Gene cloning, characterization, and heterologous expression of levansucrase from Bacillus amyloliquefaciens.

Although levan produced by Bacillus amyloliquefaciens is known to have efficient immunostimulant property which gives 100% survival of common carp when infected with Aeromonas hydrophila, no detailed reports are available describing kinetic studies of D: -glucose production and levan formation. In this study, we cloned and characterized the enzymatic kinetics using levansucrase expressed in Escherichia coli. Optimum pH for D: -glucose production and levan formation was 6.0 and 8.0, respectively, whereas optimum temperature was 30 degrees C and 4 degrees C, respectively. The K (m) and V (max) values for levansucrase were calculated to be 47.81 mM sucrose and 57.47 1mole/min mg protein, respectively. Prominent expression of levansucrase was obtained through xylose induction in Bacillus megaterium, where most of the His(6)-tagged protein was secreted into the culture broth, giving levansucrase activity of 12,906 U/l. Response-surface methodology (RSM) was further employed to optimize the fermentation conditions and improve the level of levansucrase production. Maximum levansucrase activity of 20,251 U/l was obtained in 12 h of fermentation carried out at 28 degrees C, starting induction with 0.735% xylose when A (600) was 1.2, which was 1.6- and 62-fold higher than those obtained in the nonoptimized conditions for the recombinant strain and the native strain, respectively.