A novel pathway for O-polysaccharide biosynthesis in Salmonella enterica serovar Borreze

Abstract

The plasmid-encoded gene cluster for O:54 O-polysaccharide synthesis in Salmonella enterica serovar Borraze (rfb(O:54)) contains three genes that direct synthesis of a ManNAc homopolymer with alternating β1,3 and β1,4 linkages. In Escherichia coli K-12, RfbA(O:54) adds the first ManNAc residue to the Rfe (UDP-GlcpNAc::undecaprenylphosphate GlcpNAc-1-phosphate transferase)-modified lipopolysaccharide core. Hydrophobic cluster analysis of RfbA(O:54) indicates this protein belongs to the ExoU family of nonprocessive β-glycosyltransferases. Two putative catalytic residues and a potential substrate-binding motif were identified in KfbA(O:54). Topological analysis of RfbB(O:54) predicts four transmembrane domains and a large central cytoplasmic domain. The latter shares homology with a similar domain in the processive β-glycosyltransferases Cps3S of Streptococcus pneumoniae and HasA of Streptococcus pyogenes. Hydrophobic cluster analysis of RfbB(O:54) and Cps3S indicates both possess the structural features characteristic of the HasA family of processive β-glycosyltransferases. Four potential catalytic residues and a putative substrate-binding motif were identified in RfbB(O:54). In Δrfb E. coli K-12, RfbA(O:54) and RfbB(O: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.