title: Identification and Characterization of MtoA: A Decaheme c-Type Cytochrome of the Neutrophilic Fe(II)-Oxidizing Bacterium Sideroxydans lithotrophicus ES-1.
abstract: The Gram-negative bacterium Sideroxydans lithotrophicus ES-1 (ES-1) grows on FeCO(3) 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 CymA(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)Cl(2) >  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)Cl(2) 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 CymA(ES-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.
authors.affiliation: Pacific Northwest National Laboratory Richland, WA, USA.
authors.forename: Juan
authors.initials: J
authors.lastname: Liu
authors.forename: Zheming
authors.initials: Z
authors.lastname: Wang
authors.forename: Sara M
authors.initials: SM
authors.lastname: Belchik
authors.forename: Marcus J
authors.initials: MJ
authors.lastname: Edwards
authors.forename: Chongxuan
authors.initials: C
authors.lastname: Liu
authors.forename: David W
authors.initials: DW
authors.lastname: Kennedy
authors.forename: Eric D
authors.initials: ED
authors.lastname: Merkley
authors.forename: Mary S
authors.initials: MS
authors.lastname: Lipton
authors.forename: Julea N
authors.initials: JN
authors.lastname: Butt
authors.forename: David J
authors.initials: DJ
authors.lastname: Richardson
authors.forename: John M
authors.initials: JM
authors.lastname: Zachara
authors.forename: James K
authors.initials: JK
authors.lastname: Fredrickson
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Rosso
authors.forename: Liang
authors.initials: L
authors.lastname: Shi