Extracellular cytochrome nanowires appear to be ubiquitous in prokaryotes

Electrically conductive appendages from the anaerobic bacterium Geobacter sulfurreducens, recently iden-tified as extracellular cytochrome nanowires (ECNs), have received wide attention due to numerous potential applications. However, whether other organisms employ similar ECNs for electron transfer remains unknown. Here, using cryoelectron microscopy, we describe the atomic structures of two ECNs from two major orders of hyperthermophilic archaea present in deep-sea hydrothermal vents and terrestrial hot springs. Homologs of Archaeoglobus veneficus ECN are widespread among mesophilic methane-oxidizing Methanoperedena-ceae, alkane-degrading Syntrophoarchaeales archaea, and in the recently described megaplasmids called Borgs. The ECN protein subunits lack similarities in their folds; however, they share a common heme arrange-ment, suggesting an evolutionarily optimized heme packing for efficient electron transfer. The detection of ECNs in archaea suggests that filaments containing closely stacked hemes may be a common and wide-spread mechanism for long-range electron transfer in both prokaryotic domains of life.