Practice question

Catabolite repression couples glucose availability to cyclic AMP synthesis via adenylate cyclase. When glucose transport is low, EIIA phosphorylated activates cyclase, raising cAMP levels. Cyclic AMP binds catabolite activator protein CAP, inducing conformational shift allowing dimerization and DNA recognition. The CAP-cAMP complex binds conserved sites near catabolite-sensitive promoters including lac, ara, gal, facilitating RNA polymerase recruitment through alpha-CTD contact. When glucose abundant, cAMP falls, CAP remains inactive, transcription of alternative sugar operons reduced. Thus CAP-cAMP binding specifically signals carbon starvation and glucose limitation, coordinating hierarchical sugar utilization for energy efficiency.