In the presence of tryptophan, tryptophan binds to the ________.

• A. RNA polymerase
• B. Operator
• C. Promoter
• D. Trp repressor

Answer: (D)

In this scenario, tryptophan acts as a signal that turns off the production of its own biosynthetic enzymes in bacteria. It does this by binding to the Trp repressor protein, changing its shape so the repressor can bind to the operator region of the trp operon. Once the repressor–tryptophan complex sits on the operator, RNA polymerase is blocked from transcribing the genes needed to synthesize tryptophan, effectively repressing expression when the amino acid is plentiful. The operator is the DNA site where this complex binds; the promoter is the site where RNA polymerase would bind to start transcription, and RNA polymerase itself is the enzyme that carries out transcription, not the target of tryptophan binding.