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Research Highlights

December 2007

Shewanella Shows Cytochromes Convert Contaminant

S. oneidensis outer membrane cytochromes make technetium less mobile, less soluble, less risky

Results: In the first study of its kind, a research team at Pacific Northwest National Laboratory has investigated the mechanisms the bacterium Shewanella oneidensis uses to reduce the radionuclide contaminant pertechnetate to a less soluble form. Pertechnetate (99Tc(VII)O4-) is a highly mobile contaminant at the U.S. Department of Energy's Hanford Site. Experts think future migration of significant quantities of pertechnetate in the subsurface is a major risk factor to the Columbia River bordering the site. Reducing pertechnetate to a less soluble form could greatly decrease its mobility.

Previous studies showed S. oneidensis MR-1 reduced pertechnetate enzymatically to form the poorly soluble form Tc(IV)O2(s), although the precise biomolecular mechanisms remained poorly understood. Scientists have generally thought that pertechnetate reduction by microorganisms is catalyzed by hydrogenases, enzymes that catalyze hydrogen (H2) oxidation and formation. However, the PNNL team showed that, in addition to hydrogenase's involvement, the outer membrane c-type cytochromes—electron transfer proteins that the bacteria localize to their cell surface—can play a novel and crucial role in forming and localizing Tc(IV)O2 nanoparticles. Their results appear in the January 2008 issue of Environmental Microbiology.

photo:
Transmission electron microscopy micrographs show hydrogen-driven pertechnetate localization in Shewanella oneidensis MR-1 at 24 h (A and B) and the nickel-iron (NiFe) hydrogenase mutant cells at 48 h (C and D). Locations of the cell membrane (CM), periplasm (P) and outer membrane (OM) are shown.

Why it matters: This finding is significant because it demonstrates that Shewanella can efficiently use either organic matter (reduced carbon) or hydrogen as an electron donor for pertechnetate reduction. At the Hanford Site, environments exist where electron donor (energy source) availability may select for either the hydrogenase-mediated or outer membrane c-type cytochrome-mediated pertechnetate reduction. This is the first study to demonstrate that c-type cytochromes, particularly the outer membrane cytochromes, of any organism are directly responsible for the electron transfer to pertechnetate. The results of this work provide the scientific community an important insight into the biomolecular mechanisms of electron transfer by Shewanella to pertechnetate.

Methods: To better understand the biochemical mechanisms of pertechnetate reduction in S. oneidensis MR-1, the researchers tested several mutants that lacked either c-type cytochromes or functional hydrogenases. Using kinetic studies and transmission electron microscopy, they compared their ability to reduce pertechnetate to that of wild-type MR-1.

What's next: While these results were generated using Shewanella as a model organism, analogous proteins (hydrogenases and multiheme c-type cytochromes) are present in the genomes of other dissimilatory iron-reducing bacteria, including other species of Shewanella, Geobacter and Anaeromyxobacter. The results from this study could apply to these other model systems as well.

Acknowledgments: The research team included Matt Marshall, Andy Plymale, Dave Kennedy, Liang Shi, Zheming Wang, Bree Reed, Alice Dohnalkova, Chongxuan Liu, Margie Romine, John Zachara, Alex Beliaev and Jim Fredrickson, all PNNL; and Daad Saffarini, University of Wisconsin, Milwaukee. This work was supported by DOE's Environmental Remediation Sciences Program, and research was also performed within the Environmental Molecular Sciences Laboratory Scientific Grand Challenge and DOE's Genomics: GTL program. High-resolution Transmission Electron Microscopic imaging was performed at EMSL through the Scientific User Proposal system. EMSL is a DOE national scientific user facility located at PNNL.

Reference

Marshall MJ, AE Plymale, DW Kennedy, L Shi, Z Wang, SB Reed, AC Dohnalkova, CJ Simonson, C Liu, DA Saffarini, MF Romine, JM Zachara, AS Beliaev, and JK Fredrickson. 2008. "Hydrogenase- and outer membrane c-type cytochrome-facilitated reduction of technetium(VII) by Shewanella oneidensis MR-1." Environmental Microbiology 10(1):125-136 (doi:10.1111/j.1462-2920.2007.01438.x).


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