DIFFERENTIAL REGULATION BY IRON OF ERWINIA-CHRYSANTHEMI PECTATE LYASES - PATHOGENICITY OF IRON TRANSPORT REGULATORY (CBR) MUTANTS

To incite systemic soft rot symptoms on African violets, Erwinia chrysanthemi 3937 requires a number of pectinolytic activities, including five pectate lyase isoenzymes (PLa to PLe) expressed from independent cistrons (pelA to pelE), and a functional high-affinity iron transport system mediated by chrysobactin. The low iron level in leaf intercellular fluids led us to consider this metal as a potential regulatory signal of PL production. This study shows that synthesis of PL activities is stimulated under low-iron conditions. The effect is independent of the presence of the catabolic inducer of pel gene expression and is mediated by the cbrAB operon, which negatively controls the transcription of iron transport genes in strain 3937. Study of transcriptional GUS fusions generated to each of the pel genes in various cbr backgrounds demonstrated that cbrAB positively controls the expression of pelB, pelC, and pelE under low iron. The pathogenicity of the wild-type strain on African violets was analyzed in relation to the iron status of the preinoculation growth medium and in comparison to that of a cbr mutant; wild-type cells appeared to be more aggressive in the first phase of the disease when grown under iron-deficient conditions before inoculation, but a delay was observed with the chrysobactin constitutive cbr mutant cells. This result is discussed in regard to the pleiotropic effect of the cbr mutation.