Highly Efficient Production of D-xylonic Acid From Corn Stover Hydrolysate in Gluconobacter Oxydans By mGDH Overexpression

Background: D-xylonic acid is a versatile platform chemical with wide potential applications as the water reducer and disperser for cement and the precursor for 1,4-butanediol and 1,2,4-tributantriol. Microbial production of D-xylonic acid with bacteria like Gluconobacter oxydans from cheap lignocellulosic feedstock is generally regarded as one of the most promising and cost-effective methods to industrial production, but xylonic acid productivity is reduced by high substrate inhibition and hydrolysate inhibitors. Results: D-xylonic acid productivity of G. oxydans DSM2003 was improved by overexpressing the mGDH gene, which encoded the membrane-bound glucose dehydrogenase. Using the mutated plasmids based on pBBR1MCS-5 in our previous work, the recombinant strain G. oxydans/pBBR-R3510-mGDH with the significant improvement in D-xylonic acid production and the strengthened tolerance to hydrolysate inhibitors was obtained. The fed-batch biotransformation of D-xylose by this recombinant strain reached the record high titer (588.7 g/L), yield (99.4%), and space-time yield (8.72 g/L/h). Moreover, up to 246.4 g/L D-xylonic acid was produced directly from corn stover hydroxylates without detoxification at a yield of 98.9% and a space-time yield of 11.2 g/L/h. In addition, G. oxydans/pBBR-R3510-mGDH performed strong tolerance to typical inhibitors, i.e., formic acid, furfural, and 5-hydroxymethylfurfural. Conclusion: Through overexpression of mgdh in G. oxydans, we obtained a recombinant strain G. oxydans/pBBR-R3510-mGDH. It was capable to efficiently produce xylonic acid from the corn stover hydrolysate under high concentrations of inhibitors. The high D-xylonic acid productivity made G. oxydans/pBBR-R3510-mGDH an attractive choice for biotechnical production.

Negative effects characterization and comparative transcriptomics elucidation on the lag phase of an industrial S. cerevisiae under the corn stover hydrolysate stress

During biofuels fermentation from pretreated lignocellulosic biomass, the strong toxicity of the lignocellulose hydrolysate is resulted from the synergistic effect of multiple lignocellulosic inhibitors, which far exceeds the sum of effects caused by every single inhibitor. Meanwhile, the synergistic effect is unclear and the underlying response mechanism of the industrial yeast towards the actual pretreated lignocellulose hydrolysate is still under exploration. Here, we employed an industrial S. cerevisiae for the transcriptomic analysis in two time points (early and late) of the lag phase under the corn stover hydrolysate stress. As investigation, the corn stover hydrolysate caused the accumulation of reactive oxygen species (ROS), damages of mitochondrial membrane and endoplasmic reticulum (ER) membrane in the industrial S. cerevisiae YBA_08 during the lag phase, especially these negative effects were more significant at the early lag phase. Based on the transcriptome profile, the industrial S. cerevisiae YBA_08 might recruit stress-related transcription factors (MSN4, STE12, SFL1, CIN5, COM2, MIG3, etc.) through the mitogen-activated protein kinase (MAPK)-signaling pathway to induce a transient G1/G2 arrest, and to activate defense bioprocesses like protectants metabolism, sulfur metabolism, glutaredoxin system, thioredoxin system, heat shock proteins chaperone and oxidoreductase detoxification, resisting those compounded stresses including oxidative stress, osmotic stress and structural stress. Surprisingly, this defense system might be accompanied with the transient repression of several bioprocesses like fatty acid metabolism, purine de novo biosynthesis and ergosterol biosynthesis.ImportanceThis research systematically demonstrated the lag phase response of an industrial yeast to the lignocellulosic hydrolysate in transcriptional level, providing a molecular fundament for understanding the synergistic effect of various lignocellulosic inhibitors and the regulatory mechanism of tolerance for industrial yeasts under this stress.