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Citrate-mediated iron uptake in Pseudomonas aeruginosa: involvement of the citrate-inducible FecA receptor and the FeoB ferrous iron transporter

¹ Department of Microbiology and Immunology, Queen's University, Kingston, Ontario K7L 3N6, Canada
² Ottawa Institute of Systems Biology, Department of Biochemistry, Microbiology and Immunology, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada
³ Laboratoire de Microbiologie et Génétique, Université Louis Pasteur/CNRS FRE 2326, 28 rue Goethe, 67083 Strasbourg, France

Correspondence
Keith Poole
poolek{at}queensu.ca

In an attempt to identify components of a ferric citrate uptake system in Pseudomonas aeruginosa, a mutant library of a siderophore-deficient strain (IA614) was constructed and screened for defects in citrate-promoted growth in an Fe-restricted medium. A mutant disrupted in gene PA3901, encoding a homologue of the outer-membrane ferric citrate receptor, FecA, of Escherichia coli (FecA_E.c.), was recovered and shown to be deficient in citrate-promoted growth and citrate-mediated Fe uptake. A mutant disrupted in gene PA4825, encoding a homologue of the MgtA/MgtB Mg²⁺ transporters in Salmonella enterica, was similarly deficient in citrate-promoted growth, though this was due to a citrate sensitivity of the mutant apparently resulting from citrate-promoted acquisition of Fe²⁺ and resultant oxidative stress. Consistent with citrate delivering Fe to cells as Fe²⁺, a P. aeruginosa mutant lacking the FeoB Fe²⁺ transporter homologue, PA4358, was compromised for citrate-promoted growth in Fe-restricted medium and showed markedly reduced citrate-mediated Fe uptake. Subsequent elimination of two Fe³⁺ transporter homologues, PA5216 and PA4687, in the feoB mutant failed to further compromise citrate-promoted growth or Fe uptake, though the additional loss of pcoA, encoding a periplasmic ferroxidase implicated in Fe²⁺ acquisition, completely abrogated citrate-mediated Fe uptake. Fe acquisition mediated by other siderophores (e.g. pyoverdine) was, however, unaffected in the quadruple knockout strain. These data indicate that Fe delivered to P. aeruginosa by citrate is released as Fe²⁺, probably in the periplasm, prior to its transport into cells via Fe transport components.

Supplementary material with details of primers and cycling parameters for construction of deletion mutants is available with the online version of this paper.

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				A. D. Buch, G. Archana, and G. N. Kumar Enhanced citric acid biosynthesis in Pseudomonas fluorescens ATCC 13525 by overexpression of the Escherichia coli citrate synthase gene Microbiology, August 1, 2009; 155(8): 2620 - 2629. [Abstract] [Full Text] [PDF]