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

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Henri Gerken

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Phu Vuong

Ketaki Soparkar

Rajeev Misra

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Escherichia coli secretes high-affinity Fe3+ chelators to solubilize and transport chelated Fe3+ via specific outer membrane receptors. In microaerobic and anaerobic growth environments, where the reduced Fe2+ 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 Fe2+ acts as a repressor. Work carried out in this paper 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 (EPR) spectroscopy, by inducing the OmpC/FeoB-mediated ferrous transport pathway. Elevated levels of intracellular iron in turn activated Fur, which inhibited the ferric 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 de-repression 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.

Key Words
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Iron Homeostasis
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Microbiology
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Porins
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Ferric
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