In the absence of tryptophan, the trp repressor is ______.

In the absence of tryptophan, the trp repressor is ______.

• A. Inactive and cannot bind to the operator
• B. Active and binds to the operator
• C. Inactive but can bind to the operator
• D. Active and cannot bind to the operator

Answer: (A)

The main idea here is that the trp repressor needs a corepressor to become active. In the tryptophan operon, tryptophan binds to the repressor and activates it, allowing the repressor to bind the operator and block transcription. Without tryptophan, the repressor stays inactive and cannot bind to the operator, so RNA polymerase can proceed with transcribing the genes needed to make tryptophan. That’s why the statement describing the repressor as inactive and unable to bind the operator best fits the situation when tryptophan is absent.

The main idea here is that the trp repressor needs a corepressor to become active. In the tryptophan operon, tryptophan binds to the repressor and activates it, allowing the repressor to bind the operator and block transcription. Without tryptophan, the repressor stays inactive and cannot bind to the operator, so RNA polymerase can proceed with transcribing the genes needed to make tryptophan. That’s why the statement describing the repressor as inactive and unable to bind the operator best fits the situation when tryptophan is absent.