Rewiring glycerol metabolism for enhanced production of poly-γ-glutamic acid in Bacillus licheniformis

BackgroundPoly--glutamic acid (-PGA) is a natural polymer with great potential applications in areas of agriculture, industry, and pharmaceutical. The biodiesel-derived glycerol can be used as an attractive feedstock for -PGA production due to its availability and low price; however, insufficient production of -PGA from glycerol is limitation.ResultsThe metabolic pathway of Bacillus licheniformis WX-02 was rewired to improve the efficiency of glycerol assimilation and the supply of NADPH for -PGA synthesis. GlpK, GlpX, Zwf, and Tkt1 were found to be the key enzymes for -PGA synthesis using glycerol as a feedstock. Through combinational expression of these key enzymes, the -PGA titer increased to 19.201.57g/L, which was 1.50-fold of that of the wild-type strain. Then, we studied the flux distributions, gene expression, and intracellular metabolites in WX-02 and the recombinant strain BC4 (over-expression of the above quadruple enzymes). Our results indicated that over-expression of the quadruple enzymes redistributed metabolic flux to -PGA synthesis. Furthermore, using crude glycerol as carbon source, the BC4 strain showed a high productivity of 0.38g/L/h, and produced 18.41g/L -PGA, with a high yield of 0.46g -PGA/g glycerol.Conclusions The approach to rewiring of metabolic pathways enables B. licheniformis to efficiently synthesize -PGA from glycerol. The -PGA productivity reported in this work is the highest obtained in glutamate-free medium. The present study demonstrates that the recombinant B. licheniformis strain shows significant potential to produce valuable compounds from crude glycerol.