Previous studies have shown that bacteriophage X (SfX) encodes a glucosyltransferase (GtrX, formerly Gtr), which is involved in O antigen modification (serotype Y to serotype X). However, GtrX alone can only mediate a partial conversion. More recently, a three-gene cluster has been identified next to the attachment site in the genome of two other bacteriophages (i.e. SfV and SfII). This gene cluster was postulated to be responsible for a full O antigen conversion. Here it is reported that besides the gene, the other two genes in the locus of SfX were also involved in the O antigen modification process. The first gene in the cluster () encodes a small highly hydrophobic protein which appears to be involved in the translocation of lipid-linked glucose across the cytoplasmic membrane. The second gene in the cluster () encodes an enzyme catalysing the transfer of the glucose residue from UDP-glucose to a lipid carrier. The third gene () encodes a bacteriophage-specific glucosyltransferase which is largely responsible for the final step, i.e. attaching the glucosyl molecules onto the correct sugar residue of the O antigen repeating unit. A three-step model for the glucosylation of bacterial O antigen has been proposed.


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