Abundant tryptophan attenuates transcription of the trp operon

Model for attenuation in the trp operon. (a) Proposed secondary structures in the conformation of trp leader mRNA that favors termination of transcription. Four regions can base-pair to form three stem-and-loop structures, but only two regions base-pair with one another at a given time. Thus, region 2 can base-pair with either region 1 or region 3. (b) When tryptophan is abundant, segment 1 of the trp mRNA is translated. Segment 2 enters the ribosome (although it is not translated), which enables segments 3 and 4 to base-pair. This base-paired region causes RNA polymerase to terminate transcription. (c) In contrast, when tryptophan is scarce, the ribosome is stalled at the codons of segment 1. Segment 2 interacts with segment 3 instead of being drawn into the ribosome, and so segments 3 and 4 cannot pair. Consequently, transcription continues.

[Data from D. L. Oxender, G. Zurawski, and C. Yanofsky, Proc. Natl. Acad. Sci. USA 76, 1979, 5524]