Molecular genetic basis for the variable expression of Lewis Y antigen in Helicobacter pylori: analysis of the alpha (1,2) fucosyltransferase gene.

Helicobacter pylori lipopolysaccharides (LPS) express human oncofetal antigens Lewis X and Lewis Y. The synthesis of Lewis Y involves the actions of alpha (1,3) and alpha (1,2) fucosyltransferases (FucTs). Here, we report the molecular cloning and characterization of genes encoding H. pylori alpha (1,2) FucT (Hp fucT2) from various H. pylori strains. We constructed Hp fucT2 knock-out mutants and demonstrated the loss of Lewis Y production in these mutants by enzyme-linked immunosorbent assay (ELISA) and immunoelectron microscopy. The Hp fucT2 gene contains a hypermutable sequence [poly (C) and TAA repeats], which provides a possibility of frequent shifting into and out of coding frame by a polymerase slippage mechanism. Thus, the Hp fucT2 gene displays two major genotypes, consisting of either a single full-length open reading frame (ORF; as in the strain UA802) or truncated ORFs (as in the strain 26695). In vitro expression of Hp fucT2 genes demonstrated that both types of the gene have the potential to produce the full-length protein. The production of the full-length protein by the 26695 fucT2 gene could be attributed to translational-1 frameshifting, as a perfect translation frameshift cassette resembling that of the Escherichia coli dnaX gene is present. Examination of the strain UA1174 revealed that its fucT2 gene has a frameshifted ORF at the DNA level, which cannot be compensated by translation frameshifting, accounting for its Lewis Y off phenotype. In another strain, UA1218, the fucT2 gene is apparently turned off because of the loss of its promoter. Based on these data, we proposed a model for the variable expression of Lewis Y by H. pylori, in which regulation at the level of replication slippage (mutation), transcription and translation of the fucT2 gene may all be involved.