Abundant tryptophan attenuates transcription of the trp operon
Figure 11-23: 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]
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