A trans‐outer membrane porin‐cytochrome protein complex for extracellular electron transfer by G eobacter sulfurreducens PCA

Abstract

Summary: The multi‐heme, outer membrane c ‐type cytochrome ( c ‐ C yt) OmcB of G eobacter sulfurreducens was previously proposed to mediate electron transfer across the outer membrane. However, the underlying mechanism has remained uncharacterized. In G . sulfurreducens , the omcB gene is part of two tandem four‐gene clusters, each is predicted to encode a transcriptional factor ( OrfR / OrfS ), a porin‐like outer membrane protein ( OmbB / OmbC ), a periplasmic c‐ type cytochrome ( OmaB / OmaC ) and an outer membrane c ‐ C yt ( OmcB / OmcC ) respectively. Here, we showed that OmbB / OmbC , OmaB / OmaC and OmcB / OmcC of G . sulfurreducens PCA formed the porin‐cytochrome ( P cc) protein complexes, which were involved in transferring electrons across the outer membrane. The isolated P cc protein complexes reconstituted in proteoliposomes transferred electrons from reduced methyl viologen across the lipid bilayer of liposomes to F e( III )‐citrate and ferrihydrite. The pcc clusters were found in all eight sequenced G eobacter and 11 other bacterial genomes from six different phyla, demonstrating a widespread distribution of P cc protein complexes in phylogenetically diverse bacteria. Deletion of ombB‐omaB‐omcB‐orfS‐ombC‐omaC‐omcC gene clusters had no impact on the growth of G . sulfurreducens PCA with fumarate but diminished the ability of G . sulfurreducens PCA to reduce F e( III )‐citrate and ferrihydrite. Complementation with the ombB‐omaB‐omcB gene cluster restored the ability of G . sulfurreducens PCA to reduce F e( III )‐citrate and ferrihydrite.