Citrate‐Dependent Iron Transport System in Escherichia coli K‐12

Abstract

Induction of the citrate-dependent Fe transport system of E. coli K-12 required 0.1 mM citrate and 0.1 .mu.M Fe in the growth medium; 5-10 times more Fe than citrate was taken up into the cells, suggesting that citrate was largely excluded from the transport. Fluorocitrate and phosphocitrate induced the citrate-dependent Fe transport system although they supported Fe uptake only very poorly. An outer membrane protein (FecA), belonging to the transport system, was induced in fecB mutants which were devoid of citrate-dependent Fe transport. The intracellular citrate and Fe concentrations were 10-100 times higher than the external concentrations required for induction of the transport system. Only exogenous ferric citrate induced the transport system, and citrate did not have to enter the cytoplasm. The Tn10 transposon, conferring tetracycline resistance, was inserted near the fec gene region which controls the expression of the citrate-dependent Fe transport system. The determination of the cotransduction frequencies of Tn10 with the fecA and fecB markers suggested the gene order fecA-fecB-Tn10.