2 research outputs found
Comparative genomics of Geobacter chemotaxis genes reveals diverse signaling function
Background Geobacter species are δ-Proteobacteria and are often the predominant species in a variety of sedimentary environments where Fe(III) reduction is important. Their ability to remediate contaminated environments and produce electricity makes them attractive for further study. Cell motility, biofilm formation, and type IV pili all appear important for the growth of Geobacter in changing environments and for electricity production. Recent studies in other bacteria have demonstrated that signaling pathways homologous to the paradigm established for Escherichia coli chemotaxis can regulate type IV pili-dependent motility, the synthesis of flagella and type IV pili, the production of extracellular matrix material, and biofilm formation. The classification of these pathways by comparative genomics improves the ability to understand how Geobacter thrives in natural environments and better their use in microbial fuel cells. Results The genomes of G. sulfurreducens, G. metallireducens, and G. uraniireducens contain multiple (~70) homologs of chemotaxis genes arranged in several major clusters (six, seven, and seven, respectively). Unlike the single gene cluster of E. coli, the Geobacter clusters are not all located near the flagellar genes. The probable functions of some Geobacter clusters are assignable by homology to known pathways; others appear to be unique to the Geobacter sp. and contain genes of unknown function. We identified large numbers of methyl-accepting chemotaxis protein (MCP) homologs that have diverse sensing domain architectures and generate a potential for sensing a great variety of environmental signals. We discuss mechanisms for class-specific segregation of the MCPs in the cell membrane, which serve to maintain pathway specificity and diminish crosstalk. Finally, the regulation of gene expression in Geobacter differs from E. coli. The sequences of predicted promoter elements suggest that the alternative sigma factors σ28 and σ54 play a role in regulating the Geobacter chemotaxis gene expression. Conclusion The numerous chemoreceptors and chemotaxis-like gene clusters of Geobacter appear to be responsible for a diverse set of signaling functions in addition to chemotaxis, including gene regulation and biofilm formation, through functionally and spatially distinct signaling pathways
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PHYSIOLOGICAL MODELS OF GEOBACTER SULFURREDUCENS AND DESULFOBACTER POSTGATEI TO UNDERSTAND URANIUM REMEDIATION IN SUBSURFACE SYSTEMS
Geobacter species are often the predominant Fe(III)-reducing microorganisms in many sedimentary environments due to their capacity for extracellular electron transfer. This exceptional physiological capability allows them to couple acetate oxidation to uranium (U(VI)) reduction, that is one of the most significant interactions between radionuclides and microorganisms that naturally takes place in uranium-contaminated environments. Although this process has been proposed as a promising strategy for the in situ bioremediation of uranium-contaminated groundwater, little is known about the molecular mechanisms involved in U(VI) reduction and the interaction between Geobacter and other microbial species.
In the first two research chapters, this dissertation aim to study the interaction between Geobacter sulfurreducens, a primary model organism to elucidate the physiological capabilities of Geobacter species, and U(VI). Our findings presented here suggest that G. sulfurreducens requires outer-surface c-type cytochromes, but not pili, for the reduction of uranium, and U(IV), the product of U(VI) reduction was precipitated at the outer cell surface. Our results also suggest that there is not one specific U(VI)-detoxification specific mechanism for uranium detoxification in G. sulfurreducens. Rather, resistance to U(VI) appears to be accomplished with multiple stress response systems, that includes detoxification and oxidative stress response, and regulatory networks that facilitate fast adaptation to rapidly changing conditions.
The third research chapter of this dissertation examines the physiology of Desulfobacter postgatei, a competitor for acetate during in situ bioremediation of subsurface systems at a uranium-contaminated site in Rifle, CO. Our findings suggest that novel enzymatic complexes, such as the energy-converting hydrogenase related complex, Ehr, the proton-translocating ferredoxin:NADP+ oxidoreductase, Rnf, and also the NADH-dependent reduced ferredoxin:NADP+ oxidoreductase, Nfn, are involved in energy conservation, making D. postgatei a major competitor for acetate in several environments