Practice question

Glucose represents most energetically efficient carbon source giving maximal ATP per bond and direct entry into glycolysis via phosphotransferase system without induction cost. Bacteria evolved catabolite repression as global control suppressing alternative substrate utilization when glucose present to conserve resources. In Escherichia coli mechanism involves drop in second messenger cAMP due to inhibition of adenylate cyclase CyaA by dephosphorylated EIIA-Glc during glucose transport and prevention of active CRP-cAMP complex formation that binds conserved DNA sequence upstream of lac, araBAD, mal operons encoding alternative carbon catabolic enzymes and transporters. Inducer exclusion also occurs via EIIA-Glc binding LacY lactose permease blocking import. Consequently alternative carbon pathways remain silent while glucose abundant; when glucose exhausted cAMP rises five- to ten-fold, CRP-cAMP binds DNA and alternative operons become inducible after brief protein synthesis lag producing diauxic growth pattern with two exponential phases. Glucose catabolism itself continues and is prioritized; sporulation is distinct Spo0A-controlled developmental program, amino acid interconversion reflects nitrogen control but textbook catabolite repression focuses on glucose-mediated repression of alternative carbon utilization to maximize growth rate and efficiency.