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Background  Metal reduction is thought to take place at or near the bacterial outer membrane and, thus, outer membrane proteins in the model dissimilatory metal-reducing organism  Geobacter sulfurreducens  are of interest to understand the mechanisms of Fe(III) reduction in the  Geobacter  species that are the predominant Fe(III) reducers in many environments. Previous studies have implicated periplasmic and outer membrane cytochromes in electron transfer to metals. Here we show that the most abundant outer membrane protein of  G. sulfurreducens , OmpJ, is not a cytochrome yet it is required for metal respiration.    Results  When outer membrane proteins of  G. sulfurreducens  were separated via SDS-PAGE, one protein, designated OmpJ ( o uter  m embrane  p rotein J), was particularly abundant. The encoding gene, which was identified from mass spectrometry analysis of peptide fragments, is present in other  Geobacteraceae , but not in organisms outside this family. The predicted localization and structure of the OmpJ protein suggested that it was a porin. Deletion of the  ompJ  gene in  G. sulfurreducens  produced a strain that grew as well as the wild-type strain with fumarate as the electron acceptor but could not grow with metals, such as soluble or insoluble Fe (III) and insoluble Mn (IV) oxide, as the electron acceptor. The heme  c  content in the mutant strain was ca. 50% of the wild-type and there was a widespread loss of multiple cytochromes from soluble and membrane fractions. Transmission electron microscopy analyses of mutant cells revealed an unusually enlarged periplasm, which is likely to trigger extracytoplasmic stress response mechanisms leading to the degradation of periplasmic and/or outer membrane proteins, such as cytochromes, required for metal reduction. Thus, the loss of the capacity for extracellular electron transport in the mutant could be due to the missing  c -type cytochromes, or some more direct, but as yet unknown, role of OmpJ in metal reduction.    Conclusion  OmpJ is a putative porin found in the outer membrane of the model metal reducer  G. sulfurreducens  that is required for respiration of extracellular electron acceptors such as soluble and insoluble metals. The effect of OmpJ in extracellular electron transfer is indirect, as OmpJ is required to keep the integrity of the periplasmic space necessary for proper folding and functioning of periplasmic and outer membrane electron transport components. The exclusive presence of  ompJ  in members of the  Geobacteraceae  family as well as its role in metal reduction suggest that the  ompJ  sequence may be useful in tracking the growth or activity of  Geobacteraceae  in sedimentary environments.

bmc microbiology

A novel Geobacteraceae-specific outer membrane protein J (OmpJ) is essential for electron transport to Fe (III) and Mn (IV) oxides in Geobacter sulfurreducens