Transport of the iron ionophore ferrichrome in Escherichia coli K‐12 and Salmonella typhimurium LT2

Iron(III) is actively transported as a ferrichrome complex into the cells of Escherichia coli and Salmonella typhimurium [1,2]. We have shown that in E. coli the ligand is modified and excreted as the iron-free form into the medium [3]. The modified ligand could be converted back into ferrichrome by mild acid hydrolysis. 1 mol of acetate was released per mol of ferrichrome; therefore it was proposed that an N-hydroxyl group, one of the six binding sites of iron(Ill) in ferrichrome [4], was acetylated. This could explain the low affinity of the modified ligand for iron(III). It was likely that reduction before modification took place. We suggested that the inactivation of ferrichrome prevented the intracellular withdrawal of iron by the ligand from sites of synthesis and function of the many iron containing proteins. We now describe a cell-free system that modifies ferrichrome in the same way as transporting cells. Reduction was a prerequisite for modification to occur. Both reduction and modification were membrane-bound processes. It is therefore likely that ferrichrome does not have to enter the cytoplasm in order to function as an iron(III) ionophore. To test this conclusion, we converted an analogue of ferri-