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

Division of Biochemistry and Molecular Biology, Faculty of Science, The Australian National University, Canberra, ACT 0200, Australia ABSTRACT Previous studies have shown that Shigella flexneri 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 S. flexneri 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 gtrX gene, the other two genes in the gtr locus of SfX were also involved in the O antigen modification process. The first gene in the cluster ( gtrA ) 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 ( gtrB ) encodes an enzyme catalysing 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 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. Author for correspondence: Naresh K. Verma. Tel: + 61 2 6249 2666. Fax: + 61 2 6249 0313. e-mail: Naresh.Verma@anu.edu.au Keywords: LPS, O antigen serotype conversion, glucosyltransferase, bactoprenol glucose transferase, bacteriophage Present address: School of Human and Biomedical Sciences, Faculty of Applied Science, University of Canberra, ACT, Australia.