Question

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20. (Problem 16) in the bacterium Escherichia coll, the amino acid tryptophan is synthesized in a multistep pathway, beginning with an organic precursor called chorismate: Chorismate ——Y-Tryptophan 2 3 The numbers denote steps in the pathway, each catalyzed by an enzyme. There are five bacterial genes that encode polypeptides associated with these enzymes, called trpa, trp8, trpc, trp, and trpe. A researcher isolates a series of mutations that eliminate the capacity of the cells to synthesize tryptophan, each mutation affecting one of the five genes. Each mutant cell is tested for its ability to grow on a medium containing either tryptophan, chorismate, or intermediate X or Y. The following results are obtained (+ means growth; - means absence of growth; WT means wild type): trpA trp trpc trpD trpe WT Molecule in growth medium Tryptophan Chorismate + . . + + + + <xol + + . Which genes encode the enzymes involved in steps 1, 2, and 3 of the tryptophan biosynthetic pathway? Given this information, what can you say about the structure of these enzymes? Genes trp and trpe are involved in step 1, trp in step 2, and trpA and trp in step 3. The results suggest that the enzymes catalyzing steps 1 and 3 each consist of at least two polypeptides encoded by separate genes. Genes trpA and trp8 are involved in step 1, trp in step 2, and trpd and trpE in step 3. The results suggest that the enzymes catalyzing steps 1 and 3 each consist of at least two polypeptides encoded by separate genes. Genes trpA and trpe are involved in step 1, trp in step 2, and trp and trp in step 3. The results suggest that the enzymes catalyzing steps 1 and 3 each consist of at least two polypeptides encoded by separate genes. Gene trp is involved in step 1, trpc and trp in step 2, and trpA and trp8 in step 3. The results suggest that the enzymes catalyzing steps 1 and 3 cach consist of at least two polypeptides encoded by separate genes.