Roles of the EnvZ/OmpR Two-Component System and Porins in Iron Acquisition in Escherichia coli

Escherichia coli secretes high-affinity Fe(3+) chelators to solubilize and transport chelated Fe(3+) via specific outer membrane receptors. In microaerobic and anaerobic growth environments, where the reduced Fe(2+) form is predominant, ferrous transport systems fulfill the bacterial need for iron. Expression of genes coding for iron metabolism is controlled by Fur, which when bound to Fe(2+) acts as a repressor. Work carried out here shows that the constitutively activated EnvZ/OmpR two-component system, which normally controls expression of the ompC and ompF porin genes, dramatically increases the intracellular pool of accessible iron, as determined by whole-cell electron paramagnetic resonance spectroscopy, by inducing the OmpC/FeoB-mediated ferrous transport pathway. Elevated levels of intracellular iron in turn activated Fur, which inhibited the ferric transport pathway but not the ferrous transport pathway. The data show that the positive effect of constitutively activated EnvZ/OmpR on feoB expression is sufficient to overcome the negative effect of activated Fur on feoB. In a tonB mutant, which lacks functional ferric transport systems, deletion of ompR severely impairs growth on rich medium not supplemented with iron, while the simultaneous deletion of ompC and ompF is not viable. These data, together with the observation of derepression of the Fur regulon in an OmpC mutant, show that the porins play an important role in iron homeostasis. The work presented here also resolves a long-standing paradoxical observation of the effect of certain mutant envZ alleles on iron regulon.