Metabolic Engineering of Zymomonas mobilis for Xylonic Acid Production from Lignocellulosic Hydrolysate

Bio-based xylonic acid produced from inexpensive lignocellulosic biomass has enormous market potential and enhances the overall economic benefits of biorefinery processes. In this study, the introduction of genes encoding xylose dehydrogenase driven by the promoter Ppdc into Z. mobilis using a plasmid vector resulted in the accumulation of xylonic acid at a titer of 16.8 ± 1.6 g/L. To achieve stable xylonic acid production, a gene cassette for xylonic acid production was integrated into the genome at the chromosomal locus of ZMO0038 and ZMO1650 using the endogenous type I-F CRISPR-Cas system. The titer of the resulting recombinant strain XA3 reduced to 12.2 ± 0.56 g/L, which could be the copy number difference between the plasmid and chromosomal integration. Oxygen content was then identified to be the key factor for xylonic acid production. To further increase xylonic acid production capability, a recombinant strain, XA9, with five copies of a gene cassette for xylonic acid production was constructed by integrating the gene cassette into the genome at the chromosomal locus of ZMO1094, ZMO1547, and ZMO1577 on the basis of XA3. The titer of xylonic acid increased to 51.9 ± 0.1 g/L with a maximum yield of 1.10 g/g, which is close to the theoretical yield in a pure sugar medium. In addition, the recombinant strain XA9 is genetically stable and can produce 16.2 ± 0.14 g/L of xylonic acid with a yield of 1.03 ± 0.01 g/g in the lignocellulosic hydrolysate. Our study thus constructed a recombinant strain, XA9, of Z. mobilis for xylonic acid production from lignocellulosic hydrolysate, demonstrating the capability of Z. mobilis as a biorefinery chassis for economic lignocellulosic biochemical production.