Osmoregulated periplasmic glucans (OPGs) are general constituents of many Proteobacteria. Synthesis of these oligosaccharides is repressed by increased osmolarity of the medium. OPGs are important factors required for full virulence in many zoo- or phytopathogens including Dickeya dadantii. The phytopathogen enterobacterium D. dadantii causes soft-rot disease on a wide range of plant species. The total loss of virulence of opg-negative strains of D. dadantii is linked to the constitutive activation of the RcsCD RcsB phosphorelay highlighting relationship between this phosphorelay and OPGs. Here we show that OPGs control the RcsCD RcsB activation in a concentration-dependent manner, are required for proper activation of this phosphorelay by medium osmolarity, and a high concentration of OPGs in planta is maintained to achieve the low level of activation of the RcsCD RcsB phosphorelay required for full virulence in D. dadantii.

Osmoregulated periplasmic glucans (OPGs) are oligosaccharides found in the periplasm of many Gram-negative bacteria. Glucose is the sole constitutive sugar and this backbone may be substituted by various kinds of molecules depending on the species. In E. coli, OPG are substituted by phosphoglycerol and phosphoethanolamine derived from membrane phospholipids and by succinyl residues. In this study, we describe the isolation of the opgE gene encoding the phosphoethanolamine transferase by a screen previously used for the isolation of the opgB gene encoding the phosphoglycerol transferase. Both genes show structural and functional similarities without sequence similarity.