Phenazine-1-Carboxylic Acid-Producing Bacteria Enhance the Reactivity of Iron Minerals in Dryland and Irrigated Wheat Rhizospheres.

Phenazine-1-carboxylic acid (PCA) is a broad-spectrum antibiotic produced by rhizobacteria in the dryland wheat fields of the Columbia Plateau. PCA and other phenazines reductively dissolve Fe and Mn oxyhydroxides in bacterial culture systems, but the impact of PCA upon Fe and Mn cycling in the rhizosphere is unknown. Here, concentrations of dithionite-extractable and poorly crystalline Fe were approximately 10% and 30-40% higher, respectively, in dryland and irrigated rhizospheres inoculated with the PCA-producing (PCA+) strain Pseudomonas synxantha 2-79 than in rhizospheres inoculated with a PCA-deficient mutant. However, rhizosphere concentrations of Fe(II) and Mn did not differ significantly, indicating that PCA-mediated redox transformations of Fe and Mn were transient or were masked by competing processes. Total Fe and Mn uptake into wheat biomass also did not differ significantly, but the PCA+ strain significantly altered Fe translocation into shoots. X-ray absorption near edge spectroscopy revealed an abundance of Fe-bearing oxyhydroxides and phyllosilicates in all rhizospheres. These results indicate that the PCA+ strain enhanced the reactivity and mobility of Fe derived from soil minerals without producing parallel changes in plant Fe uptake. This is the first report that directly links significant alterations of Fe-bearing minerals in the rhizosphere to a single bacterial trait.

Structure of a bacterial cell surface decaheme electron conduit.

Some bacterial species are able to utilize extracellular mineral forms of iron and manganese as respiratory electron acceptors. In Shewanella oneidensis this involves decaheme cytochromes that are located on the bacterial cell surface at the termini of trans-outer-membrane electron transfer conduits. The cell surface cytochromes can potentially play multiple roles in mediating electron transfer directly to insoluble electron sinks, catalyzing electron exchange with flavin electron shuttles or participating in extracellular intercytochrome electron exchange along "nanowire" appendages. We present a 3.2-Å crystal structure of one of these decaheme cytochromes, MtrF, that allows the spatial organization of the 10 hemes to be visualized for the first time. The hemes are organized across four domains in a unique crossed conformation, in which a staggered 65-Å octaheme chain transects the length of the protein and is bisected by a planar 45-Å tetraheme chain that connects two extended Greek key split ß-barrel domains. The structure provides molecular insight into how reduction of insoluble substrate (e.g., minerals), soluble substrates (e.g., flavins), and cytochrome redox partners might be possible in tandem at different termini of a trifurcated electron transport chain on the cell surface.

Global detection and characterization of hypothetical proteins in Shewanella oneidensis MR-1 using LC-MS based proteomics.

The availability of whole genome sequences has enabled the application of powerful tools for assaying global expression patterns in environmentally relevant bacteria such as Shewanella oneidensis MR-1. A large number of genes in prokaryote genomes, including MR-1, have been annotated as hypothetical, indicating that no similar protein has yet been identified in other organisms. Using high-sensitivity MS coupled with accurate mass and time (AMT) tag methodology, 1078 tryptic peptides were collectively detected in MR-1 cultures, 671 of which were unique to their parent protein. Using only these unique tryptic peptides and a minimum of two peptides per protein, we identified, with high confidence, the expression of 258 hypothetical proteins. These proteins ranged from 3.5 to 139 kDa, with 47 being 100 amino acid residues or less. Using a combination of information including detection in cells grown under specific culture conditions, presence within a specific cell fraction, and predictive algorithms such as PSORT and PSORT-B, possible/plausible functions are proposed for some hypothetical proteins. Further, by applying this approach a number of proteins were found not only to be expressed, but only expressed under certain culturing conditions, thereby suggesting function while at the same time isolating several proteins to distinct locales of the cell. These results demonstrate the utility of the AMT tag methodology for comprehensive profiling of the microbial proteome while confirming the expression of a large number of hypothetical genes.