FIGURE 9-7 Transcriptional control by regulation of RNA polymerase elongation and termination in the E. coli trp operon. (a) Diagram of the 140-nucleotide trp leader RNA. The numbered regions are critical to attenuation. (b) Translation of the trp leader sequence begins near the 5′ end soon after it is transcribed, while transcription of the rest of the polycistronic trp mRNA molecule continues. At high concentrations of charged tRNA^Trp, formation of the 3–4 stem-loop followed by a series of uracils causes termination of transcription. At low concentrations of charged tRNA^Trp, region 3 is sequestered in the 2–3 stem-loop and cannot base-pair with region 4. In the absence of the stem-loop structure required for termination, transcription of the trp operon continues. See C. Yanofsky, 1981, Nature 289:751.