A decade ago, a novel mechanism to drive thermodynamically unfavorable redox reactions was discovered that is used in prokaryotes to drive endergonic electron transfer reactions by a direct coupling to an exergonic redox reaction in one soluble enzyme complex. This process is referred to as flavin-based electron bifurcation, or FBEB. An important function of FBEB is that it allows the generation of reduced low-potential ferredoxin (Fd) from comparably high-potential electron donors such as NADH or molecular hydrogen (H). Fd is then the electron donor for anaerobic respiratory chains leading to the synthesis of ATP. In many metabolic scenarios, Fd is reduced by metabolic oxidoreductases and Fd then drives endergonic metabolic reactions such as H production by the reverse, electron confurcation. FBEB is energetically more economical than ATP hydrolysis or reverse electron transport as a driving force for endergonic redox reactions; thus, it does "save" cellular ATP. It is essential for autotrophic growth at the origin of life and also allows for heterotrophic growth on certain low-energy substrates.

is one of the very few thermophilic acetogenic microorganisms. It grows optimally at 66°C on sugars but also lithotrophically with H + CO or with CO, producing acetate as the major product. While a genome-derived model of acetogenesis has been developed, only a few physiological or biochemical experiments regarding the function of important enzymes in carbon and energy metabolism have been carried out. To address this issue, we developed a method for targeted markerless gene deletions and for integration of genes into the genome of The strain naturally took up plasmid DNA in the exponential growth phase, with a transformation frequency of up to 3.9 × 10 A nonreplicating plasmid and selection with 5-fluoroorotate was used to delete the gene encoding the orotate phosphoribosyltransferase (), resulting in a Δ uracil-auxotrophic strain, TKV002. Reintroduction of on a plasmid or insertion of into different loci within the genome restored growth without uracil. We subsequently studied fructose metabolism in The gene (TKV_c23150) encoding 1-phosphofructosekinase (1-PFK) was deleted, using as a selective marker via two single homologous recombination events. The resulting Δ strain, TKV003, did not grow on fructose; however, growth on glucose (or on mannose) was unaffected. The combination of as a selective marker and the natural competence of the strain for DNA uptake will be the basis for future studies on CO reduction and energy conservation and their regulation in this thermophilic acetogenic bacterium. Acetogenic bacteria are currently the focus of research toward biotechnological applications due to their potential for synthesis of carbon compounds such as acetate, butyrate, or ethanol from H + CO or from synthesis gas. Based on available genome sequences and on biochemical experiments, acetogens differ in their energy metabolism. Thus, there is an urgent need to understand the carbon and electron flows through the Wood-Ljungdahl pathway and their links to energy conservation, which requires genetic manipulations such as deletion or overexpression of genes encoding putative key enzymes. Unfortunately, genetic systems have been reported for only a few acetogenic bacteria. Here, we demonstrate proof of concept for the genetic modification of the thermophilic acetogenic species The genetic system will be used to study genes involved in biosynthesis and energy metabolism, and may further be applied to metabolically engineer to produce fuels and chemicals.

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Bioethanol production is achieved by only two metabolic pathways and only at moderate temperatures. Herein a fundamentally different synthetic pathway for bioalcohol production at 70°C was constructed by insertion of the gene for bacterial alcohol dehydrogenase (AdhA) into the archaeon Pyrococcus furiosus. The engineered strain converted glucose to ethanol via acetate and acetaldehyde, catalyzed by the host-encoded aldehyde ferredoxin oxidoreductase (AOR) and heterologously expressed AdhA, in an energy-conserving, redox-balanced pathway. Furthermore, the AOR/AdhA pathway also converted exogenously added aliphatic and aromatic carboxylic acids to the corresponding alcohol using glucose, pyruvate, and/or hydrogen as the source of reductant. By heterologous coexpression of a membrane-bound carbon monoxide dehydrogenase, CO was used as a reductant for converting carboxylic acids to alcohols. Redirecting the fermentative metabolism of P. furiosus through strategic insertion of foreign genes creates unprecedented opportunities for thermophilic bioalcohol production. Moreover, the AOR/AdhA pathway is a potentially game-changing strategy for syngas fermentation, especially in combination with carbon chain elongation pathways.Archaea | metabolic engineering | hyperthermophile | carbon monoxide | aldehydes

The three major components of plant biomass, cellulose, hemicellulose and lignin, are highly recalcitrant and deconstruction involves thermal and chemical pretreatment. Microbial conversion is a possible solution, but few anaerobic microbes utilize both cellulose and hemicellulose and none are known to solubilize lignin.Herein, we show that the majority (85%) of insoluble switchgrass biomass that had not been previously chemically treated was degraded at 78 C by the anaerobic bacterium Caldicellulosiruptor bescii.Remarkably, the glucose/xylose/lignin ratio and physical and spectroscopic properties of the remaining insoluble switchgrass were not significantly different than those of the untreated plant material. C. bescii is therefore able to solubilize lignin as well as the carbohydrates and, accordingly, lignin-derived aromatics were detected in the culture supernatants. From mass balance analyses, the carbohydrate in the solubilized switchgrass quantitatively accounted for the growth of C. bescii and its fermentation products, indicating that the lignin was not assimilated by the microorganism. Immunoanalyses of biomass and transcriptional analyses of C. bescii showed that the microorganism when grown on switchgrass produces enzymes directed at key plant cell wall moieties such as pectin, xyloglucans and rhamnogalacturonans, and that these and as yet uncharacterized enzymes enable the degradation of cellulose, hemicellulose and lignin at comparable rates. This unexpected finding of simultaneous lignin and carbohydrate solubilization bodes well for industrial conversion by extremely thermophilic microbes of biomass that requires limited or, more importantly, no chemical pretreatment. Broader contextThe three major components of plant biomass are cellulose (a glucose polymer), hemicellulose (a polymer of xylose and a variety of other sugars) and lignin (a complex polymer of aromatic units). The sugar polymers are potential feedstocks for the production of biofuels by anaerobic microorganisms. However, plant biomass is highly recalcitrant and harsh and inefficient chemical treatments are required to solubilize the biomass and release the sugars. Moreover, no anaerobic microorganism is known that can degrade the highly recalcitrant lignin. Herein it is shown that switchgrass, a model plant for bioenergy production, can be degraded at moderate temperatures (78 C) by an anaerobic bacterium that solubilizes lignin as well as cellulose and hemicellulose. The microorganism produces a range of both known and as yet uncharacterized enzymes that degrade at comparable rates all of the major components of the plant cell wall. Such thermophilic microbes could potentially be developed to enable the direct conversion of plant biomass to biofuels without the need for any chemical pretreatment.