Suppose an operon has the following characteristics: (1) The operon codes for structural proteins that convert compound Q to compound B. (2) The operon is controlled by a constitutively expressed regulatory gene called reg. (3) In wild-type individuals, the operon is transcribed in the absence of compound B but not in the presence of compound B. (4) In reg- mutants, the operon is constitutively transcribed. Is this operon inducible or repressible? Why?
The Correct Answer and Explanation is:
The correct answer is: Repressible operon.
Explanation:
An operon can be either inducible or repressible depending on how it is regulated by external or internal stimuli.
- Inducible operons are usually off but can be turned on in the presence of a specific inducer (e.g., the lac operon).
- Repressible operons are typically on but can be turned off when a specific corepressor is present (e.g., the trp operon).
Let’s analyze the scenario:
- The operon codes for enzymes that convert compound Q to compound B.
This suggests compound B is the end product of the pathway. - The operon is regulated by a constitutively expressed gene called reg.
This means the reg gene is always transcribed and translated, producing a regulatory protein continuously. - In wild-type individuals, the operon is transcribed in the absence of compound B but not in its presence.
This is key. It indicates that compound B, when present, inhibits transcription. Thus, compound B is likely acting as a corepressor. It binds to the regulatory protein (produced by reg), converting it into an active repressor that blocks operon transcription. - In reg⁻ mutants, the operon is constitutively transcribed.
In these mutants, the regulatory protein is nonfunctional or absent. Without it, compound B can’t exert its repressive effect. As a result, the operon remains on regardless of compound B’s presence, confirming that reg encodes a repressor.
This system aligns with a repressible operon model, where the operon is normally on but can be turned off in the presence of the end product (compound B), which acts as a feedback inhibitor.
Thus, the operon is repressible, and compound B serves as a corepressor working with the regulatory protein to inhibit transcription when sufficient product has been synthesized.