Specificity of siderophore-mediated transport of iron in rhizobia

Abstract

The trihydroxamate siderophore, hydroxamate K, has been purified from culture filtrates of iron-deficient Rhizobium leguminosarum biovar viciae MNF710. The iron complex has a molecular weight of 828 and an absorption maximum at 443 nm (ε_M=1510). ⁵⁵Fe complexed to purified hydroxamate K was taken up by MNF710, its hydroxamate-negative mutant MNF7102 and Rhizobium leguminosarum biovar trifolii WU95 via an iron-regulated transport system, but Rhizobium meliloti U45 failed to take up the iron-siderophore complex under any conditions. A similar pattern of iron uptake was observed with ferrioxamine B. MNF710, MNF7102, U45 and WU95 all transported ⁵⁵Fe-ferrichrome but only the first three strains took up ⁵⁵Fe-ferrichrome A. All these ⁵⁵Fe-trihydroxamate uptake systems were ironregulated in MNF710, MNF7102 and WU95. In contrast, uptake of ⁵⁵Fe-rhodotorulate, a dihydroxamate, was essentially constitutive in all four organisms. Similarly, uptake of ⁵⁵Fe-citrate and ⁵⁵Fe-nitrilotriacetic acid was constitutive. None of the strains took up ⁵⁵Fe complexed with enterobactin or with pyoverdins from Pseudomonas aeruginosa ATCC15692 (PAO1) and Pseudomonas fluorescens ATCC17400.