In the trp operon, presence of tryptophan results in which effect on transcription?

In the trp operon, presence of tryptophan results in which effect on transcription?

• A. Repression of transcription
• B. Activation of transcription
• C. No effect
• D. Induction

Answer: (A)

The question hinges on how a repressible operon is controlled by a co-repressor. In the trp operon, when tryptophan is plentiful, it acts as a co-repressor by binding to the trp repressor protein and activating it. The activated repressor then binds to the operator and blocks RNA polymerase from initiating transcription, so the genes needed to synthesize tryptophan are not transcribed. If tryptophan levels drop, the repressor remains inactive and transcription proceeds to make more tryptophan. So, the presence of tryptophan leads to repression of transcription.

The question hinges on how a repressible operon is controlled by a co-repressor. In the trp operon, when tryptophan is plentiful, it acts as a co-repressor by binding to the trp repressor protein and activating it. The activated repressor then binds to the operator and blocks RNA polymerase from initiating transcription, so the genes needed to synthesize tryptophan are not transcribed. If tryptophan levels drop, the repressor remains inactive and transcription proceeds to make more tryptophan. So, the presence of tryptophan leads to repression of transcription.