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

摘要

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 lacI^Q are unstable, acquiring mutations in lacI^Q, 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 ΔsdaA strain (pserACB) accumulates only traces of l-serine, with the C. glutamicum ΔpabABCΔsdaA 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.