Metabolic engineering of Corynebacterium glutamicum for L-serine production.

Although L-serine proceeds in just three steps from the glycolytic intermediate 3-phosphoglycerate, and as much as 8% of the carbon assimilated from glucose is directed via L-serine formation, previous attempts to obtain a strain producing L-serine from glucose have not been successful. We functionally identified the genes serC and serB from Corynebacterium glutamicum, coding for phosphoserine aminotransferase and phosphoserine phosphatase, respectively. The overexpression of these genes, together with the third biosynthetic serA gene, serA(delta197), encoding an L-serine-insensitive 3-phosphoglycerate dehydrogenase, yielded only traces of L-serine, as did the overexpression of these genes in a strain with the L-serine dehydratase gene sdaA deleted. However, reduced expression of the serine hydroxymethyltransferase gene glyA, in combination with the overexpression of serA(delta197), serC, and serB, resulted in a transient accumulation of up to 16 mM L-serine in the culture medium. When sdaA was also deleted, the resulting strain, C. glutamicum delta sdaA::pK18mobglyA'(pEC-T18mob2serA(delta197)CB), accumulated up to 86 mM L-serine with a maximal specific productivity of 1.2 mmol h(-1) g (dry weight)(-1). This illustrates a high rate of L-serine formation and also utilization in the C. glutamicum wild type. Therefore, metabolic engineering of L-serine production from glucose can be achieved only by addressing the apparent key position of this amino acid in the central metabolism.