Threonine Production by Dihydrodipicolinate Synthase-defective Mutants of Brevibacterium flavum

Abstract

Threonine-producing mutants of Brevibacterium flavum that lacked dihydrodipicolinate synthase (DPS) activity but had wild-type homoserine dehydrogenase (HD) sensitive to feedback inhibition by threonine were isolated as strains resistant to α-amino-β-bydroxyvaleric acid (AHV). The growth of a mutant, DK131, derived from an aspartate-producing strain was slow and markedly enhanced by diaminopimelic acid (DAP), but not by L-lysine alone. In a medium containing 10% glucose and the optimum concentration of DAP, strain DK131 produced 13.7 g/l of threonine, which was comparable to that by the previously reported mutant, BB69, with a feedback-resistant HD (HD^R). The production was more strongly inhibited by lysine in strain DK131 than in strain BB69. DPS-defective mutants derived from a lysine producer with feedback-insensitive aspartokinase were selected as those which were resistant to AHV and which produced more threonine than lysine. A representative strain, DA105, produced 10.3 g/l of threonine and 3.5 g/l of lysine•HC1, while AHV-resistant mutant, BA68, with an HD^R, derived from the same parent, produced 9.7 g/l of threonine and 12.2 g/l of lysine•HC1. Strain DA105 grew well in the absence of DAP.