Requirement of Staphylococcus aureus ATP-binding cassette-ATPase FhuC for iron-restricted growth and evidence that it functions with more than one iron transporter

In Staphylococcus aureus, fhuCBG encodes an ATP-binding cassette (ABC) transporter that is required for the transport of iron(III)-hydroxamates; mutation of either fhuB or fhuG eliminates transport. In this paper, we describe construction and characterization of an S. aureus fhuCBG deletion strain. The delta fhuCBG::ermC mutation not only resulted in a strain that was incapable of growth on iron(III)-hydroxamates as a sole source of iron but also resulted in a strain which had a profound growth defect in iron-restricted laboratory media. The growth defect was not a result of the inability to transport iron(III)-hydroxamates since S. aureus fhuG::Tn917 and S. aureus fhuD1::Km fhuD2::Tet mutants, which are also unable to transport iron(III)-hydroxamates, do not have similar iron-restricted growth defects. Complementation experiments demonstrated that the growth defect of the delta fhuCBG::ermC mutant was the result of the inability to express FhuC and that this was the result of an inability to transport iron complexed to the S. aureus siderophore staphylobactin. Transport of iron(III)-staphylobactin is dependent upon SirA (binding protein), SirB (permease), and SirC (permease). S. aureus expressing FhuC with a Walker A K42N mutation could not utilize iron(III)-hydroxamates or iron(III)-staphylobactin as a sole source of iron, supporting the conclusion that FhuC, as expected, functions with FhuB, FhuG, and FhuD1 or FhuD2 to transport iron(III)-hydroxamates and is the "genetically unlinked" ABC-ATPase that functions with SirA, SirB, and SirC to transport iron(III)-staphylobactin. Finally, we demonstrated that the delta fhuCBG::ermC strain had decreased virulence in a murine kidney abscess model.