Which statement accurately contrasts inducible and repressible operons with examples?

• A. Inducible operons are usually off and can be turned on by an inducer; repressible operons are usually on and can be shut off when a corepressor is present.
• B. Inducible operons are always on; repressible operons are always off.
• C. Inducible operons respond to repressors to reduce transcription; repressible operons respond to activators to increase transcription.
• D. Inducible operons require enhancers; repressible operons do not.

Answer: (A)

The situation tests how inducible and repressible operons differ in their default state and what changes transcription. Inducible operons are typically off because a repressor is bound to the operator, blocking transcription. An inducer molecule binds the repressor, inactivating it so RNA polymerase can proceed with transcription. A classic example is the lac operon, which is off in the absence of lactose and turned on when lactose (or an analog like IPTG) is present.

Repressible operons, on the other hand, are usually on. Their transcription is shut down when a corepressor binds to the repressor, enabling it to bind the operator and block transcription. The trp operon illustrates this: when tryptophan levels are high, tryptophan acts as a corepressor, activating the repressor to stop synthesis; when levels fall, the repressor is inactive and transcription continues.

So the correct statement captures that inducible operons start off and can be turned on by an inducer, while repressible operons start on and can be turned off by a corepressor. The other options misstate the typical states or rely on mechanisms (like enhancers) that aren’t how bacterial operons are regulated.