Variations in the molecular masses of the capsular exopolysaccharides amylovoran, pyrifolan and stewartan.

Erwinia amylovora, causing fire blight of apple, pear and some ornamentals, Erwinia pyrifoliae, causing Asian pear blight, and Pantoea stewartii, causing Stewart's wilt of sweet maize, synthesize capsular extracellular polysaccharides (EPSs) with a high molecular mass. The EPSs are virulence factors and form viscous aggregates, which participate in clogging vessels of infected plants and causing wilting. The sizes of EPSs produced under different environmental growth conditions were determined by analysis with large pore HPLC columns. Their molecular mass of ca. 5 MDa, when isolated from agar plates, decreases to ca. 1 MDa for E. amylovora amylovoran from freeze-dried supernatants from liquid cultures and to 2 MDa for freeze-dried preparations of P. stewartii stewartan. Size changes were also found following growth in various other media and for different strains. Stewartan, amylovoran and E. pyrifoliae pyrifolan were also shown to be completely degraded by a bacteriophage EPS depolymerase.

Genetics of biosynthesis and structure of the capsular exopolysaccharide from the Asian pear pathogen Erwinia pyrifoliae.

Erwinia pyrifoliae is a novel bacterial pathogen, which causes Asian pear blight and is related to Erwinia amylovora, the causative agent of fire blight. E. pyrifoliae produces exopolysaccharide (EPS) related to amylovoran in its sugar composition and sugar linkages. This was shown by degradation of the EPS with a viral depolymerase, and by methylation analysis and ESI/MS. The structure of the repeating units was confirmed by (1)H-NMR spectra. The EPS of E. pyrifoliae carried side chains, which were mainly terminated by acetyl and pyruvyl residues as found previously for amylovoran. On the other hand, a second side chain with glucose found for up to 65% of the repeating units of amylovoran was completely absent. The nucleotide sequences of five genes of the cps cluster of E. pyrifoliae encoding proteins for EPS synthesis were characterized and displayed a high homology with the corresponding ams genes. Similar functions of the gene products are assumed. As for ams mutants of E. amylovora, a cpsB mutant of E. pyrifoliae did not synthesize EPS and did not produce ooze on slices of immature pears or symptoms on pear seedlings. The cps mutant was complemented for EPS synthesis and virulence on pear slices with a gene cluster of E. amylovora that included amsB.