What happens in catabolite repression?
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
Ref: Prescott's Microbiology, 11th ed., Chapter 6: Catabolite Repression and Glucose Preference in Bacteria.
