Photosynthetic Reduction of Xylose to Xylitol Using Cyanobacteria.
Photosynthetic generation of reducing power makes cyanobacteria an attractive host for biochemical reduction compared to cell-free and heterotrophic systems, which require burning of additional resources for the supply of reducing equivalent. Here, using xylitol synthesis as an example, we demonstrated efficient uptake and reduction of xylose photoautotrophically in Synechococcus elongatus PCC7942 upon introduction of an effective xylose transporter from Escherichia coli (Ec-XylE) and the NADPH-dependent xylose reductase from Candida boidinii (Cb-XR). Simultaneous activation of xylose uptake and matching of cofactor specificity enabled an average xylitol yield of 0.9 g/g xylose and a maximum productivity of about 0.15 g/L/d/OD with increased level of xylose supply. While long-term cellular maintenance still appeared challenging, high-density conversion of xylose to xylitol using concentrated resting cell further pushed the titer of xylitol formation to 33 g/L in six days with 85% of maximum theoretical yield. While our results showed that the unknown dissipation of xylose can be minimized when coupled to a strong reaction outlet, it remained to be the major hurdle hampering the yield despite the reported inability of cyanobacteria to metabolize xylose. With xylitol productivity increasing linearly with the concentration of cyanobacterial biocatalysts, this study suggests that photosynthetic supply of NADPH is sufficient for one-step biochemical reduction even under limited light penetration. This article is protected by copyright. All rights reserved.