Ferric citrate transport in Escherichia coli requires outer membrane receptor protein fecA

Mutants of Escherichia coli K-12 AB2847 and of E. coli K-12 AN92 were isolated which were unable to grow on ferric citrate as the sole iron source. Of 22 mutants, 6 lacked an outer membrane protein, designated FecA protein, which was expressed by growing cells in the presence of 1 mM citrate. Outer membranes showed an enhanced binding of radioactive iron, supplied as a citrate complex, depending on the amount of FecA protein. The FecA protein was the most resistant of the proteins involved in ferric irion iron translocation across the outer membrane (FhuA = TonA, FepA, Cir, or 83K proteins) to the action of pronase P. It is also shown that previously isolated fec mutants (G. C. Woodrow et al., J. Bacteriol. 133:1524-1526, 1978) which are cotransducible with argF all lack the FecA protein. They were termed fecA to distinguish them from the other ferric citrate transport mutants, now designated fecB, which mapped in the same gene region at 7 min but were not cotransducible with ArgF. E. coli W83-24 and Salmonella typhimurium, which are devoid of a citrate-dependent iron transport system, lacked the FecA protein. It is proposed that the FecA protein participates in the transport of ferric citrate.