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

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 (Phec) 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 Phec plasmid.ppc− strains produced less cell mass and more acetate, pyruvate, and phenylalanine (in the Phec strains) than did isogenicppc+ strains. Lactate and ethanol production were not detected in any of the strains. Phec strains produced less acetate and pyruvate than their Phe0 homologs. Importantly,ppc−/Phec produced at least six times as much phenylalanine (0.32 g phenylalanine/g dry weight cells) asppc+/Phec. 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.