Final answer:
A trpR- mutation in the trp operon prevents the repressor from inhibiting transcription, leading to an active operon and tryptophan synthesis, even in the presence of tryptophan.
Step-by-step explanation:
In the case of the trp operon with a trpR- mutation, which either prevents the repressor from binding with tryptophan or inhibits repressor formation entirely, the operon's behavior in the presence of tryptophan would differ from normal. Normally, when tryptophan is present, it binds to the trp repressor, allowing the repressor to bind to the operator and block transcription. However, with the trpR- mutation, the repressor cannot bind to the operator, regardless of tryptophan availability. Therefore, in the presence of tryptophan, we would expect the trp operon to be transcriptionally active, since the defective repressor cannot bind to the operator to inhibit transcription.
The trp operon is typically negatively regulated by the tryptophan-bound repressor. Without a functioning repressor, the operon is active, and the structural genes are transcribed, resulting in the synthesis of tryptophan even when it is not needed. Hence, the mutation disrupts the normal feedback inhibition that regulates the synthesis of tryptophan in the cell.