Role of Outer Membrane Cytochromes in Respiration-Linked Metal Reduction

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Anaerobic respiration provides a mechanism to generate energy under oxygen-limited conditions. The Gram-negative bacterium Shewanella oneidensis MR-1 exhibits remarkable anaerobic respiratory versatility, using several soluble compounds and highly insoluble manganese(IV) and iron(III) oxides as electron acceptors. Therefore, the cells must establish a link between electron transport components and these extracellular insoluble electron acceptors. When grown under anaerobic conditions, MR-1 has a high specific cytochrome content in its outer membrane (OM). Two of its OM cytochromes, OmcA and OmcB (both c-type and having lipoprotein consensus sequences), have significant roles in Mn(IV) reduction and their surface exposure has important implications for the use of insoluble electron acceptors. The project will determine if OmcA and OmcB are lipoproteins, and if the lipid modification is essential for their localization to the OM and their ability to support electron transfer to extracellular electron acceptors. Mn(IV) reduction requires only 2 electrons, but the decaheme nature of the OM cytochromes may facilitate optimal heme arrangement for electron transfer across the OM to insoluble Mn(IV). This "electric wire" scheme implies that particular hemes are critical for electron transport. The project will determine if particular hemes of a decaheme OM cytochrome are required to support the reduction of insoluble Mn(IV). This research will significantly enhance the understanding of multiheme cytochromes and bacterial metal reduction, and will broaden the conceptual framework of anaerobic respiration to include electron transport links to the extracellular environment through OM components.