SPECIFICITY OF SIDEROPHORE-MEDIATED TRANSPORT OF IRON IN RHIZOBIA

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 (epsilon(M), = 1510). Fe-55 complexed to purified hydroxamate K was taken up by MNF710, its hydroxamate-negative mutant MNF7102 and Rhizobium leguminosal um 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 Fe-55-ferrichrome but only the first three strains took up Fe-55-ferrichrome A. All these Fe-55-trihydroxamate uptake systems were iron-regulated in MNF710, MNF7102 and WU95. In contrast, uptake of Fe-55-rhodotorulate, a dihydroxamate, was essentially constitutive in all four organisms. Similarly, uptake of Fe-55-citrate and Fe-55-nitrilotriacetic acid was constitutive. None of the strains took up Fe-55 complexed with enterobactin or with pyoverdins from Pseudomonas aeruginosa ATCC15692 (PAO1) and Pseudomonas fluorescens ATCC17400.