Full virulence of the pectinolytic enterobacterium Erwinia chrysanthemi strain 3937 depends on the production in planta of the
catechol-type
siderophore chrysobactin. Under
iron-limited conditions, E. chrysanthemi synthesizes a second
siderophore called
achromobactin belonging to the hydroxy/carboxylate class of
siderophore. In this study, we cloned and functionally characterized a 13 kb long operon comprising seven genes required for the biosynthesis (acs) and extracellular release (yhcA) of
achromobactin, as well as the gene encoding the specific outer membrane receptor for its ferric complex (acr). The promoter of this operon was negatively regulated by
iron. In a fur null mutant, transcriptional fusions to the acsD and acsA genes were constitutively expressed. Band shift assays showed that the purified E. chrysanthemi Fur repressor
protein specifically binds in vitro to the promoter region of the acsF gene confirming that the metalloregulation of the
achromobactin operon is achieved directly by Fur. The temporal production of
achromobactin in
iron-depleted bacterial cultures was determined:
achromobactin is produced before
chrysobactin and its production decreases as that of
chrysobactin increases. Pathogenicity tests performed on African violets showed that
achromobactin production contributes to the virulence of E. chrysanthemi. Thus, during
infection, synthesis of these two different
siderophores allows E. chrysanthemi cells to cope with the fluctuations of
iron availability encountered within plant tissues. Interestingly,
iron transport mediated by
achromobactin or a closely related
siderophore probably exists in other phytopathogenic bacterial species such as Pseudomonas syringae.