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 chrys anthemi 3937 requires a number of pectinolytic activities, including f ive pectate lyase isoenzymes (PLa to PLe) expressed from independent c istrons (pelA to pelE), and a functional high-affinity iron transport system mediated by chrysobactin. The low iron level in leaf intercellu lar fluids led us to consider this metal as a potential regulatory sig nal 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 m ediated by the cbrAB operon, which negatively controls the transcripti on of iron transport genes in strain 3937. Study of transcriptional GU S fusions generated to each of the pel genes in various cbr background s demonstrated that cbrAB positively controls the expression of pelB, pelC, and pelE under low iron. The pathogenicity of the wild-type stra in on African violets was analyzed in relation to the iron status of t he preinoculation growth medium and in comparison to that of a cbr mut ant; wild-type cells appeared to be more aggressive in the first phase of the disease when grown under iron-deficient conditions before inoc ulation, but a delay was observed with the chrysobactin constitutive c br mutant cells. This result is discussed in regard to the pleiotropic effect of the cbr mutation.