Cloning, Expression and the Characterization of b-amylase from a Bacillus megaterium WS06
巨大芽孢杆菌b-淀粉酶基因的克隆、表达和酶学性质分析

A b-amylase gene (amyG) was cloned from a Bacillus megaterium WS06 and expressed in the Escherichia coli. Nucleotide sequence anlysis showed the amyG gene is composed of 1638 bp (545 amino acid residues with a Mr of 60.194 kD). The AmyG shows 94.5% sequence homologies with b-amylase from Bacillus megaterium DSM319 and presents a normal b- amylase primary structure, constituted by three parts: the N-terminal signal sequence, the catalytic domain and the C-terminal starch binding domains. The deduced amino acid sequence revealed that several highly conserved regions of the glycosylhydrolase family 14. The amyG gene was overexpressed using the pET21a vector and Escherichia coli BL21(DE3). The recombinant enzyme was purified 7.4 fold to electrophoretic homogeneity and had a Mr of 57 kD (by SDS-PAGE). The enzyme was optimally active at pH 7.0 and 60oC and showed stability at the temperature below 60oC. This enzyme efficiently hydrolyzed starch to yield maltose from non-reducing chain ends by exo-cleavage mode.