Rhizobium leguminosarum bv. viciae produces a novel cyclic trihydroxamate siderophore, vicibactin

Trihydroxamate siderophores were isolated from iron-deficient cultures of three strains of Rhizobium leguminosarum biovar viciae, two from Japan (WSM709, WSM710) and one from the Mediterranean (WU235), and from a Tn5-induced mutant of WSM710 (MNF7101). The first three all produced the same compound (vicibactin), which was uncharged and could be purified by solvent extraction into benzyl alcohol. The gallium and ferric complexes of vicibactin were extractable into benzyl alcohol at pH 5.0, while metal-free vicibactin could be extracted with good yield at pH 8.0. The trihydroxamate from MNF7101 (vicibactin 7101) could not be extracted into benzyl alcohol, but its cationic nature permitted purification by chromatography on Sephadex CM-25 (NH⁺ ₄ form). Relative molecular masses and empirical formulae were obtained from fast-atom-bombardment MS. The structures were derived from one- and two-dimensional ¹H and ¹³C NMR spectroscopy, using DQF-COSY, NOESY, HMQC and HMBC techniques on the compounds dissolved in methanol-d ₄ and DMSO-d ₆. Vicibactin proves to be a cyclic molecule containing three residues each of (R)-2,5-diamino-N ²-acetyl-N ⁵-hydroxypentanoic acid (N ²-acetyl-N ⁵-hydroxy-D-ornithine) and (R)-3-hydroxybutanoic acid, arranged alternately, with alternating ester and peptide bonds. Vicibactin 7101 differed only in lacking the acetyl substitution on the N² of the N ⁵-hydroxyornithine, resulting in net positive charge; it was still functional as a siderophore and promoted ⁵⁵Fe uptake by iron-starved cells of WSM710 in the presence of an excess of phosphate. The rate of vicibactin biosynthesis by iron-deficient cells of WSM710 was essentially constant between pH 5.5 and 7.0, but much decreased at pH 5.0. When iron-starved cultures were supplemented with potential precursors for vicibactin, the rates of its synthesis were consistent with both β-hydroxybutyrate and ornithine being precursors. At least three genes seem likely to be involved in synthesis of vicibactin from ornithine and β-hydroxybutyrate: a hydroxylase adding the -OH group to the N⁵ of ornithine, an acetylase adding the acetyl group to the N² of ornithine, and a peptide synthetase system.