Modular pathway engineering of key carbon-precursor supply-pathways for improved N-acetylneuraminic acid production in Bacillus subtilis.

N-acetylneuraminic acid (NeuAc) is widely used as a nutraceutical for facilitating infant brain development, maintaining brain health, and enhancing immunity. Currently, NeuAc is mainly produced by extraction from egg yolk and milk, or via chemical synthesis. However, its low concentration in natural resources and its non-ecofriendly chemical synthesis result in insufficient NeuAc production and environmental pollution, respectively. In this study, improved NeuAc production was attained via modular pathway engineering of the supply pathways of two key precursors-N-acetylglucosamine (GlcNAc) and phosphoenolpyruvate (PEP)-and by balancing NeuAc biosynthesis and cell growth in engineered Bacillus subtilis. Specifically, we used a previously constructed GlcNAc-producing B. subtilis as the initial host for NeuAc biosynthesis. First, we constructed a de novo NeuAc biosynthetic pathway utilizing glucose by coexpressing glucosamine-6-phosphate acetyl-transferase (GNA1), N-acetylglucosamine 2-epimerase (AGE), and N-acetylneuraminic acid synthase (NeuB), resulting in 0.33 g/l NeuAc production. Next, to balance the supply of the two key precursors for NeuAc biosynthesis, modular pathway engineering was performed. The optimal strategy for balancing the GlcNAc module and PEP supply module involved the use of an engineered, unique glucose and malate coutilization pathway in B. subtilis, supplied with both glucose (for the GlcNAc moiety) and malate (for the PEP moiety) at high strength. This led to 1.65 g/L NeuAc production, representing a 5.0-fold improvement over the existing methods. Furthermore, to enhance the NeuAc yield on cell, glucose and malate coutilization pathways were engineered to balance NeuAc biosynthesis and cell growth via the blocking of glycolysis, the introduction of the Entner-Doudoroff pathway, and the overexpression of the malic enzyme YtsJ. NeuAc titer reached 2.18 g/L, with 0.38 g/g dry cell weight NeuAc yield on cell, which represented a 1.32-fold and 2.64-fold improvement over the existing methods, respectively. The strategy of modular pathway engineering of key carbon precursor supply pathways via engineering of the unique glucose-malate coutilization pathway in B. subtilis should be generically applicable for engineering of B. subtilis for the production of other important biomolecules. Our study also provides a good starting point for further metabolic engineering to achieve industrial production of NeuAc by a Generally Regarded As Safe bacterial strain.