REGULATION AND BIOCHEMISTRY OF EXOPOLYSACCHARIDE SYNTHESIS OF THE FIRE BLIGHT PATHOGEN ERWINIA AMYLOVORA AND PEAR PATHOGENS FROM KOREA AND JAPAN

Abstract

The fire blight pathogen Erwinia amylovora and the Asian pear pathogens synthesize similar capsular exopolysaccharides (EPS), which are strictly required for pathogenicity. The molecular weight of the EPS is at least 1 MDa. The repeating units of EPS synthesized by Erwinia pyrifoliae from Korea and the Erwinia strains from Japan do not bear a glucose residue as a second side chain found in 60% of the repeating units of amylovoran. Levan, a homopolymer of fructose synthesized by the secreted enzyme levansucrase of E. amylovora, is not produced by the Asian pear pathogens. Amylovoran and levan expression are controlled by regulatory genes and affected differently by the phosphate and chloride concentrations of the environment. Alignments of rcsB genes, and also of the nucleotide sequences upstream of the gene cluster for EPS synthesis, show a significant relationship between the pear pathogens and an evolutionary distance to E. amylovora. Three activator proteins for expression of levansucrase have been identified; RlsA, RlsB, and RlsC. The H-NS protein binds to a DNA fragment with the start of lsc. From a bacteriophage, a DNA fragment was sequenced with genes encoding a lysozyme and an EPS depolymerase. The enzyme was used to degrade amylovoran capsules and shown to interfere with colonization of pear slices by E. amylovora.