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 (rfb(O:54)) contains three genes that direct synthesis of a ManNAc homopolymer with alternating beta 1,3 and beta 1,4 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 beta-glycosyltransferases. Two putative catalytic residues and a potential substrate-binding motif were identified in RfbA(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 beta-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 beta-glycosyltransferases. Four potential catalytic residues and a putative substrate-binding motif were identified in RfbB(O:54). In Delta rfb 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.