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Evidence for a copper-dependent iron transport system in the marine, magnetotactic bacterium strain MV-1

Bradley L. Dubbels^1,

¹ Graduate Program in Microbiology, Iowa State University, Ames, IA 50011, USA
² Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA 50011, USA
³ Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48109, USA

Correspondence
Dennis A. Bazylinski
dbazylin{at}iastate.edu

Cells of the magnetotactic marine vibrio, strain MV-1, produce magnetite-containing magnetosomes when grown anaerobically or microaerobically. Stable, spontaneous, non-magnetotactic mutants were regularly observed when cells of MV-1 were cultured on solid media incubated under anaerobic or microaerobic conditions. Randomly amplified polymorphic DNA analysis showed that these mutants are not all genetically identical. Cellular iron content of one non-magnetotactic mutant strain, designated MV-1nm1, grown anaerobically, was 20- to 80-fold less than the iron content of wild-type (wt) MV-1 for the same iron concentrations, indicating that MV-1nm1 is deficient in some form of iron uptake. Comparative protein profiles of the two strains showed that MV-1nm1 did not produce several proteins produced by wt MV-1. To understand the potential roles of these proteins in iron transport better, one of these proteins was purified and characterized. This protein, a homodimer with an apparent subunit mass of about 19 kDa, was an iron-regulated, periplasmic protein (p19). Two potential ‘copper-handling’ motifs (MXM/MX₂M) are present in the amino acid sequence of p19, and the native protein binds copper in a 1 : 1 ratio. The structural gene for p19, chpA (copper handling protein) and two other putative genes upstream of chpA were cloned and sequenced. These putative genes encode a protein similar to the iron permease, Ftr1, from the yeast Saccharomyces cerevisiae, and a ferredoxin-like protein of unknown function. A periplasmic, copper-containing, iron(II) oxidase was also purified from wt MV-1 and MV-1nm1. This enzyme, like p19, was regulated by media iron concentration and contained four copper atoms per molecule of enzyme. It is hypothesized that ChpA, the iron permease and the iron(II) oxidase might have analogous functions for the three components of the S. cerevisiae copper-dependent high-affinity iron uptake system (Ctr1, Ftr1 and Fet3, respectively), and that strain MV-1 may have a similar iron uptake system. However, iron(II) oxidase purified from both wt MV-1 and MV-1nm1 displayed comparable iron oxidase activities using O₂ as the electron acceptor, indicating that ChpA does not supply the multi-copper iron(II) oxidase with copper.

The nucleotide sequences of the MV-1 3·5 kb p19 region and the p19 structural gene from MV-1nm1 have been deposited in GenBank/EMBL, accession numbers AY587957 and AY588947.

Present address: Department of Botany and Plant Pathology, Laboratory for Nitrogen Fixation Research, Oregon State University, Cordley Hall 2082, Corvallis, OR 97331-2902, USA.

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