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.