Iron transport in Escherichia coli: uptake and modification of ferrichrome.

During the transport of iron as ferrichrome complex into cells of Escherichia coli K-12, the ligand was modified and excreted into the medium. The rate of the formation of the modified product corresponded with the rate of iron transport. The modified product showed a decreased affinity for ferric iron and did not serve as an effective iron ionophore. After all of the ferrichrome had been converted, the modified product was taken up into the cell in an iron-free form. The uptake of ferrichrome and of the modified product depended on the transport system specified by the tonA and tonB genes. The modified product could be converted back into ferrichrome by mild acid or alkaline hydrolysis. One mole of acetate was released per mole of ferrichrome. It is proposed that one N-hydroxyl group of ferrichrome is acetylated to explain the low affinity for iron as the N-hydroxyl groups form the ligands for iron (III). A weak ester linkage by which the acetyl group is covalently bonded would account for the easy hydrolysis. The iron-free form of ferrichrome, deferri-ferrichrome, was also rapidly converted when incubated with cells with a functional transport system. It is therefore likely that iron is released from ferrichrome by reduction before modification takes place. The conversion of the ligand could be a mechanism by which cells rid themselves of a potentially deleterious ligand for iron in the cytoplasm. A possible role in ferrichrome transport is discussed.