In an inducible operon, the inducer is often the __________ in the pathway being regulated; the inducer binds to the __________, which then becomes __________.

• A. end product; corepressor; inactive
• B. end product; repressor; inactive
• C. substrate; corepressor; inactive
• D. substrate; repressor; active
• E. substrate; repressor; inactive

Answer: (E)

Verified step by step guidance

1. Understand the concept of an inducible operon: An inducible operon is a segment of DNA that can be turned on by the presence of a specific molecule, known as the inducer. This type of operon is typically involved in catabolic pathways, where the presence of a substrate triggers the expression of genes needed to metabolize that substrate.
2. Identify the role of the inducer: In an inducible operon, the inducer is usually the substrate of the pathway. The presence of this substrate indicates that the pathway should be active to break down the substrate.
3. Determine the interaction between the inducer and the repressor: The inducer binds to the repressor protein. In the absence of the inducer, the repressor is bound to the operator region of the operon, preventing transcription.
4. Analyze the effect of inducer binding: When the inducer binds to the repressor, it causes a conformational change in the repressor, rendering it inactive. This means the repressor can no longer bind to the operator, allowing transcription to proceed.
5. Conclude the correct sequence: Based on the understanding of the inducible operon mechanism, the inducer is the substrate, it binds to the repressor, and this binding inactivates the repressor, allowing gene expression to occur.