[Engineering of a D-xylose metabolic pathway in eutropha W50].

Objective: This study aimed to broaden the substrate spectrum of Ralstonia eutropha W50 to use D-xylose, which can produce poly-beta-hydroxybutyrates (PHB) at a high level.
Methods: The D-xylose transporter gene xylE from Escherichia coli K-12 W3110 was cloned by PCR technique and integrated into the R. eutropha W50 chromosome. The recombinant strain W50-E was obtained. The D-xylose catabolic genes xylAB from E. coli K-12 W3110 and the promotor of PHA synthase gene phaC1 from R. eutropha H16 were cloned into pBBR1MCS to construct a recombinant plasmid. The plasmid was transformed into R. eutropha W50 and W50-E to generate the recombinant strains W50-AB and W50-EAB respectively. The characteristics of D-xylose utilization by W50-AB and W50-EAB were investigated.
Results: The expression of xylA and xylB genes in R. eutropha W50 was confirmed by enzyme assay. The recombinant strain W50-AB could grow on 0.1 mol/L D-xylose with the maximum specific growth rate of 0.025 h(-1), but no growth and D-xylose consumption were observed when cultivated on 0.01 mol/L D-xylose. The recombinant strain W50-EAB exhibited a faster growth than W50-AB on 0.1 mol/L D-xylose, with the maximum specific growth rate of 0.035 h(-1). Furthermore, it exhibited a slow but defined growth and D-xylose consumption on 0.01 mol/L D-xylose. The PHB content assay showed that both recombinant strains accumulated a small amount of PHB, with a proportion of 15.07 +/- 1.01% and 15.07 +/- 1.64% on the basis of dry cell weight respectively, by using D-xylose (0.1 mol/L) as substrate. And their final D-xylose-PHB conversion rates were 0.0920 g x g(-1) and 0.0838 g x g(-1) respectively, which were much lower than their glucose-PHB conversion rates( > 0.22 g x g(-1)). However, the recombinant strains W50-AB and W50-EAB exhibited better fermentation performance and more PHB accumulation when using glucose(0.01 mol/L) and D-xylose (0.09 mol/L) mixed sugars as fermentative substrate.
Conclusion: The recombinant strain W50-AB can metabolize D-xylose by the expression of xylAB genes, and the further expression of xylE gene is able to improve its D-xylose consumption rate. Meanwhile, the two recombinant strains can accumulate a small amount of PHB by using D-xylose as the sole carbon source.