Gene order in the trp operon corresponds to reaction order in the biosynthetic pathway trpE trpD trpC trpB trpA H H coOOHO HO-ç-c-c-CH,O®- CHO N-CH H NH, C-c-COOH COOH COOH COOH -CH,O® нн HO L-Glutamine NH, PRPP H,C-č-COOH L-Tryptophan нн Indole-3-glycerol phosphate Anthranilic CDRP Phosphoribosyl anthranilic acid L-Šerine Chorismic acid acid Indole binds an operator, preventing the initiation of transcription. This repressor is the Trp repressor, the product of the trpR gene. The Trp repressor binds tryptophan when adequate levels of the amino acid are present, and only after binding tryp- tophan will the Trp repressor bind to the operator and switch off transcription of the operon. This simple mechanism ensures that the cell does not waste energy producing tryptophan when the amino acid is sufficiently abundant. E. coli strains with mutations in trpR continue to express the trp mRNA and thus continue to produce tryptophan when the amino acid is abundant. In studying these trpR mutant strains, Charles Yanofsky discovered that, when tryptophan was removed from the medium, the production of trp mRNA further increased several-fold. This finding was evidence that, in addition to the Trp repressor, a second control mechanism existed to negatively regulate transcrip- tion. This mechanism is called attenuation because mRNA production is nor- mally attenuated, meaning "decreased," when tryptophan is plentiful. Unlike the other bacterial control mechanisms described thus far, attenuation acts at a step after transcription initiation. The mechanisms governing attenuation were discovered by identifying muta- tions that reduced or abolished attenuation. Strains with these mutations pro- duce trp mRNA at maximal levels even in the presence of tryptophan. Yanofsky FIGURE 11-21 The chromosomal order of genes in the trp operon of E. col and the sequence of reactions catalyzed by the enzyme products of the trp structural genes. The products of genes trpD and trpE form a complex that catalyzes specific steps, as do the products of genes trpB and trpA. Tryptophan synthetase is a tetrameric enzyme formed by the products of trpB and trpA. It catalyzes a two-step process leading to the formation of tryptophan. Abbreviations: PRPP, phosphoribosylpyrophosphate; CDRP, 1-40-carboxyphenylamino)-1-deoxyribulose 5-phosphate. [Data from S. Tanemura and R. H. Bauerle, Genetics 95, 1980, 545]