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Iron Transport in Azospirillum brasilense: Role of the Siderophore Spirilobactin

SUMMARY: A catechol-type siderophore was secreted by Azospirillum brasilense in the growth medium when the cells became iron-deficient. The siderophore, which we call spirilobactin, was purified and a partial characterization of its structure by hydrolysis revealed that it contained 2,3-dihydroxybenzoic acid, ornithine and serine in equimolar amounts. A high-affinity iron transport system capable of 59Fe uptake from a 59Fe(III)-spirilobactin complex was also induced in A. brasilense grown under iron deficiency. Kinetic studies on the iron uptake system revealed that it was carrier-mediated, with K
m and V
max values of 0·23 μm and 0·27 nmol Fe min−1(mg cell protein)−1, respectively. Dependence of iron uptake on an energized membrane, its insensitivity to arsenate and inhibition by protonophores suggest that transport is driven by the proton gradient across the membrane. Iron-deficient Azospirillum lipoferum transported 59Fe from the 59Fe(III)-spirilobactin complex (at a rate 3-fold lower than that of A. brasilense) but A. amazonense failed to show uptake of iron under the same conditions.