MgtE homolog FicI acts as a secondary ferrous iron importer in Shewanella oneidensis strain MR-1

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.