NADPH-Auxotrophic E. coli: A Sensor Strain for Testing in Vivo Regeneration of NADPH.

Insufficient rate of NADPH regeneration often limits the activity of biosynthetic pathways. Expression of NADPH-regenerating enzymes is commonly used to address this problem and increase cofactor availability. Here, we construct an Escherichia coli NADPH-auxotroph strain, which is deleted in all reactions that produce NADPH with the exception of 6-phosphogluconate dehydrogenase. This strain grows on a minimal medium only if gluconate is added as NADPH source. When gluconate is omitted, the strain serves as a "biosensor" for the capability of enzymes to regenerate NADPH in vivo. We show that the NADPH-auxotroph strain can be used to quantitatively assess different NADPH-regenerating enzymes and provide essential information on expression levels and concentrations of reduced substrates required to support optimal NADPH production rate. The NADPH-auxotroph strain thus serves as an effective metabolic platform for evaluating NADPH regeneration within the cellular context.