Endo-1,4-β-xylanase B from Aspergillus cf. niger BCC14405 Isolated in Thailand: Purification, Characterization and Gene Isolation

During the screening of xylanolytic enzymes from locally isolated fungi, one strain BCC14405, exhibited high enzyme activity with thermostability. This fugal strain was identified as Aspergillus cf. niger based on its morphological characteristics and internal transcribed spacer (ITS) sequences. An enzyme with xylanolytic activity from BCC14405 was later purified and characterized. It was found to have a molecular mass of ca. 21 kDa, an optimal pH of 5.0, and an optimal temperature of 55 o C. When tested using xylan from birchwood, it showed Km and Vmax values of 8.9 mg/ml and 11,100 U/mg, respectively. The enzyme was inhibited by CuSO4, EDTA, and by FeSO4. The homology of the 20-residue N-terminal protein sequence showed that the enzyme was an endo-1,4β-xylanase. The full-length gene encoding endo-1,4-βxylanase from BCC14405 was obtained by PCR amplification of its cDNA. The gene contained an open reading frame of 678 bp, encoding a 225 amino acid protein, which was identical to the endo-1,4-â-xylanase B previously identified in A. niger.