Practice question

Lac operon regulation illustrates classic inducible catabolic control characterized by negative repression in absence of substrate to conserve energy. Default configuration maintains LacI repressor bound to operator, occluding polymerase progression and keeping transcription minimal and economy focused. Induction occurs when lactose metabolism generates allolactose, which allosterically inactivates LacI, allowing full derepression and transcription. This logic distinguishes inducible systems, normally OFF without inducer and ON with inducer, from repressible biosynthetic operons like tryptophan that are normally ON and shut by excess product, and from constitutive housekeeping genes expressed continuously regardless of external environment signals.