Genetic control of hydroxamate-mediated iron uptake in Escherichia coli

Cells of Escherichia coli can derive iron from a variety of chelators (siderophores) in addition to enterochelin, the catechol derivative excreted by many enteric bacteria. The genetic control of hydroxamate siderophore utilization was investigated in mutants of E. coli K-12 selected for resistance to lethal agents which adsorb to the tonA protein of the outer membrane (albomycin, colicin M, and phages T5 and phi80). Many of the mutants were unable to utilize hydroxamate siderophores as an iron source. This phenotype was termed Fhu, for ferric hydroxamate uptake. Mutants carrying lesions in the tonA region of the chromosome were studied and fell into several types. Members of one class had lost some or all of the tonA receptor protein's functions in that they were resistant to the lethal agents and unable to utilize ferrichrome and its analogs, although able to respond to the hydroxamate rhodotorulic acid (FhuA phenotype). Other mutants were unable to utilize any of the hydroxamate siderophores tested and were resistant to albomycin, althogh many were sensitive to the other lethal agents (FhuB phenotype). Members of these classes lacked ferrichrome-mediated iron uptake. Strains carrying transposon Tn10 insertions in fhuA (previously termed tonA) lacked the 78,000-molecular-weight outer membrane protein previously described; insertions in fhuB retained this protein and had no detectable change in outer membrane composition. Three-point transduction crosses revealed the gene order to be pan-fhuA-fhuB-metD in the min 3.5 region of the chromosome map. Complementation analyses with F' merodiploid strains showed that fhuA and fhuB comprise separate transcription units which are both required for utilization of ferrichrome. Response of diploid strains to rhodotorulic acid suggested the existence of a third gene, fhuC, required for utilization of this siderophore, but not ferrichrome. It is suggested that fhuB encodes a transport component in the cytoplasmic membrane that is necessary for the uptake of all hydroxamate siderophores following their receptor-mediated passage across the outer membrane.