Relative contribution of three transporters to D-xylose uptake in Aspergillus niger

The production of biofuels and chemicals from D-xylose is a promising option as D-xylose is the second most abundant sugar after D-glucose in lignocellulosic biomass. In microbes, efficient D-xylose uptake is a prerequisite for its utilization. Therefore, increasing D-xylose uptake efficiency by manipulation of D-xylose transporters would be an attractive strategy to improve fungal cell factories that use D-xylose as a substrate. In this study, we compared the contribution of three D-xylose transporters (XltA, XltB, XltD) from Aspergillus niger to overall D-xylose uptake at two D-xylose concentrations. XltA and XltD contributed similarly to D-xylose uptake, while the role of XltB was minimal. However, even in the absence of all three transporters, D-xylose uptake still occurred, indicating the involvement of additional transporters. Surprisingly, there was no clear correlation between the kinetic characteristics of the transporters nor the expression profile of their corresponding genes with their influence on D-xylose transport. This suggests that selection of transporters for metabolic engineering of filamentous fungal cell factories based solely on kinetic parameters originating from heterologous expression of the transporters in yeast may not be a very efficient and reliable strategy.