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The tandem gene clusters  orfR - ombB-omaB-omcB  and  orfS - ombC-omaC-omcC  of the metal-reducing bacterium  Geobacter sulfurreducens  PCA are responsible for trans-outer membrane electron transfer during extracellular reduction of Fe(III)-citrate and ferrihydrite [a poorly crystalline Fe(III) oxide]. Each gene cluster encodes a putative transcriptional factor (OrfR/OrfS), a porin-like outer-membrane protein (OmbB/OmbC), a periplasmic  c -type cytochrome ( c -Cyt, OmaB/OmaC) and an outer-membrane  c -Cyt (OmcB/OmcC). The individual roles of OmbB, OmaB and OmcB in extracellular reduction of Fe(III), however, have remained either uninvestigated or controversial. Here, we showed that replacements of  ombB ,  omaB ,  omcB , and  ombB-omaB  with an antibiotic gene in the presence of  ombC-omaC-omcC  had no impact on reduction of Fe(III)-citrate by  G. sulfurreducens  PCA. Disruption of  ombB ,  omaB ,  omcB , and  ombB-omaB  in the absence of  ombC-omaC-omcC , however, severely impaired the bacterial ability to reduce Fe(III)-citrate as well as ferrihydrite. These results unequivocally demonstrate an overlapping role of  ombB-omaB-omcB  and  ombC-omaC-omcC  in extracellular Fe(III) reduction by  G. sulfurreducens  PCA. Involvement of both  ombB-omaB-omcB  and  ombC-omaC-omcC  in extracellular Fe(III) reduction reflects the importance of these trans-outer membrane protein complexes in the physiology of this bacterium. Moreover, the kinetics of Fe(III)-citrate and ferrihydrite reduction by these mutants in the absence of  ombC-omaC-omcC  were nearly identical, which suggests that absence of any protein subunit eliminates function of OmaB/OmbB/OmcB protein complex. Finally,  orfS  was found to have a negative impact on the extracellular reduction of Fe(III)-citrate and ferrihydrite in  G. sulfurreducens  PCA probably by serving as a transcriptional repressor.

Direct involvement of ombB, omaB, and omcB genes in extracellular reduction of Fe(III) by Geobacter sulfurreducens PCA