Iron transport in Escherichia coli: roles of energy-dependent uptake and 2,3-dihydroxybenzoylserine.

Escherichia coli strains B/r and 2276 contain an active transport system for iron. The system is energy-dependent, repressed by excess iron in the growth medium, and capable of accumulating iron inside of the cells at concentrations 2,000-fold higher than those in the medium. Two tonB-trp deletion mutants, strains B/rlt and B/lt7, which are sensitive to chromic ion and require high levels of iron for normal growth, are deficient in this active transport system. A point mutant, strain Chr2, which is also sensitive to chromic ion and requires high levels of iron for growth, has the active uptake system but cannot synthesize a specific chelator for iron, 2,3-dihydroxybenzoylserine (DHBS). Evidence is presented to support the hypothesis that both the active uptake system and chelation of iron by DHBS play a role in iron uptake from iron-deficient medium. The chromium sensitivity of the mutants can be explained by inhibition of uptake of exogenous iron.