Metabolic engineering of Escherichia coli to efficiently produce monophosphoryl lipid A

Monophosphoryl lipid A (MPL), mainly isolated from Salmonella minnesota R595, has been used as adjuvant in several vaccines. In this study, an Escherichia coli strain that can efficiently produce the MPL has been constructed. The gene clusters related to the biosynthesis of O-antigen, core oligosaccharide, enterobacterial common antigen, and colanic acid were sequentially removed to save the carbon source and to increase the activity of PagP in E. coli MG1655. Then, the genes pldA, mlaA, and mlaC related to the phospholipid transport system were further deleted, resulting in the strain MW012. Finally, the genes lpxE from Francisella novicida and pagP and pagL from Salmonella were overexpressed in MW012 to modify the structure of lipid A, resulting in the strain MW012/pWEPL. Lipid A species were isolated from MW012/pWEPL and analyzed by thin-layer chromatography and liquid chromatography-mass spectrometry. The results showed that mainly two MPL species were produced in E. coli MW012/pWEPL, one is hexa-acylated, and the other is penta-acylated. More importantly, the proportion of the hexa-acylated MPL, which is the most effective component of lipid A vaccine adjuvant, reached 75%. E. coli MW012/pWEPL constructed in this study provided a good alternative for the production of lipid A vaccine adjuvant MPL.