Lac Operon

Lac Operon

The lac operon is an operon required for the transport and metabolism of lactose in Escherichia coli and some other enteric bacteria. It consists of three adjacent structural genes, lacZ, lacY and lacA. The genes encode β-galactosidase, lactose permease, and thiogalactoside transacetylase (or galactoside O-acetyltransferase), respectively.

In its natural environment, the lac operon allows for the effective digestion of lactose. The lactose permease, which sits in the cytoplasmic membrane, transports lactose into the cell. β-galactosidase, a cytoplasmic enzyme, subsequently cleaves lactose into glucose and galactose. However 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. Gene regulation of the lac operon was the first genetic regulatory mechanism to be understood clearly and is one of the foremost examples of prokaryotic gene regulation.

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. It achieves this primarily with the lac repressor, which halts the production in the absence of lactose, and EIIAGlc, which shuts down lactose permease when glucose is being transported into the cell. This dual control mechanism causes the sequential utilization of glucose and lactose in two distinct growth phases, known as diauxie.