Divergent Nrf Family Proteins and MtrCAB Homologs Facilitate Extracellular Electron Transfer in Aeromonas hydrophila.

Extracellular electron transfer (EET) is a strategy for respiration in which electrons generated from metabolism are moved outside the cell to a terminal electron acceptor, such as iron or manganese oxide. EET has primarily been studied in two model systems, and Metal reduction has also been reported in numerous microorganisms, including spp., which are ubiquitous found in aquatic ecosystems, with some species capable of pathogenesis in humans and fish. Genomic comparisons of spp. revealed a potential outer membrane conduit homologous to MtrCAB. While the ability to respire metals and mineral oxides is not widespread in the genus , 90% of the sequenced isolates contain MtrCAB homologs. ATCC 7966 mutants lacking are unable to reduce metals. Expression of in an mutant lacking homologous components restored metal reduction. Although the outer membrane conduits for metal reduction were similar, homologs of the inner membrane and periplasmic EET components CymA, FccA, and CctA were not found in We characterized a cluster of genes predicted to encode components related to a formate-dependent nitrite reductase (NrfBCD), here named NetBCD (for rf-like lectron ransfer), and a predicted diheme periplasmic cytochrome, PdsA (eriplasmic iheme huttle). We present genetic evidence that proteins encoded by this cluster facilitate electron transfer from the cytoplasmic membrane across the periplasm to the MtrCAB conduit and function independently from an authentic NrfABCD system. mutants lacking and were unable to reduce metals, while heterologous expression of these genes could restore metal reduction in an mutant background. EET may therefore allow and other species of to persist and thrive in iron- or manganese-rich oxygen-limited environments. Metal-reducing microorganisms are used for electricity production, bioremediation of toxic compounds, wastewater treatment, and production of valuable compounds. Despite numerous microorganisms being reported to reduce metals, the molecular mechanism has primarily been studied in two model systems, and We have characterized the mechanism of extracellular electron transfer in , which uses the well-studied system, MtrCAB, to move electrons across the outer membrane; however, most spp. appear to use a novel mechanism to transfer electrons from the inner membrane through the periplasm and to the outer membrane. The conserved use of MtrCAB in spp. and spp. for metal reduction and conserved genomic architecture of metal reduction genes in spp. may serve as genomic markers for identifying metal-reducing microorganisms from genomic or transcriptomic sequencing. Understanding the variety of pathways used to reduce metals can allow for optimization and more efficient design of microorganisms used for practical applications.