Which statement correctly describes a repressible operon?

Which statement correctly describes a repressible operon?

• A. Transcription is always on regardless of small molecules
• B. Transcription is usually on but can be inhibited when a specific small molecule binds allosterically to a regulatory protein
• C. Transcription is turned off only when an inducer binds the operator
• D. Transcription is inhibited by the inducer

Answer: (B)

Repressible operons are typically active, and their expression is down-regulated by a small molecule acting as a corepressor that binds to a regulatory protein. When this molecule binds, it changes the regulatory protein’s shape (allosteric change) so the protein can bind to the operator and block RNA polymerase, turning transcription off. That’s why the statement describes transcription as usually on and being inhibited when a specific small molecule binds to the regulatory protein. A classic example is the tryptophan (trp) operon: when tryptophan levels are high, tryptophan binds the repressor, activating it to bind the operator and stop producing the enzymes needed to synthesize more tryptophan. This contrasts with inducible operons, where an inducer binds to the repressor to relieve repression and turn on transcription.

Repressible operons are typically active, and their expression is down-regulated by a small molecule acting as a corepressor that binds to a regulatory protein. When this molecule binds, it changes the regulatory protein’s shape (allosteric change) so the protein can bind to the operator and block RNA polymerase, turning transcription off. That’s why the statement describes transcription as usually on and being inhibited when a specific small molecule binds to the regulatory protein.

A classic example is the tryptophan (trp) operon: when tryptophan levels are high, tryptophan binds the repressor, activating it to bind the operator and stop producing the enzymes needed to synthesize more tryptophan. This contrasts with inducible operons, where an inducer binds to the repressor to relieve repression and turn on transcription.