Thiamin biosynthesis and its influence on exopolysaccharide production:: A new component of virulence identified on Erwinia amylovora plasmid pEA29

The ubiquitous plasmid pEA29 of the fire blight pathogen Erwinia amylovora has been well documented as affecting virulence on apple and pear; however, the mechanism of its influence was unknown. Plasmid-cured strains of E. amylovora exhibit reduced virulence, delayed symptom expression and altered exopolysaccharide (EPS) production on minimal medium with out thiamin (MM-T). EPS in E. amylovora is composed of two separate fractions, amylovoran (a pathogenicity factor) and levan (a virulence factor), and we have established a link between carriage of pEA29 and elevated expression of the amylovoran biosynthetic operon (ams) during immature pear fruit infection. To examine the role of individual pEA29 genes in virulence, the plasmid-cured strain Ea110- was complemented with five individual genes or the thiOGF operon from pEA29 and examined for restoration of virulence and of normal EPS synthesis on MM-T. These experiments indicated that only the thiOGF operon was sufficient for restoration of virulence in pear fruit and normal EPS production on MM-T. In addition, the presence of the thiOGF operon was required for increased expression of the ams operon promoter during infection of immature pear fruit.