Reduced Folate Supply as a Key to Enhanced l -Serine Production by Corynebacterium glutamicum

The amino acid L-serine is required for pharmaceutical purposes, and the availability of a sugar-based microbial process for its production is desirable. However, a number of intracellular utilization routes prevent overproduction of L-serine, with the essential serine hydroxymethyltransferase (SHMT) (glyA) probably occupying a key position. We found that constructs of Corynebacterium glutamicum strains where chromosomal glyA expression is dependent on Ptac and lacIQ are unstable, acquiring mutations in lacIQ, for instance. To overcome the inconvenient glyA expression control, we instead considered controlling SHMT activity by the availability of 5,6,7,8-tetrahydrofolate (THF). The pabAB and pabC genes of THF synthesis were identified and deleted in C. glutamicum, and the resulting strains were shown to require folate or 4-aminobenzoate for growth. Whereas the C. glutamicum DeltasdaA strain (pserACB) accumulates only traces of L-serine, with the C. glutamicum DeltapabABCDeltasdaA strain (pserACB), L-serine accumulation and growth responded in a dose-dependent manner to an external folate supply. At 0.1 mM folate, 81 mM L-serine accumulated. In a 20-liter controlled fed-batch culture, a 345 mM L-serine accumulation was achieved. Thus, an efficient and highly competitive process for microbial l-serine production is available.