Overexpression and characterization of alkaliphilic Bacillus firmus strain K-1 xylanase

The alkaliphilic Bacillus firmus strain K-1 produces an alkaliphilic xylanase (Xyn11A) which can be stable across a wide pH range and active at high temperatures. However, the enzyme suffers from low activity when isolated directly from the culture broth using corn husk. A method was developed which employed recombinant DNA technology to produce recombinant Xyn11A (rXyn11A) in an Escherichia coli (E. coli) expression system. The thioredoxin-fused Xyn11A (Thio-rXyn11A) could be expressed as a soluble form in BL21 (DE3). The expressed protein was tobacco etch virus (TEV) cleavaged to remove the thioredoxin tag and subjected to further purification using Ni2+-NTA affinity chromatography followed by gel-filtration chromatography. Activity of rXyn11A was shown to be under the same conditions as the native enzyme isolated directly from the K-1, having a broad range of active pH (5.0-12.0), with the maximum activity obtained from pH 5.0 at 60 °C. Interestingly, the obtained rXyn11A exhibited a very large increase in specific activity (3034 U/mg), which was 84-times higher than that reported in the native enzyme when observed under the same conditions. Also seen in the xylan hydrozation assay, the rXyn11A hydrolyzed insoluble xylans around 100-times more effectively than the native enzyme. The results from this study demonstrated a successful method for generating the enzyme rXyn11A with much improved activity, making it feasible for industrial applications.