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

Authors

Juan Liu, Pacific Northwest National Laboratory
Zheming Wang, Pacific Northwest National LaboratoryFollow
Sara M. Belchik, Pacific Northwest National Laboratory
Marcus J. Edwards, University of East Anglia
Chongxuan Liu, Pacific Northwest National LaboratoryFollow
David W. Kennedy, Pacific Northwest National LaboratoryFollow
Eric D. Merkley, Pacific Northwest National Laboratory
Mary S. Lipton, Pacific Northwest National Laboratory
Julea N. Butt, University of East Anglia
David J. Richardson, University of East AngliaFollow
John M. Zachara, Pacific Northwest National LaboratoryFollow
James K. Fredrickson, Pacific Northwest National LaboratoryFollow
Kevin M. Rosso, Pacific Northwest National Laboratory
Liang Shi, Pacific Northwest National LaboratoryFollow

Date of this Version

2-8-2012

Citation

Microbiological Chemistry, February 2012, Volume 3 Article 37; doi: 10.3389/fmicb.2012.00037

Abstract

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 formicrobial extracellular Fe(II) oxidation revealed that it contained a three-genecluster 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 andCymA_ES-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 6.3×10⁻³μM⁻¹s⁻¹ for oxidation of Fe(II) Cl₂ to 1.0 × 10⁻³ μM⁻¹s⁻¹ for oxidation of Fe (II)–EDTA.