Variation at the common polysaccharide antigen locus drives lipopolysaccharide diversity within the P. syringae species complex

Abstract

The common polysaccharide antigen (CPA) from the lipopolysaccharide (LPS) component of cell walls from the species complex Pseudomonas syringae is highly variable both in structure and immunological specificity, but the genetic basis for this is not well understood. We have characterised the CPA locus from P. syringae pv. actinidiae (Psa). This locus has a modular structure with genes for both L- and D- rhamnose (Rha) biosynthesis and that of an unknown sugar. It also contains an operon coding for ABC transporter subunits, a bifunctional glycosyltransferase and an O-methyltransferase. This operon is predicted to have a role in transport, elongation and termination of the Rha backbone of the CPA oligosaccharide and is referred to as the TET operon. This is the first report of the identification of this operon in P. syringae. Two alleles of the TET operon were present amongst the different biovars of Psa and lineages of the closely related pathovar P. syringae pv. actinidifoliorum. This allelic variation was reflected in the electrophoretic properties of purified LPS from the different isolates. Gene knockout of the TET operon allele from biovar 1 and replacement with that from biovar 3, demonstrated the link between the genetic locus and the electrophoretic and immunogenic properties of the LPS molecules in Psa. Sequence analysis of the TET operon from a wide range of P. syringae and P. viridiflava isolates displayed a phylogenetic history which is incongruent with core gene phylogeny, but correlates with previously reported tailocin sensitivity, suggesting a functional relationship between LPS structure and tailocin susceptibility.