Characterization of Pyoverdin pss , the Fluorescent Siderophore Produced by Pseudomonas syringae pv. syringae

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Abstract
Pseudomonas syringae pv. syringae B301D produces a yellow-green, fluorescent siderophore, pyoverdin pss , in large quantities under iron-limited growth conditions. Maximum yields of pyoverdin pss of approximately 50 μg/ml occurred after 24 h of incubation in a deferrated synthetic medium. Increasing increments of Fe(III) coordinately repressed siderophore production until repression was complete at concentrations of ≥ 10 μM. Pyoverdin pss was isolated, chemically characterized, and found to resemble previously characterized pyoverdins in spectral traits (absorbance maxima of 365 and 410 nm for pyoverdin pss and its ferric chelate, respectively), size (1,175 molecular weight), and amino acid composition. Nevertheless, pyoverdin pss was structurally unique since amino acid analysis of reductive hydrolysates yielded β-hydroxyaspartic acid, serine, threonine, and lysine in a 2:2:2:1 ratio. Pyoverdin pss exhibited a relatively high affinity constant for Fe(III), with values of 10 25 at pH 7.0 and 10 32 at pH 10.0. Iron uptake assays with [ 55 Fe]pyoverdin pss demonstrated rapid active uptake of 55 Fe(III) by P. syringae pv. syringae B301D, while no uptake was observed for a mutant strain unable to acquire Fe(III) from ferric pyoverdin pss . The chemical and biological properties of pyoverdin pss are discussed in relation to virulence and iron uptake during plant pathogenesis.