Production of phenylalanine and organic acids by phosphoenolpyruvate carboxylase-deficient mutants ofEscherichia coli

Summary Isogenic strains ofEscherichia coli were grown aerobically in minimal medium in a 2-liter airlift fermentor to determine whether appc (phosphoenolpyruvate carboxylase) mutation had the effect of directing glucose carbon into phenylalanine synthesis. Two host strains, YMC9 (ppc⁺) and KB285 (ppc^–) were used, either with (Phe^c) or without (Phe⁰) a plasmid which determines constitutive phenylalanine production. Carbon consumption and metabolic products were monitored. Phenylalanine production occurred only in strains carrying the Phe^c plasmid.ppc^– strains produced less cell mass and more acetate, pyruvate, and phenylalanine (in the Phe^c strains) than did isogenicppc⁺ strains. Lactate and ethanol production were not detected in any of the strains. Phe^c strains produced less acetate and pyruvate than their Phe⁰ homologs. Importantly,ppc^–/Phe^c produced at least six times as much phenylalanine (0.32 g phenylalanine/g dry weight cells) asppc⁺/Phe^c. Even in this case, however, phenylalanine was produced at ten-fold lower levels than acetate. Thus, although theppc^– mutation stimulates phenylalanine production, it also stimulates the production of unwanted by-products such as acetate and pyruvate.