Biosynthesis of the Common Polysaccharide Antigen of Pseudomonas aeruginosa PAO1: Characterization and Role of GDP-d-Rhamnose:GlcNAc/GalNAc-Diphosphate-Lipid alpha 1,3-d-Rhamnosyltransferase WbpZ

The opportunistic pathogen Pseudomonas aeruginosa produces two major cell surface lipopolysaccharides, characterized by distinct O antigens, called common polysaccharide antigen (CPA) and O-specific antigen (OSA). CPA contains a polymer of d-rhamnose (d-Rha) in alpha 1-2 and alpha 1-3 linkages. Three putative glycosyltransferase genes, wbpX, wbpY, and wbpZ, are part of the CPA biosynthesis cluster. To characterize the enzymatic function of the wbpZ gene product, we chemically synthesized the donor substrate GDP-d-Rha and enzymatically synthesized GDP-d-[3H]Rha. Using nuclear magnetic resonance (NMR) spectroscopy, we showed that WbpZ transferred one d-Rha residue from GDP-d-Rha in alpha 1-3 linkage to both GlcNAc- and GalNAc-diphosphate-lipid acceptor substrates. WbpZ is also capable of transferring d-mannose (d-Man) to these acceptors. Therefore, WbpZ has a relaxed specificity with respect to both acceptor and donor substrates. The diphosphate group of the acceptor, however, is required for activity. WbpZ does not require divalent metal ion for activity and exhibits an unusually high pH optimum of 9. WbpZ from PAO1 is therefore a GDP-d-Rha:GlcNAc/GalNAc-diphosphate-lipid alpha 1,3-d-rhamnosyltransferase that has significant activity of GDP-d-Man:GlcNAc/GalNAc-diphosphate-lipid alpha 1,3-d-mannosyltransferase. We used site-directed mutagenesis to replace the Asp residues of the two DXD motifs with Ala. Neither of the mutant constructs of wbpZ (D172A or D254A) could be used to rescue CPA biosynthesis in the Delta wbpZ knockout mutant in a complementation assay. This suggested that D172 and D254 are essential for WbpZ function. This work is the first detailed characterization study of a d-Rha-transferase and a critical step in the development of CPA synthesis inhibitors. IMPORTANCE This is the first characterization of a d-rhamnosyltransferase and shows that it is essential in Pseudomonas aeruginosa for the synthesis of the common polysaccharide antigen.