Practice question

Maximal lac expression requires both absence of repressing signal and presence of activating signal, implementing AND logic for metabolic economy. Lactose generates allolactose that removes LacI barrier by conformational change, while glucose absence raises cyclic AMP, producing active CAP complex that stimulates polymerase recruitment. Combined regulatory outputs yield approximately thousand-fold induction over basal repressed level, enabling rapid lactose utilization when glucose scarce. If glucose present despite lactose, cAMP remains low giving partial induction only; if lactose absent, repressor maintains OFF state regardless of cAMP levels, illustrating dual control integration principle underlying operon behavior.