Directed expression of halophilic and acidophilic β-glucosidases by introducing homologous constitutive expression cassettes in marine Aspergillus niger

The hydrolysis step by β-glucosidase (BGL) is generally recognized as the major limiting step in cellulose degradation and the BGLs with prominent enzymatic properties are of great importance for efficient utilization of lignocellulosic biomass. In order to identify some salt-tolerant BGLs, two BGL genes were cloned from marine Aspergillus niger ZJUBE-1 genome. Then two bgl expression cassettes driven by gpdA promoter were respectively transformed into marine A. niger for homologous constitutive expression. Directed expression was achieved for the domination of target BGLs in fermentation broth. Conveniently, two BGLs were purified to homogeneity by two separation steps, ultrafiltration and anion exchange chromatography. The purified BGL1 and BGL2 showed maximum activity at pH 3.0-4.0 and 3.5-4.5, respectively, suggesting these two BGLs were relatively acidophilic, especially for BGL1. Besides, BGL1 was stable to most of metal ions, while BGL2 was sensitive to Cu2+, Fe3+ and Ag+. Most specially, BGL2 activity increased by 44% in the presence of 4 M NaCl, suggesting BGL2 was halophilic. Homology modeling revealed that longer loops and linkers as well as polymerous Glu492 may contribute to the halophilism of BGL2. At last, the medium for directed expression was optimized and the content as well as the purity of target protein was improved.