Cometabolism of a Nongrowth Substrate:l-Serine Utilization byCorynebacterium glutamicum

ABSTRACT Despite its key position in central metabolism, l -serine does not support the growth of Corynebacterium glutamicum . Nevertheless, during growth on glucose, l -serine is consumed at rates up to 19.4 ± 4.0 nmol min −1 (mg [dry weight]) −1 , resulting in the complete consumption of 100 mM l -serine in the presence of 100 mM glucose and an increased growth yield of about 20%. Use of 13 C-labeled l -serine and analysis of cellularly derived metabolites by nuclear magnetic resonance spectroscopy revealed that the carbon skeleton of l -serine is mainly converted to pyruvate-derived metabolites such as l -alanine. The sdaA gene was identified in the genome of C. glutamicum , and overexpression of sdaA resulted in (i) functional l -serine dehydratase ( l -SerDH) activity, and therefore conversion of l -serine to pyruvate, and (ii) growth of the recombinant strain on l -serine as the single substrate. In contrast, deletion of sdaA decreased the l -serine cometabolism rate with glucose by 47% but still resulted in degradation of l -serine to pyruvate. Cystathionine β-lyase was additionally found to convert l -serine to pyruvate, and the respective metC gene was induced 2.4-fold under high internal l -serine concentrations. Upon sdaA overexpression, the growth rate on glucose is reduced 36% from that of the wild type, illustrating that even with glucose as a single substrate, intracellular l -serine conversion to pyruvate might occur, although probably the weak affinity of l -SerDH (apparent K m , 11 mM) prevents substantial l -serine degradation.