Respiratory processes in bacteria are remarkable because of their ability to use a variety of compounds, including insoluble minerals, as terminal electron acceptors1. Although much is known about microbial electron transport to soluble electron acceptors, little is understood about electron transport to insoluble compounds such as ferric oxides2,3. In anaerobic environments, humic substances can serve as electron acceptors and also as electron shuttles to ferric oxides4,5,6. To explore this process, we identified mutants in Shewanella putrefaciens that are unable to respire on humic substances. Here we show that these mutants contain disruptions in a gene that is involved in the biosynthesis of menaquinone. During growth, the wild type releases a menaquinone-related redox-active small molecule into the medium that complements the mutants. This finding raises the possibility that electron transfer to a variety of oxidants, including poorly soluble minerals, may be mediated by microbially excreted quinones that have yet to be identified.
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We thank C. Myers for providing S. putrefaciens strain MR-1 and members of the Kolter lab, P. Belshaw, F. Morel, B. Schink and D. Lovley for advice and encouragment. Preliminary sequence data was obtained from The Institute for Genomic Research website (http://www.tigr.org). Sequencing of S. putrefaciens was accomplished with support from the Department of Energy. This research was supported by the Office of Naval Research, NIH and a postdoctoral fellowship to D.K.N. by the Cancer Research Fund of the Damon Runyon Walter Winchell Foundation.
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Newman, D., Kolter, R. A role for excreted quinones in extracellular electron transfer. Nature 405, 94–97 (2000). https://doi.org/10.1038/35011098
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