Identification of a molecular signature unique to metal-reducing Gammaproteobacteria

Functional genes required for microbial (dissimilatory) metal reduction display high sequence divergence, which limits their utility as molecular biomarkers for tracking the presence and activity of metal-reducing bacteria in natural and engineered systems. In the present study, homologs of the outer membrane beta-barrel protein MtrB of metal-reducing Gammaproteobacteria were found to contain a unique N-terminal CXXC motif that was missing from MtrB homologs of nonmetal-reducing Gammaproteobacteria and metal- and nonmetal-reducing bacteria outside the Gammaproteobacteria. To determine whether the N-terminal CXXC motif of MtrB was required for dissimilatory metal reduction, each cysteine in the CXXC motif of the representative metal-reducing gammaproteobacterium Shewanella oneidensis was replaced with alanine, and the resulting site-directed mutants were tested for metal reduction activity. Anaerobic growth experiments demonstrated that the first, but not the second, conserved cysteine was required for metal reduction by S. oneidensis. The ability to predict metal reduction by Gammaproteobacteria with unknown metal reduction capability was confirmed with Vibrio parahaemolyticus, a pathogen whose genome encodes an MtrB homolog with an N-terminal CXXC motif. MtrB homologs with an N-terminal CXXC motif may thus represent a molecular signature unique to metal-reducing members of the Gammaproteobacteria.