Levansucrase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are sucrose and (2,6-beta-D-fructosyl)<sub>n</sub>, whereas its two products are glucose and (2,6-beta-D-fructosyl)<sub>n+1</sub>.
This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is sucrose:2,6-beta-D-fructan 6-beta-D-fructosyltransferase. Other names in common use include sucrose 6-fructosyltransferase, beta-2,6-fructosyltransferase, and beta-2,6-fructan:D-glucose 1-fructosyltransferase. This enzyme participates in starch and sucrose metabolism and two-component system - general.
As of late 2007, 3 structures have been solved for this class of enzymes, with PDB accession codes , , and .
SacB counter-selection relies on the toxic product produced by the sacB gene. sacB comes from the gram-positive bacteria Bacillus subtilis and encodes the enzyme levansucrase that converts sucrose into a toxic metabolite in gram-negative bacteria. Plating on sucrose medium will select for cells that contain constructs that have lost the sacB gene.