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Original Research ARTICLE

Front. Microbiol., 08 February 2012 | http://dx.doi.org/10.3389/fmicb.2012.00037

Identification and characterization of MtoA: a decaheme c-type cytochrome of the neutrophilic Fe(II)-oxidizing bacterium Sideroxydans lithotrophicus ES-1

Juan Liu1, Zheming Wang1, Sara M. Belchik1, Marcus J. Edwards2, Chongxuan Liu1, David W. Kennedy1, Eric D. Merkley1, Mary S. Lipton1, Julea N. Butt2, David J. Richardson2, John M. Zachara1, James K. Fredrickson1, Kevin M. Rosso1 and Liang Shi1*
  • 1 Pacific Northwest National Laboratory, Richland, WA, USA
  • 2 Centre for Molecular and Structural Biochemistry, School of Biological Sciences and School of Chemistry, University of East Anglia Norwich Research Park, Norwich, UK

The Gram-negative bacterium Sideroxydans lithotrophicus ES-1 (ES-1) grows on FeCO3 or FeS at oxic–anoxic interfaces at circumneutral pH, and the ES-1-mediated Fe(II) oxidation occurs extracellularly. However, the molecular mechanisms underlying ES-1’s ability to oxidize Fe(II) remain unknown. Survey of the ES-1 genome for candidate genes for microbial extracellular Fe(II) oxidation revealed that it contained a three-gene cluster encoding homologs of Shewanella oneidensis MR-1 (MR-1) MtrA, MtrB, and CymA that are involved in extracellular Fe(III) reduction. Homologs of MtrA and MtrB were also previously shown to be involved in extracellular Fe(II) oxidation by Rhodopseudomonas palustris TIE-1. To distinguish them from those found in MR-1, the identified homologs were named MtoAB and CymAES-1. Cloned mtoA partially complemented an MR-1 mutant without MtrA with regards to ferrihydrite reduction. Characterization of purified MtoA showed that it was a decaheme c-type cytochrome and oxidized soluble Fe(II). Oxidation of Fe(II) by MtoA was pH- and Fe(II)-complexing ligand-dependent. Under conditions tested, MtoA oxidized Fe(II) from pH 7 to pH 9 with the optimal rate at pH 9. MtoA oxidized Fe(II) complexed with different ligands at different rates. The reaction rates followed the order Fe(II)Cl2 > Fe(II)–citrate > Fe(II)–NTA > Fe(II)–EDTA with the second-order rate constants ranging from 6.3 × 10−3 μM−1 s−1 for oxidation of Fe(II)Cl2 to 1.0 × 10−3 μM−1 s−1 for oxidation of Fe(II)–EDTA. Thermodynamic modeling showed that redox reaction rates for the different Fe(II)-complexes correlated with their respective estimated reaction-free energies. Collectively, these results demonstrate that MtoA is a functional Fe(II)-oxidizing protein that, by working in concert with MtoB and CymAES-1, may oxidize Fe(II) at the bacterial surface and transfer released electrons across the bacterial cell envelope to the quinone pool in the inner membrane during extracellular Fe(II) oxidation by ES-1.

Keywords: Sideroxydans lithotrophicus ES-1, extracellular Fe(II) oxidation, decaheme c-type cytochrome MtoA, pH-dependent, ligand complexation

Citation: Liu J, Wang Z, Belchik SM, Edwards MJ, Liu C, Kennedy DW, Merkley ED, Lipton MS, Butt JN, Richardson DJ, Zachara JM, Fredrickson JK, Rosso KM and Shi L (2012) Identification and characterization of MtoA: a decaheme c-type cytochrome of the neutrophilic Fe(II)-oxidizing bacterium Sideroxydans lithotrophicus ES-1. Front. Microbio. 3:37. doi: 10.3389/fmicb.2012.00037

Received: 22 October 2011; Accepted: 23 January 2012;
Published online: 08 February 2012.

Edited by:

David Emerson, Bigelow Laboratory for Ocean Sciences, USA

Reviewed by:

Violaine Bonnefoy, Centre National de la Recherche Scientifique, France
Yongqin Jiao, Lawrence Livermore National Laboratory, USA

Copyright: © 2012 Liu, Wang, Belchik, Edwards, Liu, Kennedy, Merkley, Lipton, Butt, Richardson, Zachara, Fredrickson, Rosso and Shi. This is an open-access article distributed under the terms of the Creative Commons Attribution Non Commercial License, which permits non-commercial use, distribution, and reproduction in other forums, provided the original authors and source are credited.

*Correspondence: Liang Shi, Microbiology Group, Pacific Northwest National Laboratory, 902 Battelle Blvd., P.O. Box 999, Richland, WA 99352, USA. e-mail: liang.shi@pnnl.gov