Inducer exclusion is a mechanism that controls gene expression in the lac operon, which is responsible for lactose metabolism in bacteria like Escherichia coli. The lac operon consists of three structural genes: lacZ, lacY, and lacA, as well as regulatory elements.
Inducer exclusion works by preventing the transport of the inducer molecule, typically lactose or allolactose, into the bacterial cell. This mechanism is mediated by a process called phosphoenolpyruvate-dependent phosphotransferase system (PTS).
The PTS is a complex system involved in the transport and phosphorylation of sugars in bacteria. In the absence of glucose, the PTS is activated and phosphorylates incoming sugars, including lactose. However, when glucose is present, it is preferred over other sugars, including lactose.
In the presence of glucose, the PTS phosphorylates a protein called Enzyme IIA (EIIA) of the PTS. Phosphorylated EIIA interacts with the lactose permease, which is responsible for transporting lactose into the cell, and inhibits its activity. As a result, the transport of lactose into the cell is reduced, and the concentration of intracellular lactose remains low.
The low intracellular lactose concentration prevents the binding of lactose or allolactose to the lac repressor protein, which normally binds to the operator region of the lac operon, inhibiting transcription. When lactose or allolactose is absent or at low levels, the lac repressor remains bound to the operator, preventing RNA polymerase from transcribing the lac genes.
By excluding the inducer molecule (lactose) from entering the cell and inhibiting the binding of lactose to the lac repressor, inducer exclusion effectively regulates gene expression in the lac operon based on the presence or absence of glucose.