Fucosyltransferases (FucT) from differentHelicobacter pylori strains display distinct Type I (Gal1,3GlcNAc) or Type II (Gal1,4GlcNAc) substrate specificity. FucT from strain UA948 can transfer fucose to the OH-3 of Type II acceptors as well as to the OH-4 of Type I acceptors on the GlcNAc moiety, so it has both ␣1,3 and ␣1,4 activities. In contrast, FucT from strain NCTC11639 has exclusive ␣1,3 activity. Our domain swapping study ( Helicobacter pylori is associated with gastritis and peptic ulcer formation and is a risk factor for the development of gastric cancer and mucosa-associated lymphoid tissue lymphoma. One of the virulence factors of H. pylori is the lipopolysaccharide, which contains lipid A, core oligosaccharide and O-antigens. The O-antigens of H. pylori lipopolysaccharide contain fucosylated oligosaccharides, predominantly the Type II blood group antigens, Lewis X (Gal1,4(Fuc␣1,3)GlcNAc) and Lewis Y (Fuc␣1,2Gal1,4(Fuc␣1,3)GlcNAc) (1), but a small number of H. pylori strains also express the Type I blood group antigens, Lewis A (Gal1,3(Fuc␣1,4)GlcNAc), and Lewis B (Fuc␣1,2Gal1,3(Fuc␣1,4)-GlcNAc) (2).The role of Lewis antigens in H. pylori pathogenesis is still ambiguous. It has been suggested that Lewis antigens play a role in H. pylori adhesion to (3, 4) or internalization by (5) the gastric epithelial cells. Nevertheless, conflicting evidence argues that Lewis X and Lewis Y are not required for colonization of human gastric epithelium (6) or mouse stomach (7,8). Lewis antigens may also play an important role in the persistence of H. pylori infection by molecular mimicry, helping the bacteria to evade the host immune response (2, 9, 10). Environmental changes such as pH influence the expression of H. pylori O-antigens, particularly Lewis X and Lewis Y. This may aid in adaptation of the bacterium to its niche in the stomach (10).Fucosyltransferases (FucTs) 3 are enzymes responsible for the last steps in the synthesis of Lewis antigens in H. pylori (11,12). ␣1,2 and ␣1,3 or ␣1,3/4 FucTs have been identified and characterized in H. pylori (13-18). These FucTs catalyze the transfer of the L-fucose moiety from guanosine diphosphate -L-fucose (GDP-Fuc) to the OH-2 of the galactose moiety and the OH-3 or the OH-3 and the OH-4 positions of the GlcNAc moiety in glycoconjugate acceptors, respectively. The H. pylori genome contains two homologous ␣1,3 or ␣1,3/4 FucT genes, futA and futB (19,20), but they do not always encode functional proteins. For instance, only the futA gene encodes an active FucT in H. pylori strains NCTC11639 and UA948 (13,17). Bacterial ␣1,3/4 FucTs are functionally equivalent to the mammalian ␣1,3/4 FucTs, which have been well characterized. Mammalian FucTs are Type II membrane proteins with a short N-terminal cytoplasmic tail, transmembrane domain, stem region, and C-terminal catalytic domain. H. pylori FucTs share weak homology with their mammalian counterparts in two small segments within the catalytic domain, called ␣1,3 FucT motifs (14, 21). H. pylori ␣1,3/4 FucTs lack the N...