Under what conditions does the trp repressor block transcription of the trp operon?

The trp repressor blocks transcription of the trp operon when the repressor binds to tryptophan. The trp repressor or the tryptophan repressor is a transcription factor that is involved in controlling amino acid metabolism. In the trp operon, tryptophan binds to the repressor protein thus effectively blocking gene transcription. 

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