The lac operon (lactose operon) is an operon required for the transport and metabolism of lactose in Escherichia coli and many other enteric bacteria..............................................learn more in slides

6. 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.
It would be wasteful to produce the enzymes when there is no lactose available or if there is a more preferable energy
source available, such as glucose. The lac operon uses a two-part control mechanism to ensure that the cell expends energy
producing the enzymes encoded by the lac operon only when necessary. In the absence of lactose, the lac repressor, lacI,
halts production of the enzymes encoded by the lac operon . The lac repressor is always expressed unless a co-inducer binds
to it. In other words, it is transcribed only in the presence of small molecule co-inducer. In the presence of glucose, the
catabolite activator protein (CAP), required for production of the enzymes, remains inactive, and EIIAGlc shuts down lactose
permease to prevent transport of lactose into the cell. This dual control mechanism causes the sequential utilization of
glucose and lactose in two distinct growth phases, known as diauxie

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