Improper Localization of the OmcS Cytochrome May Explain the Inability of the xapD-Deficient Mutant of Geobacter sulfurreducens to Reduce Fe(III) Oxide

Abstract

Extracellular electron transfer through a redox-active exopolysaccharide matrix has been proposed as a strategy for extracellular electron transfer to Fe(III) oxide by Geobacter sulfurreducens, based on the phenotype of a xapD-deficient strain. Central to this model was the assertion that the xapD-deficient strain produced pili decorated with the multi-heme c-type cytochrome OmcS in manner similar to the wild-type strain. Further examination of the xapD-deficient strain with immunogold labeling of OmcS and transmission electron microscopy revealed that OmcS was associated with the outer cell surface rather than pili. PilA, the pilus monomer, could not be detected in the xapD-deficient strain under conditions in which it was readily detected in the wild-type strain. Multiple lines of evidence in previous studies have suggested that long-range electron transport to Fe(III) oxides proceeds through electrically conductive pili and that OmcS associated with the pili is necessary for electron transfer from the pili to Fe(III) oxides. Therefore, an alternative explanation for the Fe(III) oxide reduction phenotype of the xapD-deficient strain is that the pili-OmcS route for extracellular electron transport to Fe(III) oxide has been disrupted in the xapD-deficient strain.