Purification and characterization of an extracellular α-L-arabinosidase from a novel isolate Bacillus pumilus ARA and its over-expression in Escherichia coli

The alpha-L-arabinosidase, AraB, was induced when Bacillus pumilus ARA was grown at 50 degrees C in a minimal medium containing xylan. A 56-kDa protein with alpha-L-arabinosidase activity was purified from culture supernatant to gel electrophoretic homogeneity. The optimal activity was at pH 6.4 and 60 degrees C over a 10-min assay. The purified enzyme was stable over a pH range of 5.2-7.6 and had a 1-h half life at 70 degrees C. The enzyme released arabinose from oat spelt xylan. Kinetic experiments at 60 degrees C with p-nitrophenyl alpha-L-arabinofuranoside as substrate gave a Km, and V-max of 1.05 mM and 240 U per mg of protein. The NH2-terminal amino acid sequence of the enzyme was determined, and its gene araB was subsequently cloned, sequenced, and over-expressed in Escherichia coli. The open reading frame of araB consists of a 1,479-bp fragment encoding a protein of 472 amino acids, which belonged to family 51 of the glycoside hydrolases with an identity of 67% to the protein encoded by abfB of Bacillus subtilis 168.