Identification of an iron permease, cFTR1, in cyanobacteria involved in the iron reduction/re-oxidation uptake...

Cyanobacteria are globally important primary producers and abundant in many iron-limited aquatic environments. The ways in which they take up iron are largely unknown, but reduction of Fe³⁺ is an important step in the process. Here we report a special iron permease in Synechocystis, cFTR1, that is required for Fe³⁺ uptake following Fe²⁺ re-oxidation. The expression of cFTR1 is induced by iron starvation, and a mutant lacking the gene is abnormally sensitive to iron starvation. The cFTR1 protein localizes to the plasma membrane and contains the iron-binding motif "REXXE". Point-directed mutagenesis of the REXXE motif results in a sensitivity to Fe-deficiency. Measurements of iron (⁵⁵ Fe) uptake rate show that cFTR1 takes up Fe³⁺ rather than Fe²⁺ . The function of cFTR1 in Synechocystis could be genetically complemented by the iron permease, Ftr1p, of Saccharomyces cerevisiae, that is known to transport Fe³⁺ produced by the oxidation of Fe²⁺ via a multicopper oxidase. Unlike yeast Ftr1p, cyanobacterial cFTR1 probably obtains Fe³⁺ primarily from the oxidation of Fe²⁺ by oxygen. Growth assays show that the cFTR1 is required during oxygenic, photoautotrophic growth but not when oxygen production is inhibited during photoheterotrophic growth. In cyanobacteria, iron reduction/re-oxidation uptake pathway may represent their adaptation to oxygenated environments.