Improved L-lysine yield with Corynebacterium glutamicum: use of dapA resulting in increased flux combined with growth limitation.

The amino acid L-lysine is produced on a large scale using mutants of Corynebacterium glutamicum. However, as yet recombinant DNA techniques have not succeed in improving strains selected for decades by classic mutagenesis for high productivity. We here report that seven biosynthetic enzymes were assayed and oversynthesis of the dihydrodipicolinate synthase resulted in an increase of lysine accumulation from 220 mM to 270 mM. The synthase, encoded by dapA, is located at the branch point of metabolite distribution to either lysine or threonine and competes with homoserine dehydrogenase for the common substrate aspartate semialdehyde. When graded dapA expression was used, as well as quantification of enzyme activities, intracellular metabolite concentrations and flux rates, a global response of the carbon metabolism to the synthase activity became apparent: the increased flux towards lysine was accompanied by a decreased flux towards threonine. This resulted in a decreased growth rate, but increased intracellular levels of pyruvate-derived valine and alanine. Therefore, modulating the flux at the branch point results in an intrinsically introduced growth limitation with increased intracellular precursor supply for lysine synthesis. This does not only achieve an increase in lysine yield but this example of an intracellularly introduced growth limitation is proposed as a new general means of increasing flux for industrial metabolite over-production.