In the lac operon, allolactose acts as the inducer that allows E. coli to use lactose.
When lactose is available, some of it is converted into the isomer allolactose. Allolactose binds the lac repressor, changing its shape so the repressor no longer blocks the operator. RNA polymerase can then transcribe genes needed for lactose uptake and breakdown.
The lac operon contains lacZ, lacY, and lacA. Their products include beta-galactosidase, lactose permease, and thiogalactoside transacetylase. Expression is strongest when lactose is present and glucose is scarce, because low glucose raises cyclic AMP and helps the CAP–cAMP regulatory complex stimulate transcription.
A common simplification calls lactose itself the inducer, but the direct natural inducer is allolactose. The system was studied by François Jacob and Jacques Monod, whose operon model helped establish modern ideas about gene regulation.