Molecular biology and control of exopolysaccharide synthesis by Erwinia amylovora and related plant pathogens

The fire blight bacterium Erwinia amylovora was first described in North America and strains are highly related by PFGE analysis when isolated in Europe and Mediterranean region. In order to avoid recognition by plant defense mechanisms, E. amylovora, an Asian pear pathogen from Japan and E. pyrifoliae from Korea produce exopolysaccharides (EPS) of high molecular weight with repeating units of galactose and glucuronic acid. EPS synthesis is regulated by genes rcsA, rcsB and hns. The hns gene is present on the chromosome of E. amylovora and on the common plasmid pEA29. Levansucrase of E. amylovora polymerizes the fructose residues of sucrose to homopolymer Jevan, influenced by regulators RlsA, RlsB and RlsC. Gene rlsB is located upstream of lsc and rlsA at the end of hrp region. A viral EPS depolymerase degraded amylovoran capsules and interfered with colonization of pear slices by E. amylovora.