In the absence of tryptophan, ______:

• A. The inducer cannot bind to the operator, so trp operon transcription occurs.
• B. The active repressor cannot bind to the operator, so trp operon transcription is reduced.
• C. The inactive repressor cannot bind to the operator, so trp operon transcription occurs.
• D. The repressor binds to the corepressor, and trp operon transcription occurs.
• E. The active repressor binds to the operator, so trp operon transcription is repressed.

Answer: (C)

Verified step by step guidance

1. Understand the role of the trp operon: The trp operon is a group of genes used by bacteria to synthesize tryptophan. It is regulated by a repressor protein that can bind to the operator region of the operon to inhibit transcription.
2. Identify the role of tryptophan: Tryptophan acts as a corepressor. When tryptophan is present, it binds to the repressor protein, activating it. The active repressor then binds to the operator, blocking RNA polymerase and preventing transcription.
3. Consider the scenario in the absence of tryptophan: Without tryptophan, the repressor protein remains inactive because it cannot bind to the operator without the corepressor (tryptophan).
4. Determine the effect on transcription: Since the inactive repressor cannot bind to the operator, RNA polymerase can bind to the promoter and transcribe the genes of the trp operon, leading to the production of enzymes for tryptophan synthesis.
5. Conclude the correct statement: In the absence of tryptophan, the inactive repressor cannot bind to the operator, so trp operon transcription occurs.