<i>Shewanella putrefaciens mtrB</i>Encodes an Outer Membrane Protein Required for Fe(III) and Mn(IV) Reduction

<jats:title>ABSTRACT</jats:title><jats:p>Iron and manganese oxides or oxyhydroxides are abundant transition metals, and in aquatic environments they serve as terminal electron acceptors for a large number of bacterial species. The molecular mechanisms of anaerobic metal reduction, however, are not understood.<jats:italic>Shewanella putrefaciens</jats:italic>is a facultative anaerobe that uses Fe(III) and Mn(IV) as terminal electron acceptors during anaerobic respiration. Transposon mutagenesis was used to generate mutants of<jats:italic>S. putrefaciens</jats:italic>, and one such mutant, SR-21, was analyzed in detail. Growth and enzyme assays indicated that the mutation in SR-21 resulted in loss of Fe(III) and Mn(IV) reduction but did not affect its ability to reduce other electron acceptors used by the wild type. This deficiency was due to Tn<jats:italic>5</jats:italic>inactivation of an open reading frame (ORF) designated<jats:italic>mtrB. mtrB</jats:italic>encodes a protein of 679 amino acids and contains a signal sequence characteristic of secreted proteins. Analysis of membrane fractions of the mutant, SR-21, and wild-type cells indicated that MtrB is located on the outer membrane of<jats:italic>S. putrefaciens</jats:italic>. A 5.2-kb DNA fragment that contains<jats:italic>mtrB</jats:italic>was isolated and completely sequenced. A second ORF, designated<jats:italic>mtrA</jats:italic>, was found directly upstream of<jats:italic>mtrB</jats:italic>. The two ORFs appear to be arranged in an operon.<jats:italic>mtrA</jats:italic>encodes a putative 10-heme<jats:italic>c</jats:italic>-type cytochrome of 333 amino acids. The N-terminal sequence of MtrA contains a potential signal sequence for secretion across the cell membrane. The amino acid sequence of MtrA exhibited 34% identity to NrfB from<jats:italic>Escherichia coli</jats:italic>, which is involved in formate-dependent nitrite reduction. To our knowledge, this is the first report of genes encoding proteins involved in metal reduction.</jats:p>