The transport of metals into and out of cells is necessary for the maintenance of appropriate intracellular concentrations. Metals are needed for incorporation into metalloproteins but become toxic at higher concentrations. Many metal transport proteins have been discovered in bacteria, including the Mg2+ transporter E (MgtE) family of passive Mg2+/Co2+ cation-selective channels. Low sequence identity exists between members of the MgtE family, indicating that substrate specificity may differ among MgtE transporters. Under anoxic conditions, dissimilatory metalreducing bacteria, such as Shewanella and Geobacter species, are exposed to high levels of soluble metals, including Fe2+ and Mn2+. Here we characterize SO_3966, which encodes an MgtE homolog in Shewanella oneidensis that we name FicI (ferrous iron and cobalt importer) based on its role in maintaining metal homeostasis. A SO_3966 deletion mutant exhibits enhanced growth over that of the wild type under conditions with high Fe2+ or Co2+ concentrations but exhibits wild-type Mg2+ transport and retention phenotypes. Conversely, deletion of feoB, which encodes an energy-dependent Fe2+ importer, causes a growth defect under conditions of low Fe2+ concentrations but not high Fe2+ concentrations. We propose that FicI represents a secondary, less energy-dependent mechanism for iron uptake by S. oneidensis under high Fe2+ concentrations.
Bibliographical noteFunding Information:
We thank Rick Knurr and Elizabeth Lundstrom in the Department of Earth Sciences at the University of Minnesota for ICP-MS analysis and the anonymous reviewers for feedback on the manuscript. This work was supported by the Office of Naval Research (grant N00014-13-1-0552 to J.A.G.). B.D.B. was supported in part by the University of Minnesota Biotechnology Training Grant Program (National Institutes of Health). K.E.R. was supported by a MnDRIVE Seed Grant for Undergraduate Scholars (University of Minnesota).
- Iron acquisition
- Iron transport
- Shewanella oneidensis