Which statement correctly describes constitutive vs inducible promoters in bacteria?

• A. Constitutive promoters drive constant transcription; inducible promoters require a signal to initiate transcription
• B. Inducible promoters drive constant transcription
• C. Constitutive promoters drive constant transcription
• D. Inducible promoters cannot be regulated by repressors

Answer: (A)

The key idea is how promoters respond to signals to control transcription in bacteria. Constitutive promoters are active most of the time and produce RNA at a steady rate, regardless of environmental cues. Inducible promoters stay off or at a low baseline until a specific signal appears, which triggers transcription initiation. In many bacterial systems, inducible control involves a repressor bound to the promoter or operator; when the inducer is present, the repressor is inactivated or removed, allowing RNA polymerase to proceed with transcription. A classic example is the lac promoter, where the presence of lactose or an analog like IPTG relieves repression and promotes transcription.

So, describing constitutive promoters as driving constant transcription and inducible promoters as requiring a signal to initiate transcription directly contrasts the two modes of regulation and best captures their difference. Inducible promoters can indeed be regulated by repressors, and saying they cannot would be inaccurate; also, inducible promoters do not drive constant transcription, which would contradict their defining feature.