Metabolic Engineering of Corynebacterium glutamicum for l -Serine Production

Abstract

ABSTRACT 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 Δ 197 , 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 Δ 197 , 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 Δ sdaA ::pK18mob glyA ′(pEC-T18mob2 serA Δ197 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.