A PERIPLASMIC AND EXTRACELLULAR C-TYPE CYTOCHROME OF GEOBACTER-SULFURREDUCENS ACTS AS A FERRIC IRON REDUCTASE AND AS AN ELECTRON CARRIER TOOTHER ACCEPTORS OR TO PARTNER BACTERIA

An estracellular electron carrier excreted into the growth medium by c ells of Geobacter sulfurreducens was identified as a c-type cytochrome . The cytochrome was found to be distributed in about equal amounts in the membrane fraction, the periplasmic space, and the surrounding med ium during all phases of growth with acetate plus fumarate, It was iso lated from periplasmic preparations and purified to homogeneity by cat ion-exchange chromatography, gel filtration, and hydrophobic interacti on chromatography. The electrophoretically homogeneous cytochrome had a molecular mass of 9.57 +/- 0.02 kDa and exhibited in its reduced sta te absorption maxima at wavelengths of 552, 522, and 419 nm, The midpo int redox potential determined by redox titration was -0.167 V, With r espect to molecular mass, redox properties, and molecular features, th is cytochrome exhibited its highest similarity to the cytochromes c of Desulfovibrio salexigens and Desulfuromonas acetoxidans. The G, sulfu rreducens cytochrome c reduced ferrihydrite (Fe(OH)(3)), Fe(III) nitri lotriacetic acid, Fe(III) citrate, and manganese dioxide at high rates . Elemental sulfur, anthraquinone disulfonate, and humic acids were re duced more slowly, G, sulfurreducens reduced the cytochrome with aceta te as an electron donor and oxidized it dth fumarate, Wolinella succin ogens was able to reduce externally provided cytochrome c of G, sulfur -reducens with molecular hydrogen or formate as an electron donor and oxidized it with fumarate ol nitrate as an electron acceptor. A cocult ure could be established in which G, sulfurreducens reduced the cytoch rome with acetate, and the reduced cytochrome was reoxidized by W succ inogens in the presence of nitrate. We conclude that this cytochrome c an act as iron(III) reductase for electron transfer to insoluble iron hydroxides or to sulfur, manganese dioxide, or other oxidized compound s, and it can transfer electrons to partner bacteria.