Practice question

Catabolite repression represents global regulatory strategy ensuring efficient use of carbon sources by prioritizing best substrate. In presence of rapidly metabolizable glucose, intracellular cyclic AMP cAMP falls dramatically due to inhibition of adenylate cyclase CyaA by dephosphorylated EIIA-Glc component of phosphotransferase system and increased phosphodiesterase activity. Low cAMP prevents formation of active CRP-cAMP complex that binds conserved motif TGTGA upstream of catabolic operons such as lacZYA for lactose, araBAD for arabinose and malEFG for maltose required for alternative sugar uptake and catabolism. Additionally inducer exclusion occurs where EIIA-Glc directly binds lactose permease LacY and maltose transporter inhibiting them, reducing influx of secondary sugars to negligible. Net result is glucose is preferentially utilized generating maximal growth rate and secondary pathways remain repressed until glucose depletes, at which time cAMP rises and operons derepress allowing growth on alternative sugars. This regulation prevents wasteful enzyme synthesis, produces diauxic growth biphasic pattern and influences virulence and biofilm formation in pathogens adapting to host environments where glucose availability fluctuates. Blocking anaerobiosis or glycolysis broadly would harm cell so repression selectively targets alternative catabolic gene expression.