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Long-range (>10 μm) transport of electrons along networks of Geobacter sulfurreducens protein filaments, known as microbial nanowires, has been invoked to explain a wide range of globally important redox phenomena. These nanowires were previously thought to be type IV pili composed of PilA protein. Here, we report a 3.7 Å resolution cryoelectron microscopy structure, which surprisingly reveals that, rather than PilA, G. sulfurreducens nanowires are assembled by micrometer-long polymerization of the hexaheme cytochrome OmcS, with hemes packed within ∼3.5-6 Å of each other. The inter-subunit interfaces show unique structural elements such as inter-subunit parallel-stacked hemes and axial coordination of heme by histidines from neighboring subunits. Wild-type OmcS filaments show 100-fold greater conductivity than other filaments from a ΔomcS strain, highlighting the importance of OmcS to conductivity in these nanowires. This structure explains the remarkable capacity of soil bacteria to transport electrons to remote electron acceptors for respiration and energy sharing.

Extracellular electron transfer by Geobacter species through surface appendages known as microbial nanowires.sup.1 is important in a range of globally important environmental phenomena.sup.2, as well as for applications in bio-remediation, bioenergy, biofuels and bioelectronics. Since 2005, these nanowires have been thought to be type 4 pili composed solely of the PilA-N protein.sup.1. However, previous structural analyses have demonstrated that, during extracellular electron transfer, cells do not produce pili but rather nanowires made up of the cytochromes OmcS.sup.2,3 and OmcZ.sup.4. Here we show that Geobacter sulfurreducens binds PilA-N to PilA-C to assemble heterodimeric pili, which remain periplasmic under nanowire-producing conditions that require extracellular electron transfer.sup.5. Cryo-electron microscopy revealed that C-terminal residues of PilA-N stabilize its copolymerization with PilA-C (to form PilA-N-C) through electrostatic and hydrophobic interactions that position PilA-C along the outer surface of the filament. PilA-N-C filaments lack [pi]-stacking of aromatic side chains and show a conductivity that is 20,000-fold lower than that of OmcZ nanowires. In contrast with surface-displayed type 4 pili, PilA-N-C filaments show structure, function and localization akin to those of type 2 secretion pseudopili.sup.6. The secretion of OmcS and OmcZ nanowires is lost when pilA-N is deleted and restored when PilA-N-C filaments are reconstituted. The substitution of pilA-N with the type 4 pili of other microorganisms also causes a loss of secretion of OmcZ nanowires. As all major phyla of prokaryotes use systems similar to type 4 pili, this nanowire translocation machinery may have a widespread effect in identifying the evolution and prevalence of diverse electron-transferring microorganisms and in determining nanowire assembly architecture for designing synthetic protein nanowires. Structural, functional and localization studies reveal that Geobacter sulfurreducens pili cannot behave as microbial nanowires, instead functioning in a similar way to secretion pseudopili to export cytochrome nanowires that are essential for extracellular electron transfer., Author(s): Yangqi Gu [sup.1] [sup.2] , Vishok Srikanth [sup.1] [sup.3] , Aldo I. Salazar-Morales [sup.1] [sup.3] , Ruchi Jain [sup.1] [sup.3] , J. Patrick O'Brien [sup.1] [sup.3] , Sophia M. [...]

The law of demand is the basic tenet of economic theory which states that, other things being equal, more of an item will be bought, the lower its price and less will be bought, the higher its price. This was demonstrated dramatically at Jacksonville State University when the library photocopy service reduced the cost of copies from 10¢ to 5¢. Over the course of two years, the number of copies purchased more than doubled after the price was halved. After adjusting for other variables, such as fluctuating enrollments and the addition of another copier, unitary price elasticity, or a one-to-one inverse ratio of price to quantity, was evident. The user benefit of the price reduction was obvious, while the fact that the photocopy service provider continues to renew the contract at the reduced price indicates that the service remains profitable. [ABSTRACT FROM PUBLISHER]