The Lac Operon Induction

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 galactoside O-acetyltransferase, respectively.

In its natural environment, the lac operon allows for the effective digestion of lactose. Lactose permease, which is embedded 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, so it has become a foremost example of prokaryotic gene regulation. It is often discussed in introductory molecular and cellular biology classes at universities for this reason.


The repressor is an allosteric protein, i.e. it can assume either one of two slightly different shapes, which are in equilibrium with each other. In one form the repressor will bind to the operator DNA with high specificity, and in the other form it has lost its specificity. According to the classical model of induction, binding of the inducer, either allolactose or IPTG, to the repressor affects the distribution of repressor between the two shapes. Thus, repressor with inducer bound is stabilized in the non-DNA-binding conformation. However, this simple model cannot be the whole story, because repressor is bound quite stably to DNA, yet it is released rapidly by addition of inducer. Therefore it seems clear that repressor can also bind inducer while still bound to DNA.[dubious – discuss] It is still not entirely known what the exact mechanism of binding is.