Final answer:
When tryptophan concentrations are high in an E. coli cell, tryptophan activates the repressor protein that binds to the trp operon's operator, blocking transcription, and attenuation also contributes to stopping the operon's expression. Hence, the answer is D. All of the above.
Step-by-step explanation:
When tryptophan concentrations are high in an E. coli cell, several things happen:
- Tryptophan binds the repressor and it can repress trp operon transcription.
- Attenuation causes premature termination of transcription of the trp operon.
- The tryptophan synthesis genes are not transcribed.
Therefore, the correct answer is D. All of the above. This is because the presence of tryptophan activates the repressor, which then binds to the operator, blocking RNA polymerase from transcribing the operon. Additionally, attenuation also helps to ensure that when tryptophan is plentiful, the cell does not waste energy and resources by synthesizing more of it.