Final answer:
In the trpR- strain, which has a non-functional trp repressor, transcription is not halted in the presence of tryptophan because the defective repressor cannot bind to the operator of the trp operon to block transcription.
Step-by-step explanation:
The question pertains to the trpR- strain of bacteria, specifically how it regulates transcription in the presence and absence of tryptophan. In the absence of tryptophan, the trp repressor protein does not bind to the trp operator, hence RNA synthesis proceeds. However, in a trpR- strain, which lacks a functional trp repressor, transcription would not be halted by the presence of tryptophan, because the dysfunctional repressor cannot bind to the trp operator and block transcription.
When tryptophan is present in a normal strain, it binds to the trp repressor, changing its shape and enabling it to attach to the operator, blocking the RNA polymerase from transcribing the genes necessary for tryptophan synthesis. But, since the trpR- strain lacks this regulatory mechanism, it would continue transcription even when external tryptophan is available.