Practice question

Arabinose operon exemplifies inducible catabolic system controlling pentose sugar utilization in Escherichia coli. Genes remain transcriptionally silent without substrate because apo-AraC regulator forms DNA loop repressing PBAD promoter activity. Presence of L-arabinose sugar converts regulatory protein into transcriptional activator capable of recruiting polymerase. Operon therefore defaults off state, switched on only upon metabolite accumulation, similar regulatory logic to lac induction by allolactose inducer and distinct from trp repressible anabolic logic where end product shuts off. Additional control by glucose through CAP-cAMP complex ensures carbon source hierarchy maximizing growth efficiency while retaining ability to metabolize arabinose when preferred sugars depleted.