What is attenuation in the trp operon and how does it control gene expression?

• A. Attenuation uses a leader sequence with tryptophan codons to terminate transcription when tryptophan is scarce.
• B. Attenuation uses a leader sequence with tryptophan codons to terminate transcription when tryptophan is abundant.
• C. A repressor binds the operator to terminate transcription.
• D. Attenuation affects translation but not transcription in the tryptophan operon.

Answer: (B)

Attenuation hinges on a leader peptide that contains tryptophan codons and leverages transcription–translation coupling to sense amino acid levels. As RNA polymerase begins transcribing, a ribosome starts translating this leader. When tryptophan is abundant, the ribosome translates quickly through the leader, enabling formation of a terminator structure in the mRNA that causes transcription to terminate before the structural genes are transcribed. When tryptophan is scarce, the ribosome stalls at the tryptophan codons in the leader, preventing the terminator structure from forming and allowing RNA polymerase to continue into the operon. So, transcription is terminated when tryptophan is plentiful, and expression proceeds when it is scarce. This is the best description of attenuation in the trp operon. Repressor-based control or a mechanism that affects only translation would describe different regulatory modes.