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Supplemental results EBI InterPro Scan (version 4.8) protein domain analysis of RnfC reveals that S. fumaroxidans strain MPOB RnfC encodes RnfC beta-barrel sandwich hybrid domain (IPR026902), NADH:ubiquinone oxidoreductase, 51kDa subunit (IPR010208), alphahelical ferredoxin (IPR009051), C-terminal fumarate reductase (IPR012285), and 4Fe-4S ferredoxin-type, Fe-S binding domains (IPR017896/017900). In comparison, the strain UI SynarDRAFT_0709 encodes FAD-binding oxidoreductase (IPR008333), FAD/NAD(P)-binding oxidoreductase (IPR001433), ferredoxin reductasetype FAD-binding domain (IPR017927), riboflavin synthase-like beta-barrel (IPR017936), and dihydroorotate dehydrogenase, electron transfer subunit, Fe-S cluster binding-domain (IPR019480), ferredoxin reductase-like C-terminal domain (SSF52343), and a transmembrane region. In addition, SynarDRAFT_0710 encodes an alpha-helical ferredoxin (IPR009051), C-terminal fumarate reductase (IPR012285), and 4Fe-4S ferredoxin-type, Fe-S binding domains (IPR017896/017900). The RnfC and SynarDRAFT_0710-0709 clearly share features. In particular, both possess alphahelical ferredoxin, C-terminal fumarate reductase, 4Fe-4S ferredoxin-type Fe-S binding domain, and transmembrane beta-barrel domains. Strikingly, both betabarrel domains encode seven beta-strands. Transmembrane beta-barrel are known to act as selective ion channels (Smythies, 1981; Schulz, 2000; Baumeister et al., 2001; Delcour, 2002; Menestrina et al., 2003; Som et al., 2003; Moroni and Thiel, 2006; Jang et al., 2010; Mohammad et al., 2011; Garcia-Gimenez et al., 2012). Given that none of the other Rnf subunits encode discernable transmembrane ion channels, we speculate that the RnfC beta-barrel may serve as an ion channel driving reverse electron transport. Furthermore, the RnfC and SynarDRAFT_0709 beta-barrels may select for ions with similar size and charge because they are both seven-stranded. References Baumeister, B., Sakai, N., and Matile, S. (2001) p-octiphenyl β-barrels with ion channel and esterase activity. Org Lett 3: 4229-4232. Delcour, A.H. (2002) Structure and function of pore-forming β-barrels from bacteria. J Mol Microbiol Biotechnol 4: 1-10. Garcia-Gimenez, E., Alcaraz, A., and Aguilella, V.M. (2012) Divalent metal ion transport across large biological ion channels and their effect on conductance and selectivity. Biochem Res Int 2012: 245786. Jang, H., Arce, F.T., Ramachandran, S., Capone, R., Lal, R., and Nussinov, R. (2010) βBarrel topology of Alzheimer's beta-amyloid ion channels. J Mol Biol 404: 917-934. Menestrina, G., Dalla Serra, M., Comai, M., Coraiola, M., Viero, G., Werner, S. et al. (2003) Ion channels and bacterial infection: the case of β-barrel pore-forming protein toxins of Staphylococcus aureus. FEBS Lett 552: 54-60. Mohammad, M.M., Howard, K.R., and Movileanu, L. (2011) Redesign of a plugged βbarrel membrane protein. J Biol Chem 286: 8000-8013. Moroni, A., and Thiel, G. (2006) Flip-flopping salt bridges gate an ion channel. Nat Chem Biol 2: 572-573. Schulz, G.E. (2000) β-Barrel membrane proteins. Curr Opin Struct Biol 10: 443-447. Smythies, J.R. (1981) On the molecular structure of the ion channel associated with the acetylcholine receptor: a β-barrel? Med Hypotheses 7: 1095-1098. Som, A., Sakai, N., and Matile, S. (2003) Complementary characteristics of homologous p-octiphenyl β-barrels with ion channel and esterase activity. Bioorg Med Chem 11: 1363-1369.