Characterization of siderophore‐mediated iron transport from Rhizobium leguminosarum bv. phaseoli.

Abstract

A high‐affinity iron assimilation system mediated by siderophores may be required for efficient nitrogen fixation in the genus Rhizobium, in view of the role of iron enzymes in this process. Siderophores biosynthesized by three strains of Rhizobium leguminosarum bv phaseoli were purified, hydrolyzed and analyzed. Using paper chromatography, two well‐defined spots that, according to their relative mobility, were identified as the L‐amino acids: threonine and tyrosine. Using thin layer chromatography, 2,3‐dihydroxybenzoic acid (2,3‐DHBA) was identified in the hydrolyzed samples as well. The spectrophotometric profiles from 2,3‐DHBA itself and a purified siderophore preparation from one of the strains were similar. The FAB⁺ mass spectrum showed a maximum peak of 637 m/z that indicates the purified compound has a molecular mass of 636 Da. Analysis of our results indicate that the compound that matched that molecular mass may contain: 2,3‐DHBA, tyrosine and threonine in a proportion of 1:1:3, with two of the threonine bound by their methyl group. A 30 KDa cell membrane protein was identified in cells grown under iron‐starved conditions. The siderophore and membrane receptor protein, structural constituents of the high‐affinity iron transport system in this species were found different to the compounds previously described in this genus by other authors.