Single Amino Acid Residues Control Potential-Dependent Inactivation of an Inner Membrane bc-Cytochrome**

During extracellular electron transfer, Geobacter sulfurreducens constitutively expresses the bc-cytochrome CbcL, yet cells containing only this menaquinone oxidase fail to respire above −0.1 V vs. SHE. By identifying mutations within cbcL that permit growth at higher potentials, we provide evidence that this cytochrome is regulated by redox potential. Strains expressing only CbcL^V205A, CbcL^V205G, and CbcL^F525Y were capable of growth with high potential electron acceptors including Fe(III) citrate, Mn(IV) oxides, and electrodes poised at +0.1 V vs. SHE. Electrochemical characterization of wild type CbcL revealed oxidative inactivation of electron transfer above −0.1 V, while CbcL^V205A, CbcL^V205G, and CbcL^F525Y remained active. Growth yields of CbcL^V205A, CbcL^V205G, and CbcL^F525Y were only 50 % of WT, consistent with CbcL-dependent electron transfer conserving less energy. These data support the hypothesis that CbcL has evolved to rapidly shut off in response to redox potential, in order to divert electrons to higher yield oxidases that coexist in the Geobacter membrane.