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 tryptophan in the trp operon system. Tryptophan acts as a corepressor in this system.
2. Recognize that in the absence of tryptophan, the repressor protein remains inactive because it cannot bind to the operator without tryptophan.
3. Identify that an inactive repressor means it cannot bind to the operator region of the trp operon.
4. Conclude that if the repressor is not bound to the operator, RNA polymerase can bind to the promoter and initiate transcription of the trp operon.
5. Summarize that in the absence of tryptophan, the trp operon is transcribed because the inactive repressor does not block transcription.