Directed evolution of a mesophilic fungal xylanase by fusion of a thermophilic bacterial carbohydrate-binding module

Bacterial GH10 xylanase usually contains carbohydrate-binding module (CBM) that binds to insoluble xylan. Differing from GH10 xylanase, we isolated a fungal GH11 xylanase containing a single catalytic domain from Aspergilus niger (XYN). The thermophilic CBM from Thermotoga maritima (TmCBM9-1_2) might increase the mesophilic XYN's thermo-activity and catalytic efficiency on insoluble xylan, we fused it with the TmCBM9-1_2 behaving as “hand” to grasp xylan actively. The chimeric xylanase XYN-TmCBM9-1_2 exhibited an optimal activity at pH4.2 and 49 °C, 2 °C higher than the thermo-activity of XYN. The chimeric xylanase's activity was 970.1 ± 5.8 U/mg on insoluble oat-spelt xylan, 4.2-fold of that on soluble birchwood xylan (228.1 ± 1.1 U/mg). In contrast, the XYN's activity was 226.9 ± 1.2 U/mg on insoluble oat-spelt xylan, only 40% of that on soluble birchwood xylan (567.2 ± 3.0 U/mg). Fusing with the TmCBM9-1_2 increased the XYN's property, indicating that we can direct to evolve a molecule's function through fusing domains of bacteria and fungi.