Functional analysis of the O antigen glucosylation gene cluster of Shigella flexneri bacteriophage SfX

Previous studies have shown that Shigella flexneri bacteriophage X (SfX) en codes a glucosyltransferase (GtrX, formerly Gtr), which is involved in O an tigen 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 S. flexn eri 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 b esides the gtrX gene, the other two genes in the gtr focus of SfX were also involved in the O antigen modification process. The first gene in the clus ter (gtrA) encodes a small highly hydrophobic protein which appears to be i nvolved in the translocation of lipid-linked glucose across the cytoplasmic membrane. The second gene in the cluster (gtrB) encodes an enzyme catalysi ng the transfer of the glucose residue from UDP-glucose to a lipid carrier. The third gene (gtrX) encodes a bacteriophage-specific glucosyltransferase which is largely responsible for the final step, i.e. attaching the glucos yl 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.