Characterization of fluorescent and non-fluorescent peptide siderophores produced by Pseudomonas syringae strains and their potential use in strain identification

Nonfluorescent highly virulent strains of Pseudomonas syringae pv. aptata i solated in different European countries and in Uruguay produce a nonfluores cent peptide siderophore, the production of which is iron repressed and spe cific to these strains. The amino acid composition of this siderophore is i dentical to that of the dominant fluorescent peptide siderophore produced b y fluorescent P. syringae strains, and the molecular masses of the respecti ve Fe(III) chelates are 1,177 and 1,175 atomic mass units. The unchelated n onfluorescent siderophore is converted into the fluorescent siderophore at pH 10, and colors and spectral characteristics of the unchelated siderophor es and of the Fe(III)-chelates in acidic conditions are similar to those of dihydro-pyoverdins and pyoverdins, respectively. The nonfluorescent sidero phore is used by fluorescent and nonfluorescent P. syringae strains. These results and additional mass spectrometry data strongly suggest the presence of a pyoverdin chromophore in the fluorescent siderophore and a dihydropyo verdin chromophore in the nonfluorescent siderophore, which are both ligate d to a succinamide residue. When chelated, the siderophores behave differen tly from typical pyoverdins and dihydropyoverdins in neutral and alkaline c onditions, apparently because of the ionization occurring around pH 4.5 of carboxylic acids present in beta -hydroxyaspartic acid residues of the pept ide chains. These differences can he detected visually by pH-dependent chan ges of the chelate colors and spectrophotochemically. These characteristics and the electrophoretic behavior of the unchelated and chelated siderophor es offer new tools to discriminate between saprophytic fluorescent Pseudomo nas species and fluorescent P. syringae and P. viridiflava strains and to d istinguish between the two siderovars in P. syringae pv. aptata.