Iron transport in Escherichia coli: relationship between chromium sensitivity and high iron requirement in mutants of Escherichia coli.

Utilization of iron (Fe(3+)) by Escherichia coli depends upon a system which is determined by at least two genetic loci. Mutants which carry a deletion of the tonB-trp region of the chromosome grow only when very high concentrations of iron are present in the medium. These strains are sensitive to chromic ion (Cr(3+)) and, unlike the parent strain, fail to grow on MnSO(4) when FeSO(4) is not added to the medium. A second type of mutant, Chr2, which was isolated on the basis of its sensitivity to chromic ion, also requires a high concentration of iron for growth. This mutant can be distinguished phenotypically from the deletion mutants since it grows normally on low concentrations of iron, provided citrate is added to the medium. The chromium sensitivity of both types of mutants can be reversed by high concentrations of exogenous iron. The data are interpreted to indicate that the E. coli mutants studied are defective in iron transport and that residual iron transport is in some way inhibited by chromic ion.