<P> The lac operon (lactose operon) is an operon required for the transport and metabolism of lactose in Escherichia coli and many other enteric bacteria . Although glucose is the preferred carbon source for most bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available through the activity of beta - galactosidase . Gene regulation of the lac operon was the first genetic regulatory mechanism to be understood clearly, so it has become a foremost example of prokaryotic gene regulation . It is often discussed in introductory molecular and cellular biology classes for this reason . This lactose metabolism system was used by Jacob and Monod to determine how a cell knows which enzyme to synthesize . Their work on the lac operon won them the Nobel Prize in Physiology in 1965 . </P> <P> Bacterial operons are polycistronic transcripts that are able to produce multiple proteins from one mRNA transcript . In this case, when lactose is required as a sugar source for the bacterium, the three genes of the lac operon can be expressed and their subsequent proteins translated: lacZ, lacY, and lacA . The gene product of lacZ is β - galactosidase which cleaves lactose, a disaccharide, into glucose and galactose . lacY encodes Beta - galactoside permease, a protein which becomes embedded in the cytoplasmic membrane to enable transport of lactose into the cell . Finally, lacA encodes β - galactoside transacetylase . </P>

The lac operator site within the lac operon is an example of a