A levansucrase (SacB) of Zymomonas mobilis was purified to electrophoretic homogeneity from a recombinant Escherichia coli. The 55 kDa enzyme hydrolysed beta-fructosides but not alpha-glucosides and catalysed levan formation from sucrose as well as raffinose. The optimum temperature for polymerase activity (30 degrees C) was lower than that for hyrolase activity (50 degrees C). In contrast to other levansucrases, polymerase activity of levansucrase was inhibited by para-chloromercuribenzoate (1 mM) but with little or no effect on hydrolase activity. Selective modulation of polymerase activity by this inhibitor will be useful in revealing the mechanism of levansucrase catalysis.