Practice question

Intracellular inducer identification required distinguishing external substrate supplied in medium from effector that physically contacts LacI repressor protein. Beta-galactosidase at low basal levels converts a small fraction of imported lactose via intramolecular transgalactosylation from beta-1-4 to beta-1-6 linkage, generating allolactose, 6-O-beta-D-galactopyranosyl-D-glucose. Allolactose binds core domain of LacI tetramer, reducing operator DNA affinity by three orders of magnitude through allosteric shift propagating to recognition helix. Lactose itself binds poorly, glucose never acts as inducer for lac, and IPTG is synthetic mimic, confirming allolactose as physiological inducer enabling positive feedback loop amplifying expression.