Long isomalto-oligosaccharides (IMOs) were generated via an engineered fusion enzyme of dextransucrase and dextranase (DSXR). To increase the expression level, response surface methodology (RSM) was utilized for optimization of protein expression conditions for enhancement of protein production by the effects of three-level-three-factors and their mutual interaction in Escherichia coli. Seventeen experiments were designed and conducted for investigation of cell density to start induction, induction temperature, and induction time. Optimal induction conditions included a cell density to start induction (A(600)) of 0.76 at 12.16 degrees C for 18 h for dextransucrase activity and a cell density to start induction (A(600)) of 0.75 at 10.5 degrees C for 20.9 h for dextranase activity. The produced dextransucrase or dextranase activity was obtained at 120.1 +/- 7.2 or 871 +/- 58 U, respectively, from 1 L cultures. (c) 2010 Elsevier Inc. All rights reserved.