Fig. 1: Major metabolic pathways associated with PN biosynthesis in E. coli, metabolic engineering approaches applied to overproduce PN, and carbon flux from glycerol toward PNP starting from glycerol.

Blue arrows indicate the DXP-dependent PN biosynthetic routes in E. coli. The orange dotted box represents a heterologous DXP-independent pathway. The deleted gene is labeled with a red cross. Gray squares represent the metabolic fluxes for maximum biomass formation, and the numbers in squares represent the metabolic fluxes of reactions. The uptake rate of glycerol was defined as 10 mmol gDCW⁻¹ h⁻¹. Source data are provided as a Source Data file. Enzymes: Epd erythrose 4-phosphate dehydrogenase, PdxB 4-phosphoerythronate dehydrogenase, SerC 3-phosphoserine aminotransferase, PdxA 4-phosphohydroxy-l-threonine dehydrogenase, PdxJ PNP synthase, Dxs 1-deoxyxylulose 5-phosphate synthase, PdxP PNP phosphatase, PdxH PNP oxidase, PdxK PN/PL/PM kinase, PdxY PL kinase, PdxI PL reductase. Metabolites: E4P erythrose 4-phosphate, 4PE 4-phosphoerythronate, OHPB 2-oxo-3-hydroxy-4-phosphobutanoate. 4HTP 4-phosphohydroxy-_L-threonine, PHA 3-phosphohydroxy-1-aminoacetone, DHAP dihydroxyacetone phosphate, G3P glyceraldehyde 3-phosphate, 1,3DPG 1,3-bisphospho-d-glycerate, Pyr pyruvate, AcCoA acetyl-CoA, Glu glutamate, Gln glutamine, R5P ribose 5-phosphate, Ru5P d-ribulose 5-phosphate. Species: Eme Ensifer meliloti.