Enhancing tryptophan production by balancing precursors in Escherichia coli

Tryptophan, an essential aromatic amino acid, is widely used in animal feed, food additives, and pharmaceuticals. Although sustainable and environmentally friendly, microbial tryptophan production from renewable feedstocks is limited by low biosynthesis and transport rates. Here, an Escherichia coli strain capable of efficient tryptophan production was generated by improving and balancing the supply of precursors and by engineering membrane transporters. Tryptophan biosynthesis was increased by eliminating negative regulatory factors, blocking competing pathways, and preventing tryptophan degradation. Promoter engineering balanced the supply of the precursors erythrose-4-phosphate and phosphoenolpyruvate, as well as the availability of serine. Finally, the engineering of tryptophan transporters prevented feedback inhibition and growth toxicity. Fed-batch fermentation of the final strain (TRP12) in a 5 L bioreactor produced 52.1 g·L^-1 of tryptophan, with a yield of 0.171 g·g^-1 glucose and productivity of 1.45 g·L^-1 ·h^-1 . The metabolic engineering strategy described here paves the way for high-performance microbial cell factories aimed at the production of tryptophan as well as other valuable chemicals.