Characterization of a tonB mutation in Erwinia chrysanthemi 3937: TonB(Ech) is a member of the enterobacterial TonB family

The pectinolytic enterobacterium Erwinia chrysanthemi 3937 causes a systemi c disease in its natural host, the African violet (Saintpaulia ionantha). I t produces two structurally unrelated siderophores, chrysobactin and achrom obactin. Chrysobactin makes a large contribution to invasive growth of the bacterium in its host. Insertion mutants of a chrysobactin-defective strain were constructed and screened on the universal CAS-agar medium used for si derophore detection. A set of mutants affected in the production of achromo bactin were identified. This paper describes a mutant affected in the trans port of all the ferrisiderophores used by the bacterium as iron sources. Mo lecular analysis revealed that the insertion mutation disrupts the tonB gen e. The predicted Er. chrysanthemi TonB protein has a molecular mass of 27 6 00 Da and shares 20-58 % identity with the TonB proteins from 20 other bact erial species. The pathogenicity of the tonB mutant was assessed by inocula tion of African violets. The impairment in the spread of symptoms was simil ar in the tonB mutant to that in chrysobactin-defective mutants. However, t he pectinolytic activity, the major pathogenicity determinant in Er. chrysa nthemi, appeared to be stimulated twofold in the tonB mutant.