When Does the Trp Repressor Block Transcription of the Trp

Question

When does the trp repressor block transcription of the trp operon?
A)when TrpR binds to the inducer
B)when TrpR binds to tryptophan
C)when TrpR is not bound to tryptophan
D)when TrpR is not bound to the operator
E)when TrpR binds to the promoter

Accepted Answer

The trp repressor protein (TrpR) binds to the operator region of the trp operon's promoter when it is not bound to tryptophan. This blocks RNA polymerase from binding to the promoter and transcribing the genes involved in tryptophan biosynthesis. As the concentration of tryptophan increases in the cell, some of it binds to TrpR, causing a conformational change that allows the repressor to dissociate from the operator, thus allowing transcription to occur. Therefore, when TrpR binds to tryptophan, it blocks transcription of the trp operon.