Characterization of Bacterial Electron Transport to Extracellular Electron Acceptors
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Photosynthesis in the cyanobacteria Synechocystis sp. PCC 6803 requires both manganese and a large quantity of iron to maintain its electron transport chain and oxygen evolution; however, in most natural cyanobacterial environments these elements are present in the insoluble form of oxides. Whether these oxides can be used by cyanobacteria, and the whether there are any molecular mechanisms involved is yet to be completely defined. There is evidence mounting suggesting that bacterial type IV pili (nanowires) facilitate extracellular donation of electrons to electron acceptors, like iron oxides in non-photosynthetic bacteria. The donation of electrons to iron oxides in these organisms may be crucial for maintaining anaerobic respiration. Since the iron quota of the cyanobacterium Synechocystis sp. PCC 6803 is in the 106 atoms per cell range (one order of magnitude higher than that of the similarly sized non-photosynthetic organism, Escherichia coli), it would therefore be desirable for Synechocystis sp. PCC 6803 to conserve the metal reduction properties of these type IV pili. This study investigates if any of the type IV PilA pilin proteins in Synechocystis sp. PCC 6803 (including PilA1, the main pilin protein) may also provide a mechanism for converting insoluble forms of metal elements into soluble forms for uptake by membrane transport systems. Growth experiments, spectroscopic data and oxygen evolution of strains deficient in the pilA1 gene suggest the presence of the pilA1 gene enhances growth ability on various iron and manganese oxides. This provides a novel biological function of PilA1 and the pili it forms in Synechocystis sp. PCC 6803 iron and manganese acquisition.