented by binding of a repressor molecule to the operator site. Another molecule, the inducer, can bind to the repressor molecule, preventing it from binding to the operator and thus allowing the promoter site to bind the enzyme RNA polymerase, thereby initiating transcription. The repressor molecule is encoded by a regulator gene, which may be close to or distant from the operon. Some operons also have an attenuator region (see attenuation) preceding the first structural gene, where transcription may either stall or proceed according to the amount of end-product in the cell. See also lac operon.