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Abstract:
The genetics of the two major exopolysaccharides of Erwinia amylovora, levan and amylovoran, were determined. Transposon mutants lacking levan synthesis were fully virulent on immature pears and showed retarded fireblight symptoms on shoots of pear seedlings. The gene encodes a polypeptide of 46.6 kD, which is close to the molecular weight of the purified enzyme. Mutants deficient in amylovoran synthesis were non-pathogenic. The ams-genes, involved in synthesis of the acidic EPS, were mapped, and a DNA fragment of the region was sequenced. At least six genes seem to be involved in amylovoran synthesis. Among other possible factors, EPS-synthesis is regulated by the rcsA protein and by the sugar concentration in the environment. Lectin staining of the capsule confirmed the results about EPS production on plates and revealed loose association of EPS to cells in ooze of immature pears. The rcsA gene is also involved in regulation of levan synthesis. Levansucrase is expressed independently from the carbon source and secreted into the culture medium. Two loci were found for galactose metabolism which are separate from each other and not part of the cloned ams-region.
