Practice question

Catabolite repression establishes hierarchical sugar utilization favoring glucose over lactose for optimal growth efficiency. Elevated glucose transport through phosphotransferase system converts EIIA-Glc to dephosphorylated form that directly inhibits LacY permease activity and reduces adenylate cyclase activation, substantially decreasing intracellular cyclic AMP concentration. Without sufficient cAMP, catabolite activator protein CAP remains as inactive dimer unable to bind its target DNA site centered at -61.5 relative to lac promoter transcription start. RNA polymerase holoenzyme affinity for promoter falls, initiation frequency declines approximately fiftyfold, and lactose metabolism remains silenced even when lactose present, ensuring preferential glucose consumption.