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The lac operon (lactose operon) is an operon required for the transport and metabolism of lactose in Escherichia coli and some other enteric bacteria. It has 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 is one of the most basic methods for explanation of how a repressor enzyme works within a cell on DNA and for that reason is discussed in many introductory molecular and cellular biology classes at universities.
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 presence of lactose, the lac repressor halts production of the enzymes encoded by the lac operon. 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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