, 2002; Gorby et al., 2006); (2) nonreductive dissolution of metal oxides to form more readily reducible organic metal complexes (Taillefert et al., 2007; Fennessey et al., 2010; Jones et al., Pirfenidone molecular weight 2010); and (3) delivery of
electrons to external metals via endogenous or exogenous electron shuttles (Hernandez et al., 2004; Marsili et al., 2008; Roden et al., 2010). Shewanella oneidensis contains an electron transport chain that consists of IM-localized primary dehydrogenases, menaquinone, and CymA, a menaquinol-oxidizing c-type cytochrome that functions as a central branch point in electron transport to Fe(III), Mn(IV), nitrate (), nitrite (), dimethyl sulfoxide (DMSO), and fumarate (Myers & Myers, 1997). CymA transfers electrons to the periplasmic c-type cytochrome MtrA (Schuetz et al., 2009), which interacts with outer membrane (OM)-localized protein complexes composed of transmembrane β-barrel protein MtrB (Beliaev & Saffarini, 1998; Myers & Myers,
2002) and decaheme c-type cytochrome MtrC (Shi et al., 2006; Ross et al., 2007). Purified MtrC reduces Fe(III) (Hartshorne et al., 2007; Eggleston et al., 2008), and in proteoliposomes, purified MtrB, MtrC, and MtrA form a lipid-embedded ‘porin–cytochrome’ complex (Richardson Inhibitor Library order et al., 2012) that transfers electrons from internal reduced methyl viologen to external Fe(III) substrates (Hartshorne et al., 2009; White et al., 2013). Previous nucleotide sequence analyses indicated that the N-terminus of S. oneidensis MtrB contained a unique CXXC motif (Beliaev & Saffarini, 1998). The identification of a CXXC motif in S. oneidensis MtrB was unusual because CXXC motifs are generally not found in OM β-barrel Rucaparib proteins, most likely to avoid protein-folding problems caused by redox-reactive cysteines during passage across the intermembrane space in eukaryotes or the periplasmic space in bacteria (Tamm et al., 2004; Schleiff & Soll, 2005; Denoncin et al., 2010). The identification of an unusual CXXC motif in the N-terminus of MtrB led us to
hypothesize that this motif may represent a molecular signature unique to metal-reducing γ-proteobacteria. To test this hypothesis, nucleotide sequence analyses were carried out to correlate dissimilatory metal reduction capability with the presence of MtrB homologs containing an N-terminal CXXC motif. Site-directed mutational analyses were performed to determine whether the N-terminal CXXC motif of MtrB was required for metal reduction by the representative metal-reducing γ-proteobacterium S. oneidensis. The ability to predict dissimilatory metal reduction by a γ-proteobacterium with unknown metal reduction capability was then tested with Vibrio parahaemolyticus, a human pathogen whose genome encodes an MtrB homolog with an N-terminal CXXC motif. Bacterial strains and plasmids used in this study are listed in Table 1. For genetic manipulations, all Escherichia coli and S.