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 P-tac and lacI(Q) 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 Delta sdaA strain (pserACB) accumulates only traces Of L-serine, with the C. glutamicam Delta pabABC Delta sdaA 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.