A siderophore from Pseudomonas putida type A1: structural and biological characterization

A siderophore from a root-colonizing, plant-beneficial fluorescent Pseudomo nas (P. putida type A1) isolated from chickpea rhizosphere was studied. Cul ture conditions required for optimal production of the chromophore by the o rganism were standardized. The compound was purified by gel filtration, ion exchange and RP-HPLC chromatographic procedures. The purified compound exh ibited siderophore activity for P. putida and antifungal activity on phytop athogens, Fusarium oxysporum f. sp. ciceri and Helminthosporium oryzae. Gro wth inhibition of the pathogens was observed under iron-deficient condition s. Complete acid hydrolysis of the compound revealed that it is a peptide c ontaining Asx, Thr, Glx, Val, His, Lys, Ser and Gly. Spectral analysis reve aled that it contains hydroxyquinoline-based chromophore in addition to an aromatic residue and the molecular weight of the compound was 1.5 kDa. EPR analysis of the peptide-chromophore-iron complex showed that the compound b inds to iron and the bound iron was in the Fe3+ oxidation state having a hi gh spin d(5) system. The peptide-chromophore-iron complex takes a turn stru cture in solution as shown by circular dichroism spectroscopy, a feature wh ich was hitherto not known for other siderophores. The siderophore studied here is unique in this respect but otherwise strikingly similar to other ps eudobactin-type siderophores of plant growth-promoting and plant-deleteriou s pseudomonads. The possible functional significance of the compound is dis cussed in relation to the secondary structure described earlier for siderop hores. (C) 1999 Elsevier Science B.V. All rights reserved.