Practice question

Glucose preference is mediated by phosphotransferase system regulation of cyclic AMP metabolism and inducer exclusion. When extracellular glucose high, EIIA-Glc component remains dephosphorylated, inhibiting adenylate cyclase and activating phosphodiesterase, resulting in sharp drop in intracellular cAMP to low micromolar levels. Low cAMP leaves CAP mainly apo unable to bind upstream site, so even if LacI derepressed by lactose, activation remains poor, producing catabolite repression and diauxic growth. High glucose therefore corresponds to low cAMP and reduced lac transcription despite presence of lactose inducer signal integration.