This chapter contains a lengthy discussion of the trp operon involved in regulating the synthesis of tryptophan. In the case of E. coli, the bacterium may need to synthesize tryptophan, or its host, a human, may supply all the tryptophan it needs. Why do both bacteria and humans need tryptophan?

• A. It is important in inducing sleep.
• B. It is an important constituent of cell walls in both humans and bacteria.
• C. Tryptophan induces the lac operon.
• D. It is an important enzyme in the glycolysis pathway.
• E. It is one of the 20 amino acids used by all organisms to synthesize proteins.