The environmental and genetic regulation of virulence mechanisms of Erwinia amylovora

Virulence of E. amylovora, the causal agent of fireblight, depends on the synthesis of harpin and amylovoran, an acidic exopolysaccharide and is enhanced by levan, produced from sucrose by the secreted enzyme levansucrase. Amylovoran synthesis and capsule formation were influenced by the presence of various carbon sources. Sorbitol, the major transport sugar of rosaceous plants, showed the strongest induction. Amylovoran synthesis by E. amylovora in the presence of sorbitol or sucrose was dependent on the ionic strength of the medium.;The importance of sorbitol for E. amylovora was further shown by mutations in genes involved in its uptake leading to a non-virulent phenotype on apple seedlings. Sorbitol uptake in E. amylovora is achieved via a phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). Sequence analysis identified an operon encoding six proteins that shared a high similarity to the PTS enzyme II complex for sorbitol uptake in E. coli.;The genetic regulation of amylovoran synthesis by RcsV required the expression of rcsV as a fusion protein, which could also suppress an rcsA mutation. The intact rcsV promoter sequence separated from the gene induced amylovoran synthesis via the RcsA/RcsB pathway. The titration effect and the lack of transcription from the rcsV promoter indicated that rcsV was negatively regulated by several proteins.;The gene encoding for H-NS, a global negative regulator, was isolated from a genomic library of E. amylovora and found to have significant homology to hns genes of several bacteria. Overexpression of hns reduced both amylovoran and levan production. A mutation of the E. amylovora hns gene did not alter amylovoran production or virulence but levan formation was increased. The study of the E. amylovora hns allele is the first time this gene has been characterised from a plant pathogen