Abstract:A soluble ferric reductase, SfrAB, which catalysed the NADPH-dependent reduction of chelated Fe(III), was previously purified from the dissimilatory Fe(III)-reducing micro-organism Geobacter sulfurreducens, suggesting that reduction of chelated forms of Fe(III) might be cytoplasmic. However, metabolically active spheroplast suspensions could not catalyse acetate-dependent Fe(III) citrate reduction, indicating that periplasmic and/or outer-membrane components were required for Fe(III) citrate reduction. Further… Show more
“…Adaptive evolution of the coculture for enhanced ethanol metabolism was associated with the formation of large aggregates of the two species. Although G. sulfurreducens is capable of utilizing either H 2 or formate as an electron donor for fumarate reduction when acetate is available as a carbon source (9), cells within the aggregates were not effective in H 2 or formate metabolism, and cocultures were readily initiated with a mutant strain of G. sulfurreducens that was unable to use H 2 as an electron donor (37). These results suggested that the coculture was functioning via an alternative to interspecies H 2 or formate transfer.…”
Direct interspecies electron transfer (DIET) is an alternative to interspecies H 2 /formate transfer as a mechanism for microbial species to cooperatively exchange electrons during syntrophic metabolism. To understand what specific properties contribute to DIET, studies were conducted with Pelobacter carbinolicus, a close relative of Geobacter metallireducens, which is capable of DIET. P. carbinolicus grew in coculture with Geobacter sulfurreducens with ethanol as the electron donor and fumarate as the electron acceptor, conditions under which G. sulfurreducens formed direct electrical connections with G. metallireducens. In contrast to the cell aggregation associated with DIET, P. carbinolicus and G. sulfurreducens did not aggregate. Attempts to initiate cocultures with a genetically modified strain of G. sulfurreducens incapable of both H 2 and formate utilization were unsuccessful, whereas cocultures readily grew with mutant strains capable of formate but not H 2 uptake or vice versa. The hydrogenase mutant of G. sulfurreducens compensated, in cocultures, with significantly increased formate dehydrogenase gene expression. In contrast, the transcript abundance of a hydrogenase gene was comparable in cocultures with that for the formate dehydrogenase mutant of G. sulfurreducens or the wild type, suggesting that H 2 was the primary electron carrier in the wild-type cocultures. Cocultures were also initiated with strains of G. sulfurreducens that could not produce pili or OmcS, two essential components for DIET. The finding that P. carbinolicus exchanged electrons with G. sulfurreducens via interspecies transfer of H 2 /formate rather than DIET demonstrates that not all microorganisms that can grow syntrophically are capable of DIET and that closely related microorganisms may use significantly different strategies for interspecies electron exchange.
“…Adaptive evolution of the coculture for enhanced ethanol metabolism was associated with the formation of large aggregates of the two species. Although G. sulfurreducens is capable of utilizing either H 2 or formate as an electron donor for fumarate reduction when acetate is available as a carbon source (9), cells within the aggregates were not effective in H 2 or formate metabolism, and cocultures were readily initiated with a mutant strain of G. sulfurreducens that was unable to use H 2 as an electron donor (37). These results suggested that the coculture was functioning via an alternative to interspecies H 2 or formate transfer.…”
Direct interspecies electron transfer (DIET) is an alternative to interspecies H 2 /formate transfer as a mechanism for microbial species to cooperatively exchange electrons during syntrophic metabolism. To understand what specific properties contribute to DIET, studies were conducted with Pelobacter carbinolicus, a close relative of Geobacter metallireducens, which is capable of DIET. P. carbinolicus grew in coculture with Geobacter sulfurreducens with ethanol as the electron donor and fumarate as the electron acceptor, conditions under which G. sulfurreducens formed direct electrical connections with G. metallireducens. In contrast to the cell aggregation associated with DIET, P. carbinolicus and G. sulfurreducens did not aggregate. Attempts to initiate cocultures with a genetically modified strain of G. sulfurreducens incapable of both H 2 and formate utilization were unsuccessful, whereas cocultures readily grew with mutant strains capable of formate but not H 2 uptake or vice versa. The hydrogenase mutant of G. sulfurreducens compensated, in cocultures, with significantly increased formate dehydrogenase gene expression. In contrast, the transcript abundance of a hydrogenase gene was comparable in cocultures with that for the formate dehydrogenase mutant of G. sulfurreducens or the wild type, suggesting that H 2 was the primary electron carrier in the wild-type cocultures. Cocultures were also initiated with strains of G. sulfurreducens that could not produce pili or OmcS, two essential components for DIET. The finding that P. carbinolicus exchanged electrons with G. sulfurreducens via interspecies transfer of H 2 /formate rather than DIET demonstrates that not all microorganisms that can grow syntrophically are capable of DIET and that closely related microorganisms may use significantly different strategies for interspecies electron exchange.
“…The nuoA gene appears to be the first gene of the operon encoding other subunits of NADH dehydrogenase I. It appears likely that the sfrB gene composes an operon with the sfrA gene (43). Figure 3.Genes containing the repressor binding site.…”
Geobacter species play important roles in bioremediation of contaminated environments and in electricity production from waste organic matter in microbial fuel cells. To better understand physiology of Geobacter species, expression and function of citrate synthase, a key enzyme in the TCA cycle that is important for organic acid oxidation in Geobacter species, was investigated. Geobacter sulfurreducens did not require citrate synthase for growth with hydrogen as the electron donor and fumarate as the electron acceptor. Expression of the citrate synthase gene, gltA, was repressed by a transcription factor under this growth condition. Functional and comparative genomics approaches, coupled with genetic and biochemical assays, identified a novel transcription factor termed HgtR that acts as a repressor for gltA. Further analysis revealed that HgtR is a global regulator for genes involved in biosynthesis and energy generation in Geobacter species. The hgtR gene was essential for growth with hydrogen, during which hgtR expression was induced. These findings provide important new insights into the mechanisms by which Geobacter species regulate their central metabolism under different environmental conditions.
“…Subcellular fractions were prepared using modifications of previously described methods (9,21,41). Wild-type and strain ZKI cells in their mid-log and stationary growth phases in 100 ml of acetate-fumarate medium were harvested by centrifugation for 15 min at 6,000 ϫ g at 4°C.…”
Previous studies have demonstrated that Geobacter sulfurreducens requires the c-type cytochrome OmcZ, which is present in large (OmcZ L ; 50-kDa) and small (OmcZ S ; 30-kDa) forms, for optimal current production in microbial fuel cells. This protein was further characterized to aid in understanding its role in current production. Subcellular-localization studies suggested that OmcZ S was the predominant extracellular form of OmcZ. N-and C-terminal amino acid sequence analysis of purified OmcZ S and molecular weight measurements indicated that OmcZ S is a cleaved product of OmcZ L retaining all 8 hemes, including 1 heme with the unusual c-type heme-binding motif CX 14 CH. The purified OmcZ S was remarkably thermally stable (thermaldenaturing temperature, 94.2°C). Redox titration analysis revealed that the midpoint reduction potential of OmcZ S is approximately ؊220 mV (versus the standard hydrogen electrode [SHE]) with nonequivalent heme groups that cover a large reduction potential range (؊420 to ؊60 mV). OmcZ S transferred electrons in vitro to a diversity of potential extracellular electron acceptors, such as Fe(III) citrate, U(VI), Cr(VI), Au(III), Mn(IV) oxide, and the humic substance analogue anthraquinone-2,6-disulfonate, but not Fe(III) oxide. The biochemical properties and extracellular localization of OmcZ suggest that it is well suited for promoting electron transfer in current-producing biofilms of G. sulfurreducens.
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