Inducible expression requires what to turn on or off?

• A. A promoter constant regardless of conditions
• B. Only a gene coding region determines induction
• C. A stimulus such as a chemical inducer is required to turn expression on or off
• D. Expression is never regulated

Answer: (C)

Inducible expression hinges on a regulatory signal that controls transcription in response to conditions. In these systems, a stimulus—often a chemical inducer—binds to a regulatory protein or interacts with the promoter to relieve repression or enhance activation, turning transcription on or off. Without this signal, the gene remains off or at a low baseline level. So the inducer is the trigger that makes expression switch.

If the promoter were constant and always active, expression would be constitutive and not inducible. If only the coding region determined induction, there would be no regulatory input to turn expression on or off. And saying expression is never regulated would ignore the regulatory mechanisms that control inducible systems.