Cloning of the aapT gene and characterization of its product, alpha-amylase-pullulanase (AapT), from thermophilic and alkaliphilic Bacillus sp. strain XAL601

Abstract

A thermophilic and alkaliphilic Bacillus sp. strain, XAL601, was isolated from soil. It produces a thermostable and alkaline-stable enzyme with both alpha-amylase and pullulanase activities. The alpha-amylase-pullulanase gene (aapT) from this Bacillus strain was cloned, and its nucleotide sequence was determined (GenBank accession number D28467). A very large open reading frame composed of 6,096 bases, which encodes 2,032 amino acid residues with an M(r) of 224,992, was found. The deduced amino acid sequence revealed that the four highly conserved regions that are common among amylolytic enzymes were well conserved. These include an active center and common substrate-binding sites of various amylases. In the C-terminal region, a six-amino-acid sequence (Gly-Ser-Gly-Thr-Thr-Pro) is repeated 12 times. The aapT gene was then subcloned in Escherichia coli and overexpressed under the control of the lac promoter. Purification of AapT from this recombinant E. coli was performed, and it was shown that the aapT gene product exhibits both alpha-amylase and pullulanase activities with one active site. The optimum temperature and pH for enzyme activity were found to be 70 degrees C and pH 9, respectively. Furthermore, AapT was found to strongly adsorb to crystalline cellulose (Avicel) and raw corn starch. Final hydrolyzed products from soluble starch range from maltose (G2) to maltotetraose (G4). Only maltotriose (G3) was produced from pullulan. The enzyme also hydrolyzes raw starch under a broad range of conditions (60 to 70 degrees C and pH 8 to 9).