Escherichia Coli FeoC Binds a Redox-Active, Rapidly Oxygen-Sensitive [4Fe-4S] Cluster
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Date
2019-02-07
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Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)
Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)
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Abstract
The acquisition of iron is essential to establishing virulence among most pathogens.
Under acidic and/or anaerobic conditions, most bacteria utilize the widely-distributed ferrous
iron (Fe²⁺) uptake (Feo) system to import metabolically-required iron. The Feo system is
inadequately understood at the atomic, molecular, and mechanistic levels, but we do know it is
comprised of a main membrane component (FeoB) essential for iron translocation, as well as two
small, cytosolic proteins (FeoA and FeoC) hypothesized to function as accessories to this
process. FeoC has many hypothetical functions, including that of an iron-responsive
transcriptional regulator. Here, we demonstrate for the first time that Escherichia coli FeoC
(EcFeoC) binds an [Fe-S] cluster. Using electronic absorption, X-ray absorption, and electron
paramagnetic resonance spectroscopies, we extensively characterize the nature of this cluster.
Under strictly anaerobic conditions after chemical reconstitution, we demonstrate that EcFeoC
binds a redox-active [4Fe-4S]²⁺ᐟ⁺ cluster that is rapidly oxygen-sensitive (t₁/₂ ≈ 20 s), similar to
the [Fe-S] cluster in the fumarate and nitrate reductase (FNR) transcriptional regulator. In a
manner similar to FNR, we further probed the nature of the oxygen-induced cluster decay
products and report conversion of a [4Fe-4S]²⁺ cluster to a [2Fe-2S]²⁺ cluster. In contrast to FNR,
we show that [4Fe-4S]²⁺ cluster binding to EcFeoC is associated with modest conformational
changes of the polypeptide, but not protein dimerization. Finally, we posit a working hypothesis
in which the cluster-binding FeoCs may function as oxygen-sensitive iron sensors that fine-tune
pathogenic ferrous iron acquisition.