Iron reduction and uptake was studied in wild-type and haem-deficient strains of . Haem-deficient strains lacked inducible ferri-reductase activity and were unable to take up iron from different ferric chelates such as Fe(III)-citrate or rhodotorulic acid. In contrast, ferrioxamine B was taken up actively by the mutants as well as by the wild-type strains. At a low extracellular concentration, uptake was insensitive to ferrozine and competitively-inhibited by Ga(III)-desferrioxamine B. Extracellular reductive dissociation of the siderophore occurred at higher extracellular concentrations. Two mechanisms appear to contribute to the uptake of ferrioxamine B by : one with high affinity, by which the siderophore is internalized as such and another with lower affinity by which iron is dissociated from the ligand prior to uptake.


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