Production of Aspartic Acid and Lysine by Citrate Synthase Mutants of Brevibacterium flavum

Abstract

A prototrophic revertant was derived from a citrate synthase-defective glutamate auxotroph of Brevibacterium flavum. It showed low citrate synthase activity and overproduced L-aspartic acid. The maximum production was 10.6 g/liter (about a 30% yield) when the strain was cultured in medium containing 36 g/liter of glucose for 48 hr. The amount of aspartic acid produced depended greatly on the concentration of biotin in the medium, as the case of glutamate production by the original wild strain. When the revertant strain was cultured with excess biotin, acetic acid accumulated instead of aspartic acid. A mutant resistant to S-(2-aminoethyl)-L-cysteine plus threonine derived from the revertant strain produced 36 g/liter of L-lysine (as hydrochloride salt) when it was cultured in medium containing 100 g/liter of glucose for 72 hr (36% yield). The amount of lysine production by this mutant was greater than those by the resistant mutants derived directly from the original wild strain. The frequency of the appearance of mutants which produced more than 30 g/liter of lysine was about one per 250 strains of resistant mutants. A high concentration of ammonium sulfate was required for a high yield of lysine production.

A homoserine auxotroph derived from the revertant strain produced 33 g/liter of L-lysine (33% yield), which was almost the same amount as that derived directly from the original wild strain.