Structure of OmcZ filaments suggests extracellular cytochrome polymers evolved independently multiple times.

While early genetic and low-resolution structural observations suggested that extracellular conductive filaments on metal-reducing organisms such as were composed of type IV pili, it has now been established that bacterial -type cytochromes can polymerize to form extracellular filaments capable of long-range electron transport. Atomic structures exist for two such cytochrome filaments, formed from the hexaheme cytochrome OmcS and the tetraheme cytochrome OmcE. Due to the highly conserved heme packing within the central OmcS and OmcE cores, and shared pattern of heme coordination between subunits, it has been suggested that these polymers have a common origin.