A Novel Pathway for O-Polysaccharide Biosynthesis in Salmonella enterica Serovar Borreze

The plasmid-encoded gene cluster for O:54 O-polysaccharide synthesis in Salmonella enterica serovar Borreze (rfbO:54) contains three genes that direct synthesis of a ManNAc homopolymer with alternating beta1,3 and beta1,4 linkages. In Escherichia coli K-12, RfbAO:54 adds the first ManNAc residue to the Rfe (UDP-GlcpNAc::undecaprenylphosphate GlcpNAc-1-phosphate transferase)- modified lipopolysaccharide core. Hydrophobic cluster analysis of RfbAO:54 indicates this protein belongs to the ExoU family of nonprocessive beta-glycosyltransferases. Two putative catalytic residues and a potential substrate-binding motif were identified in RfbAO:54. Topological analysis of RfbBO:54 predicts four transmembrane domains and a large central cytoplasmic domain. The latter shares homology with a similar domain in the processive beta-glycosyltransferases Cps3S of Streptococcus pneumoniae and HasA of Streptococcus pyogenes. Hydrophobic cluster analysis of RfbBO:54 and Cps3S indicates both possess the structural features characteristic of the HasA family of processive beta-glycosyltransferases. Four potential catalytic residues and a putative substrate-binding motif were identified in RfbBO:54. In Deltarfb E. coli K-12, RfbAO:54 and RfbBO:54 direct synthesis of smooth O:54 lipopolysaccharide, indicating that this O-polysaccharide involves a novel pathway for O-antigen transport. Based on sequence and structural conservation, 15 new ExoU-related and 17 new HasA-related transferases were identified.