Novel metabolic pathways in Archaea

Major evolutionary transitions before cells: A journey from molecules to organisms

Basing on logical assumptions and necessary steps of complexification along biological evolution, we propose here an evolutionary path from molecules to cells presenting four ages and three major transitions. At the first age, the basic biomolecules were formed and become abundant. The first transition happened with the event of a chemical symbiosis between nucleic acids and peptides worlds, which marked the emergence of both life and the process of organic encoding. FUCA, the first living process, was composed of self-replicating RNAs linked to amino acids and capable to catalyze their binding. The second transition, from the age of FUCA to the age of progenotes, involved the duplication and recombination of proto-genomes, leading to specialization in protein production and the exploration of protein to metabolite interactions in the prebiotic soup. Enzymes and metabolic pathways were incorporated into biology from protobiotic reactions that occurred without chemical catalysts, step by step. Then, the fourth age brought origin of organisms and lineages, occurring when specific proteins capable to stackle together facilitated the formation of peptidic capsids. LUCA was constituted as a progenote capable to operate the basic metabolic functions of a cell, but still unable to interact with lipid molecules. We present evidence that the evolution of lipid interaction pathways occurred at least twice, with the development of bacterial-like and archaeal-like membranes. Also, data in literature suggest at least two paths for the emergence of DNA biosynthesis, allowing the stabilization of early life strategies in viruses, archaeas and bacterias. Two billion years later, the eukaryotes arouse, and after 1,5 billion years of evolution, they finally learn how to evolve multicellularity via tissue specialization.

Cell-Free Protein Expression by a Reconstituted Transcription-Translation System Energized by Sugar Catabolism

Cooperation between catabolism and anabolism is crucial for maintaining homeostasis in living cells. The most fundamental systems for catabolism and anabolism are the glycolysis of sugars and the transcription-translation (TX-TL) of DNA, respectively. Despite their importance in living cells, the in vitro reconstitution of their cooperation through purified factors has not been achieved, which hinders the elucidation of the design principle in living cells. Here, we reconstituted glycolysis using sugars and integrated it with the PURE system, a commercial in vitro TX-TL kit composed of purified factors. By optimizing key parameters, such as glucokinase and initial phosphate concentrations, we determined suitable conditions for their cooperation. The optimized system showed protein synthesis at up to 33% of that of the original PURE system. We observed that ATP consumption in upstream glycolysis inhibits TX-TL and that this inhibition can be alleviated by the co-addition of glycolytic intermediates, such as glyceraldehyde 3-phosphate, with glucose. Moreover, the system developed here simultaneously synthesizes a subset of its own enzymes, that is, glycolytic enzymes, in a single test tube, which is a necessary step toward self-replication. As glycolysis and TX-TL provide building blocks for constructing cells, the integrated system can be a fundamental material for reconstituting living cells from purified factors.

Disentangling a metabolic cross-feeding in a halophilic archaea-bacteria consortium

Microbial syntrophy, a cooperative metabolic interaction among prokaryotes, serves a critical role in shaping communities, due to the auxotrophic nature of many microorganisms. Syntrophy played a key role in the evolution of life, including the hypothesized origin of eukaryotes. In a recent exploration of the microbial mats within the exceptional and uniquely extreme Cuatro Cienegas Basin (CCB), a halophilic isolate, designated as AD140, emerged as a standout due to its distinct growth pattern. Subsequent genome sequencing revealed AD140 to be a co-culture of a halophilic archaeon from the Halorubrum genus and a marine halophilic bacterium, Marinococcus luteus, both occupying the same ecological niche. This intriguing coexistence hints at an early-stage symbiotic relationship that thrives on adaptability. By delving into their metabolic interdependence through genomic analysis, this study aims to uncover shared characteristics that enhance their symbiotic association, offering insights into the evolution of halophilic microorganisms and their remarkable adaptations to high-salinity environments.

N-glycosylation in Archaea: Unusual sugars and unique modifications

Archaea are microorganisms that comprise a distinct branch of the universal tree of life and which are best known as extremophiles, residing in a variety of environments characterized by harsh physical conditions. One seemingly universal trait of Archaea is the ability to perform N-glycosylation. At the same time, archaeal N-linked glycans present variety in terms of both composition and architecture not seen in the parallel eukaryal or bacterial processes. In this mini-review, many of the unique and unusual sugars found in archaeal N-linked glycans as identified by nuclear magnetic resonance spectroscopy are described.

A non-carboxylating pentose bisphosphate pathway in halophilic archaea

Bacteria and Eucarya utilize the non-oxidative pentose phosphate pathway to direct the ribose moieties of nucleosides to central carbon metabolism. Many archaea do not possess this pathway, and instead, Thermococcales utilize a pentose bisphosphate pathway involving ribose-1,5-bisphosphate (R15P) isomerase and ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco). Intriguingly, multiple genomes from halophilic archaea seem only to harbor R15P isomerase, and do not harbor Rubisco. In this study, we identify a previously unrecognized nucleoside degradation pathway in halophilic archaea, composed of guanosine phosphorylase, ATP-dependent ribose-1-phosphate kinase, R15P isomerase, RuBP phosphatase, ribulose-1-phosphate aldolase, and glycolaldehyde reductase. The pathway converts the ribose moiety of guanosine to dihydroxyacetone phosphate and ethylene glycol. Although the metabolic route from guanosine to RuBP via R15P is similar to that of the pentose bisphosphate pathway in Thermococcales, the downstream route does not utilize Rubisco and is unique to halophilic archaea.

Archaeal contribution to carbon-functional composition and abundance in China's coastal wetlands: Not to be underestimated

Microbial diversity, together with carbon function, plays a key role in driving the wetland carbon cycle; however, the composition, driving factors of carbon-functional genes and the relationship with microbial community have not been well characterized in coastal wetlands. To understand these concerns, microbes, carbon-functional genes, and related environmental factors were investigated in twenty wetlands along China's coast. The results indicate that carbon-functional gene composition is dominated by archaeal rather than bacterial community and that Nanoarchaeaeota is the dominant archaeal phylum associated with carbon cycling in anoxic sediments. Compared with microbes, carbon-functional composition was more stable because they showed the highest Shannon diversity and archaeal functional redundancy. Deterministic processes dominated microbial community, and stochastic processes were more important for carbon-functional genes. Labile Fe governed archaeal and carbon-functional composition by coupling with nitrogen and carbon biogeochemical cycles, while bacterial community was affected by NH4-N and SOC/SON. This study highlights the predominant contributions of archaea to carbon-functional genes and to the stability of carbon-functional composition, thus providing new insights into the microbial dominance of the carbon cycle and the evaluation of carbon function in coastal wetlands.

On the evolution of coenzyme biosynthesis

Covering: up to 2022The report provides a broad approach to deciphering the evolution of coenzyme biosynthetic pathways. Here, these various pathways are analyzed with respect to the coenzymes required for this purpose. Coenzymes whose biosynthesis relies on a large number of coenzyme-mediated reactions probably appeared on the scene at a later stage of biological evolution, whereas the biosyntheses of pyridoxal phosphate (PLP) and nicotinamide (NAD⁺) require little additional coenzymatic support and are therefore most likely very ancient biosynthetic pathways.

A functional promoter from the archaeon Halobacterium salinarum is also transcriptionally active in E. coli

Background

Archaea form a third domain of life that is distinct from Bacteria and Eukarya. So far, many scholars have elucidated considerable details about the typical promoter architectures of the three domains of life. However, a functional promoter from the archaeon Halobacterium salinarum has never been studied in Escherichia coli.

Results

This paper found that the promoter of Halobacterium salinarum showed a promoter function in Escherichia coli. This Escherichia coli promoter structure contains − 10 box, -10 box extension and − 29 elements, however, no -35 box. The − 29 element is exercised by the TATA box in archaea. And we isolated the RM10 fragment that possessed the fusion characteristics of bacteria and archaea, which was overlapped with functionality of TATA box and − 29 elements.

Conclusions

The − 29 element reflects the evolutionary relationship between the archaeal promoter and the bacterial promoter. The result possibly indicated that there may be a certain internal connection between archaea and bacteria. We hypothesized that it provided a new viewpoint of the evolutionary relationship of archaea and other organisms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02489-y.

Engineering transcriptional regulation in Escherichia coli using an archaeal TetR-family transcription factor

Synthetic biology requires well-characterized biological parts that can be combined into functional modules. One type of biological parts are transcriptional regulators and their cognate operator elements, which enable to either generate an input-specific response or are used as actuator modules. A range of regulators has already been characterized and used for orthogonal gene expression engineering, however, previous efforts have mostly focused on bacterial regulators. This work aims to design and explore the use of an archaeal TetR family regulator, FadR_Sa from Sulfolobus acidocaldarius, in a bacterial system, namely Escherichia coli. This is a challenging objective given the fundamental difference between the bacterial and archaeal transcription machinery and the lack of a native TetR-like FadR regulatory system in E. coli. The synthetic σ⁷⁰-dependent bacterial promoter proD was used as a starting point to design hybrid bacterial/archaeal promoter/operator regions, in combination with the mKate2 fluorescent reporter enabling a readout. Four variations of proD containing FadR_Sa binding sites were constructed and characterized. While expressional activity of the modified promoter proD was found to be severely diminished for two of the constructs, constructs in which the binding site was introduced adjacent to the -35 promoter element still displayed sufficient basal transcriptional activity and showed up to 7-fold repression upon expression of FadR_Sa. Addition of acyl-CoA has been shown to disrupt FadR_Sa binding to the DNA in vitro. However, extracellular concentrations of up to 2 mM dodecanoate, subsequently converted to acyl-CoA by the cell, did not have a significant effect on repression in the bacterial system. This work demonstrates that archaeal transcription regulators can be used to generate actuator elements for use in E. coli, although the lack of ligand response underscores the challenge of maintaining biological function when transferring parts to a phylogenetically divergent host.

The Redox Active [2Fe-2S] Clusters: Key-Components of a Plethora of Enzymatic Reactions—Part I: Archaea

The earliest forms of life (i.e., Archaea, Bacteria, and Eukarya) appeared on our planet about ten billion years after its formation. Although Archaea do not seem to possess the multiprotein machinery constituted by the NIF (Nitrogen Fixation), ISC (Iron Sulfur Cluster), SUF (sulfur mobilization) enzymes, typical of Bacteria and Eukarya, some of them are able to encode Fe-S proteins. Here we discussed the multiple enzymatic reactions triggered by the up-to-date structurally characterized members of the archaeal family that require the crucial presence of structurally characterized [2Fe-2S] assemblies, focusing on their biological functions and, when available, on their electrochemical behavior.

Proteomic Sample Preparation and Data Analysis in Line with the Archaeal Proteome Project

Despite the ecological, evolutionary and economical significance of archaea, key aspects of their cell biology, metabolic pathways, and adaptations to a wide spectrum of environmental conditions, remain to be elucidated. Proteomics allows for the system-wide analysis of proteins, their changes in abundance between different conditions, as well as their post-translational modifications, providing detailed insights into the function of proteins and archaeal cell biology. In this chapter, we describe a sample preparation and mass spectrometric analysis workflow that has been designed for Haloferax volcanii but can be applied to a broad range of archaeal species. Furthermore, proteomics experiments provide a wealth of data that is invaluable to various disciplines. Therefore, we previously initiated the Archaeal Proteome Project (ArcPP), a community project that combines the analysis of multiple datasets with expert knowledge in various fields of archaeal research. The corresponding bioinformatic analysis, allowing for the integration of new proteomics data into the ArcPP, as well as the interactive exploration of ArcPP results is also presented here. In combination, these protocols facilitate an optimized, detailed and collaborative approach to archaeal proteomics.

Characterization of a novel mesophilic CTP-dependent riboflavin kinase and rational engineering to create its thermostable homologs

Flavins play a central role in metabolism as molecules that catalyze a wide range of redox reactions in living organisms. Several variations in flavin biosynthesis exist among the domains of life, and their analysis has revealed many new structural and mechanistic insights till date. The cytidine triphosphate (CTP)-dependent riboflavin kinase in archaea is one such example - unlike most kinases that use adenosine triphosphate, archaeal riboflavin kinases utilize CTP to phosphorylate riboflavin and produce flavin mononucleotide. In this study, we present the characterization of a new mesophilic archaeal CTP-utilizing riboflavin kinase homolog from Methanococcus maripaludis (MmpRibK), which is linked closely in sequence to the previously characterized thermophilic Methanocaldococcus jannaschii homolog. We reconstitute the activity of MmpRibK, determine its kinetic parameters and molecular factors that contribute to its unique properties, and finally establish the residues that improve its thermostability using computation and a series of experiments.  Our work advances the molecular understanding of flavin biosynthesis in archaea by the characterization of the first mesophilic CTP-dependent riboflavin kinase. Finally, it validates the role of salt bridges and rigidifying amino acid residues in imparting thermostability to this unique structural fold that characterizes archaeal riboflavin kinase enzymes, with implications in enzyme engineering and biotechnological applications.

Identification and Enzymatic Analysis of an Archaeal ATP-Dependent Serine Kinase from the Hyperthermophilic Archaeon Staphylothermus marinus

Serine kinase catalyzes the phosphorylation of free serine (Ser) to produce O-phosphoserine (Sep). An ADP-dependent Ser kinase in the hyperthermophilic archaeon Thermococcus kodakarensis (Tk-SerK) is involved in cysteine (Cys) biosynthesis and most likely Ser assimilation. An ATP-dependent Ser kinase in the mesophilic bacterium Staphylococcus aureus is involved in siderophore biosynthesis. Although proteins displaying various degrees of similarity with Tk-SerK are distributed in a wide range of organisms, it is unclear if they are actually Ser kinases. Here, we examined proteins from Desulfurococcales species in Crenarchaeota that display moderate similarity with Tk-SerK from Euryarchaeota (42 to 45% identical). Tk-serK homologs from Staphylothermus marinus (Smar_0555), Desulfurococcus amylolyticus (DKAM_0858), and Desulfurococcus mucosus (Desmu_0904) were expressed in Escherichia coli. All three partially purified recombinant proteins exhibited Ser kinase activity utilizing ATP rather than ADP as a phosphate donor. Purified Smar_0555 protein displayed activity for l-Ser but not other compounds, including d-Ser, l-threonine, and l-homoserine. The enzyme utilized ATP, UTP, GTP, CTP, and the inorganic polyphosphates triphosphate and tetraphosphate as phosphate donors. Kinetic analysis indicated that the Smar_0555 protein preferred nucleoside 5'-triphosphates over triphosphate as a phosphate donor. Transcript levels and Ser kinase activity in S. marinus cells grown with or without serine suggested that the Smar_0555 gene is constitutively expressed. The genes encoding Ser kinases examined here form an operon with genes most likely responsible for the conversion between Sep and 3-phosphoglycerate of central sugar metabolism, suggesting that the ATP-dependent Ser kinases from Desulfurococcales play a role in the assimilation of Ser. IMPORTANCE Homologs of the ADP-dependent Ser kinase from the archaeon Thermococcus kodakarensis (Tk-SerK) include representatives from all three domains of life. The results of this study show that even homologs from the archaeal order Desulfurococcales, which are the most structurally related to the ADP-dependent Ser kinases from the Thermococcales, are Ser kinases that utilize ATP, and in at least some cases inorganic polyphosphates, as the phosphate donor. The differences in properties between the Desulfurococcales and Thermococcales enzymes raise the possibility that Tk-SerK homologs constitute a group of kinases that phosphorylate free serine with a wide range of phosphate donors.

Enigmatic Evolutionary History of Porphobilinogen Deaminase in Eukaryotic Phototrophs

In most eukaryotic phototrophs, the entire heme synthesis is localized to the plastid, and enzymes of cyanobacterial origin dominate the pathway. Despite that, porphobilinogen deaminase (PBGD), the enzyme responsible for the synthesis of hydroxymethybilane in the plastid, shows phylogenetic affiliation to α-proteobacteria, the supposed ancestor of mitochondria. Surprisingly, no PBGD of such origin is found in the heme pathway of the supposed partners of the primary plastid endosymbiosis, a primarily heterotrophic eukaryote, and a cyanobacterium. It appears that α-proteobacterial PBGD is absent from glaucophytes but is present in rhodophytes, chlorophytes, plants, and most algae with complex plastids. This may suggest that in eukaryotic phototrophs, except for glaucophytes, either the gene from the mitochondrial ancestor was retained while the cyanobacterial and eukaryotic pseudoparalogs were lost in evolution, or the gene was acquired by non-endosymbiotic gene transfer from an unspecified α-proteobacterium and functionally replaced its cyanobacterial and eukaryotic counterparts.

Insights on Cadmium Removal by Bioremediation: The Case of Haloarchaea

Although heavy metals are naturally found in the environment as components of the earth’s crust, environmental pollution by these toxic elements has increased since the industrial revolution. Some of them can be considered essential, since they play regulatory roles in different biological processes; but the role of other heavy metals in living tissues is not clear, and once ingested they can accumulate in the organism for long periods of time causing adverse health effects. To mitigate this problem, different methods have been used to remove heavy metals from water and soil, such as chelation-based processes. However, techniques like bioremediation are leaving these conventional methodologies in the background for being more effective and eco-friendlier. Recently, different research lines have been promoted, in which several organisms have been used for bioremediation approaches. Within this context, the extremophilic microorganisms represent one of the best tools for the treatment of contaminated sites due to the biochemical and molecular properties they show. Furthermore, since it is estimated that 5% of industrial effluents are saline and hypersaline, halophilic microorganisms have been suggested as good candidates for bioremediation and treatment of this kind of samples. These microorganisms, and specifically the haloarchaea group, are of interest to design strategies aiming the removal of polluting compounds due to the efficiency of their metabolism under extreme conditions and their significant tolerance to highly toxic compounds such as heavy metals, bromate, nitrite, chlorate, or perchlorate ions. However, there are still few trials that have proven the bioremediation of environments contaminated with heavy metals using these microorganisms. This review analyses scientific literature focused on metabolic capabilities of haloarchaea that may allow these microbes to tolerate and eliminate heavy metals from the media, paying special attention to cadmium. Thus, this work will shed light on potential uses of haloarchaea in bioremediation of soils and waters negatively affected by heavy metals, and more specifically by cadmium.

Coenzymes and Their Role in the Evolution of Life

The evolution of coenzymes, or their impact on the origin of life, is fundamental for understanding our own existence. Having established reasonable hypotheses about the emergence of prebiotic chemical building blocks, which were probably created under palaeogeochemical conditions, and surmising that these smaller compounds must have become integrated to afford complex macromolecules such as RNA, the question of coenzyme origin and its relation to the evolution of functional biochemistry should gain new impetus. Many coenzymes have a simple chemical structure and are often nucleotide-derived, which suggests that they may have coexisted with the emergence of RNA and may have played a pivotal role in early metabolism. Based on current theories of prebiotic evolution, which attempt to explain the emergence of privileged organic building blocks, this Review discusses plausible hypotheses on the prebiotic formation of key elements within selected extant coenzymes. In combination with prebiotic RNA, coenzymes may have dramatically broadened early protometabolic networks and the catalytic scope of RNA during the evolution of life.

Coordinating precursor supply for pharmaceutical polyketide production in Streptomyces

The widely used polyketide pharmaceuticals in medicine and agriculture are mainly produced by Streptomyces species. These compounds, as secondary metabolites, are not involved in essential cellular processes and are usually produced during the stationary phase of fermentation. Consequently, their yields and productivities are often low and frequently limited by the availability of the precursors. The precursor pathways, therefore, are key entities for synthetic biology-driven design and optimization. We discuss recent advances in precursor engineering, in both Streptomyces and other bacteria, focusing on the diverse native and heterologous precursor pathways that could be rewired for polyketide titer improvement. We also highlight the coordination of other required factors to direct the precursors towards polyketide biosynthesis. The precursor-supply enhancement tools and strategies covered in this review will facilitate the design and construction of synthetic Streptomyces 'cell-factories' for efficient polyketide production.

Lysine-specific acetylated proteome from the archaeon Thermococcus gammatolerans reveals the presence of acetylated histones

Thermococcus gammatolerans EJ3 is an extremophile archaeon which was revealed as one of the most radioresistant organisms known on Earth, withstanding up to 30 kGy gamma-ray radiations. While its theoretical proteome is rather small, T. gammatolerans may enhance its toolbox by post-translational modification of its proteins. Here, we explored its extent of Nε-acetylation of lysines. For this, we immunopurified with two acetylated-lysine antibodies the acetylated peptides resulting from a proteolysis of soluble proteins with trypsin. The comparison of acetylated proteomes of two archaea highlights some common acetylation patterns but only 4 out of 26 orthologous proteins found to be acetylated in both species, are acetylated on the same lysine site. We evidenced that histone B is acetylated in T. gammatolerans at least at two different sites (K27 and K36), and a peptide common at the C-terminus of histones A and B is also acetylated. We verified that acetylation of histones is a common trait among Thermococcales after recording data on Thermococcus kodakaraensis histones and identifying three acetylated sites. This discovery reinforces the strong evolutionary link between Archaea and Eukaryotes and should be an incentive for further investigation on the extent and role of acetylation of histones in Archaea. SIGNIFICANCE: Acetylation is an important post-translational modification of proteins that has been extensively described in Eukaryotes, and more recently in Bacteria. Here, we report for the first time ever that histones in Archaea are also modified by acetylation after a systematic survey of acetylated peptides in Thermococcus gammatolerans. Structural models of histones A and B indicates that acetylation of the identified modified residues may play an important role in histone assembly and/or interaction with DNA. The in-depth protein acetylome landscape in T. gammatolerans includes at least 181 unique protein sequences, some of them being modified on numerous residues. Proteins involved in metabolic processes, information storage and processing mechanisms are over-represented categories in this dataset, highlighting the ancient role of this protein post-translational modification in primitive cells.

The host-associated archaeome

Host-associated microbial communities have an important role in shaping the health and fitness of plants and animals. Most studies have focused on the bacterial, fungal or viral communities, but often the archaeal component has been neglected. The archaeal community, the so-called archaeome, is now increasingly recognized as an important component of host-associated microbiomes. It is composed of various lineages, including mainly Methanobacteriales and Methanomassiliicoccales (Euryarchaeota), as well as representatives of the Thaumarchaeota. Host-archaeome interactions have mostly been delineated from methanogenic archaea in the gastrointestinal tract, where they contribute to substantial methane production and are potentially also involved in disease-relevant processes. In this Review, we discuss the diversity and potential roles of the archaea associated with protists, plants and animals. We also present the current understanding of the archaeome in humans, the specific adaptations involved in interaction with the resident microbial community as well as with the host, and the roles of the archaeome in both health and disease.

Importance of Initial Interfacial Steps during Chalcopyrite Bioleaching by a Thermoacidophilic Archaeon

Studies of thermophilic microorganisms have shown that they have a considerable biotechnological potential due to their optimum growth and metabolism at high temperatures. Thermophilic archaea have unique characteristics with important biotechnological applications; many of these species could be used in bioleaching processes to recover valuable metals from mineral ores. Particularly, bioleaching at high temperatures using thermoacidophilic microorganisms can greatly improve metal solubilization from refractory mineral species such as chalcopyrite (CuFeS₂), one of the most abundant and widespread copper-bearing minerals. Interfacial processes such as early cell adhesion, biofilm development, and the formation of passive layers on the mineral surface play important roles in the initial steps of bioleaching processes. The present work focused on the investigation of different bioleaching conditions using the thermoacidophilic archaeon Acidianus copahuensis DSM 29038 to elucidate which steps are pivotal during the chalcopyrite bioleaching. Fluorescent in situ hybridization (FISH) and confocal laser scanning microscopy (CLSM) were used to visualize the microorganism–mineral interaction. Results showed that up to 85% of copper recovery from chalcopyrite could be achieved using A. copahuensis. Improvements in these yields are intimately related to an early contact between cells and the mineral surface. On the other hand, surface coverage by inactivated cells as well as precipitates significantly reduced copper recoveries.

Engineering microbial pathways for production of bio-based chemicals from lignocellulosic sugars: current status and perspectives

Lignocellulose is the most abundant biomass on earth with an annual production of about 2 × 10¹¹ tons. It is an inedible renewable carbonaceous resource that is very rich in pentose and hexose sugars. The ability of microorganisms to use lignocellulosic sugars can be exploited for the production of biofuels and chemicals, and their concurrent biotechnological processes could advantageously replace petrochemicals' processes in a medium to long term, sustaining the emerging of a new economy based on bio-based products from renewable carbon sources. One of the major issues to reach this objective is to rewire the microbial metabolism to optimally configure conversion of these lignocellulosic-derived sugars into bio-based products in a sustainable and competitive manner. Systems' metabolic engineering encompassing synthetic biology and evolutionary engineering appears to be the most promising scientific and technological approaches to meet this challenge. In this review, we examine the most recent advances and strategies to redesign natural and to implement non-natural pathways in microbial metabolic framework for the assimilation and conversion of pentose and hexose sugars derived from lignocellulosic material into industrial relevant chemical compounds leading to maximal yield, titer and productivity. These include glycolic, glutaric, mesaconic and 3,4-dihydroxybutyric acid as organic acids, monoethylene glycol, 1,4-butanediol and 1,2,4-butanetriol, as alcohols. We also discuss the big challenges that still remain to enable microbial processes to become industrially attractive and economically profitable.

The Carbon Switch at the Level of Pyruvate and Phosphoenolpyruvate in Sulfolobus solfataricus P2

Sulfolobus solfataricus P2 grows on different carbohydrates as well as alcohols, peptides and amino acids. Carbohydrates such as D-glucose or D-galactose are degraded via the modified, branched Entner–Doudoroff (ED) pathway whereas growth on peptides requires the Embden–Meyerhof–Parnas (EMP) pathway for gluconeogenesis. As for most hyperthermophilic Archaea an important control point is established at the level of triosephophate conversion, however, the regulation at the level of pyruvate/phosphoenolpyruvate conversion was not tackled so far. Here we describe the cloning, expression, purification and characterization of the pyruvate kinase (PK, SSO0981) and the phosphoenolpyruvate synthetase (PEPS, SSO0883) of Sul. solfataricus. The PK showed only catabolic activity [catalytic efficiency (PEP): 627.95 mM^-1s^-1, 70°C] with phosphoenolpyruvate as substrate and ADP as phosphate acceptor and was allosterically inhibited by ATP and isocitrate (K_i 0.8 mM). The PEPS was reversible, however, exhibited preferred activity in the gluconeogenic direction [catalytic efficiency (pyruvate): 1.04 mM^-1s^-1, 70°C] and showed some inhibition by AMP and α-ketoglutarate. The gene SSO2829 annotated as PEPS/pyruvate:phosphate dikinase (PPDK) revealed neither PEPS nor PPDK activity. Our studies suggest that the energy charge of the cell as well as the availability of building blocks in the citric acid cycle and the carbon/nitrogen balance plays a major role in the Sul. solfataricus carbon switch. The comparison of regulatory features of well-studied hyperthermophilic Archaea reveals a close link and sophisticated coordination between the respective sugar kinases and the kinetic and regulatory properties of the enzymes at the level of PEP-pyruvate conversion.

Introduction to Genome Biology and Diversity

Organisms display astonishing levels of cell and molecular diversity, including genome size, shape, and architecture. In this chapter, we review how the genome can be viewed as both a structural and an informational unit of biological diversity and explicitly define our intended meaning of genetic information. A brief overview of the characteristic features of bacterial, archaeal, and eukaryotic cell types and viruses sets the stage for a review of the differences in organization, size, and packaging strategies of their genomes. We include a detailed review of genetic elements found outside the primary chromosomal structures, as these provide insights into how genomes are sometimes viewed as incomplete informational entities. Lastly, we reassess the definition of the genome in light of recent advancements in our understanding of the diversity of genomic structures and the mechanisms by which genetic information is expressed within the cell. Collectively, these topics comprise a good introduction to genome biology for the newcomer to the field and provide a valuable reference for those developing new statistical or computation methods in genomics. This review also prepares the reader for anticipated transformations in thinking as the field of genome biology progresses.

Metabolic reconstruction and experimental verification of glucose utilization in Desulfurococcus amylolyticus DSM 16532

Desulfurococcus amylolyticus DSM 16532 is an anaerobic and hyperthermophilic crenarchaeon known to grow on a variety of different carbon sources, including monosaccharides and polysaccharides. Furthermore, D. amylolyticus is one of the few archaea that are known to be able to grow on cellulose. Here, we present the metabolic reconstruction of D. amylolyticus’ central carbon metabolism. Based on the published genome, the metabolic reconstruction was completed by integrating complementary information available from the KEGG, BRENDA, UniProt, NCBI, and PFAM databases, as well as from available literature. The genomic analysis of D. amylolyticus revealed genes for both the classical and the archaeal version of the Embden-Meyerhof pathway. The metabolic reconstruction highlighted gaps in carbon dioxide-fixation pathways. No complete carbon dioxide-fixation pathway such as the reductive citrate cycle or the dicarboxylate-4-hydroxybutyrate cycle could be identified. However, the metabolic reconstruction indicated that D. amylolyticus harbors all genes necessary for glucose metabolization. Closed batch experimental verification of glucose utilization by D. amylolyticus was performed in chemically defined medium. The findings from in silico analyses and from growth experiments are discussed with respect to physiological features of hyperthermophilic organisms.

A series of new E. coli-Thermococcus shuttle vectors compatible with previously existing vectors

Hyperthermophilic microorganisms are an important asset in the toolkits of biotechnologists, biochemists and evolutionary biologists. The anaerobic archaeon, Thermococcus kodakarensis, has become one of the most useful hyperthermophilic model species, not least due to its natural competence and genetic tractability. Despite this, the range of genetic tools available for T. kodakarensis remains limited. Using sequencing and phylogenetic analyses, we determined that the rolling-circle replication origin of the cryptic mini-plasmid pTP2 from T. prieurii is suitable for plasmid replication in T. kodakarensis. Based on this replication origin, we present a novel series of replicative E. coli–T. kodakarensis shuttle vectors. These shuttle vectors have been constructed with three different selectable markers, allowing selection in a range of T. kodakarensis backgrounds. Moreover, these pTP2-derived plasmids are compatible with the single-existing E. coli–T. kodakarensis shuttle vector, pLC70. We show that both pTP2-derived and pLC70-derived plasmids replicate faithfully while cohabitating in T. kodakarensis cells. These plasmids open the door for new areas of research in plasmid segregation, DNA replication and gene expression.

A physiological perspective on the origin and evolution of photosynthesis

The origin and early evolution of photosynthesis are reviewed from an ecophysiological perspective. Earth's first ecosystems were chemotrophic, fueled by geological H2 at hydrothermal vents and, required flavin-based electron bifurcation to reduce ferredoxin for CO2 fixation. Chlorophyll-based phototrophy (chlorophototrophy) allowed autotrophs to generate reduced ferredoxin without electron bifurcation, providing them access to reductants other than H2. Because high-intensity, short-wavelength electromagnetic radiation at Earth's surface would have been damaging for the first chlorophyll (Chl)-containing cells, photosynthesis probably arose at hydrothermal vents under low-intensity, long-wavelength geothermal light. The first photochemically active pigments were possibly Zn-tetrapyrroles. We suggest that (i) after the evolution of red-absorbing Chl-like pigments, the first light-driven electron transport chains reduced ferredoxin via a type-1 reaction center (RC) progenitor with electrons from H2S; (ii) photothioautotrophy, first with one RC and then with two, was the bridge between H2-dependent chemolithoautotrophy and water-splitting photosynthesis; (iii) photothiotrophy sustained primary production in the photic zone of Archean oceans; (iv) photosynthesis arose in an anoxygenic cyanobacterial progenitor; (v) Chl a is the ancestral Chl; and (vi), anoxygenic chlorophototrophic lineages characterized so far acquired, by horizontal gene transfer, RCs and Chl biosynthesis with or without autotrophy, from the architects of chlorophototrophy-the cyanobacterial lineage.

Sulfolobus acidocaldarius Transports Pentoses via a Carbohydrate Uptake Transporter 2 (CUT2)-Type ABC Transporter and Metabolizes Them through the Aldolase-Independent Weimberg Pathway

Sulfolobus spp. possess a great metabolic versatility and grow heterotrophically on various carbon sources, such as different sugars and peptides. Known sugar transporters in Archaea predominantly belong to ABC transport systems. Although several ABC transporters for sugar uptake have been characterized in the crenarchaeon Sulfolobus solfataricus, only one homologue of these transporters, the maltose/maltooligomer transporter, could be identified in the closely related Sulfolobus acidocaldarius Comparison of the transcriptome of S. acidocaldarius MW001 grown on peptides alone and peptides in the presence of d-xylose allowed for the identification of the ABC transporter for d-xylose and l-arabinose transport and the gaining of deeper insights into pentose catabolism under the respective growth conditions. The d-xylose/l-arabinose substrate binding protein (SBP) (Saci_2122) of the ABC transporter is unique in Archaea and shares more similarity to bacterial SBPs of the carbohydrate uptake transporter-2 (CUT2) family than to any characterized archaeal one. The identified pentose transporter is the first CUT2 family ABC transporter analyzed in the domain of Archaea Single-gene deletion mutants of the ABC transporter subunits exemplified the importance of the transport system for d-xylose and l-arabinose uptake. Next to the transporter operon, enzymes of the aldolase-independent pentose catabolism branch were found to be upregulated in N-Z-Amine and d-xylose medium. The α-ketoglutarate semialdehyde dehydrogenase (KGSADH; Saci_1938) seemed not to be essential for growth on pentoses. However, the deletion mutant of the 2-keto-3-deoxyarabinoate/xylonate dehydratase (KDXD [also known as KDAD]; Saci_1939) was no longer able to catabolize d-xylose or l-arabinose, suggesting the absence of the aldolase-dependent branch in S. acidocaldarius IMPORTANCE Thermoacidophilic microorganisms are emerging model organisms for biotechnological applications, as their optimal growth conditions resemble conditions used in certain biotechnologies such as industrial plant waste degradation. Because of its high genome stability, Sulfolobus acidocaldarius is especially suited as a platform organism for such applications. For use in (ligno)cellulose degradation, it was important to understand pentose uptake and metabolism in S. acidocaldarius This study revealed that only the aldolase-independent Weimberg pathway is required for growth of S. acidocaldarius MW001 on d-xylose and l-arabinose. Moreover, S. acidocaldarius employs a CUT2 ABC transporter for pentose uptake, which is more similar to bacterial than to archaeal ABC transporters. The identification of pentose-inducible promoters will expedite the metabolic engineering of S. acidocaldarius for its development into a platform organism for (ligno)cellulose degradation.

Phosphoglycerate kinase acts as a futile cycle at high temperature

In (hyper)thermophilic organisms metabolic processes have to be adapted to function optimally at high temperature. We compared the gluconeogenic conversion of 3-phosphoglycerate via 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate at 30 °C and at 70 °C. At 30 °C it was possible to produce 1,3-bisphosphoglycerate from 3-phosphoglycerate with phosphoglycerate kinase, but at 70 °C, 1,3-bisphosphoglycerate was dephosphorylated rapidly to 3-phosphoglycerate, effectively turning the phosphoglycerate kinase into a futile cycle. When phosphoglycerate kinase was incubated together with glyceraldehyde 3-phosphate dehydrogenase it was possible to convert 3-phosphoglycerate to glyceraldehyde 3-phosphate, both at 30 °C and at 70 °C, however, at 70 °C only low concentrations of product were observed due to thermal instability of glyceraldehyde 3-phosphate. Thus, thermolabile intermediates challenge central metabolic reactions and require special adaptation strategies for life at high temperature.

Genome Sequencing of Sulfolobus sp. A20 from Costa Rica and Comparative Analyses of the Putative Pathways of Carbon, Nitrogen, and Sulfur Metabolism in Various Sulfolobus Strains

The genome of Sulfolobus sp. A20 isolated from a hot spring in Costa Rica was sequenced. This circular genome of the strain is 2,688,317 bp in size and 34.8% in G+C content, and contains 2591 open reading frames (ORFs). Strain A20 shares ~95.6% identity at the 16S rRNA gene sequence level and <30% DNA-DNA hybridization (DDH) values with the most closely related known Sulfolobus species (i.e., Sulfolobus islandicus and Sulfolobus solfataricus), suggesting that it represents a novel Sulfolobus species. Comparison of the genome of strain A20 with those of the type strains of S. solfataricus, Sulfolobus acidocaldarius, S. islandicus, and Sulfolobus tokodaii, which were isolated from geographically separated areas, identified 1801 genes conserved among all Sulfolobus species analyzed (core genes). Comparative genome analyses show that central carbon metabolism in Sulfolobus is highly conserved, and enzymes involved in the Entner-Doudoroff pathway, the tricarboxylic acid cycle and the CO₂ fixation pathways are predominantly encoded by the core genes. All Sulfolobus species encode genes required for the conversion of ammonium into glutamate/glutamine. Some Sulfolobus strains have gained the ability to utilize additional nitrogen source such as nitrate (i.e., S. islandicus strain REY15A, LAL14/1, M14.25, and M16.27) or urea (i.e., S. islandicus HEV10/4, S. tokodaii strain7, and S. metallicus DSM 6482). The strategies for sulfur metabolism are most diverse and least understood. S. tokodaii encodes sulfur oxygenase/reductase (SOR), whereas both S. islandicus and S. solfataricus contain genes for sulfur reductase (SRE). However, neither SOR nor SRE genes exist in the genome of strain A20, raising the possibility that an unknown pathway for the utilization of elemental sulfur may be present in the strain. The ability of Sulfolobus to utilize nitrate or sulfur is encoded by a gene cluster flanked by IS elements or their remnants. These clusters appear to have become fixed at a specific genomic site in some strains and lost in other strains during the course of evolution. The versatility in nitrogen and sulfur metabolism may represent adaptation of Sulfolobus to thriving in different habitats.

An archaeal ADP-dependent serine kinase involved in cysteine biosynthesis and serine metabolism

Routes for cysteine biosynthesis are still unknown in many archaea. Here we find that the hyperthermophilic archaeon Thermococcus kodakarensis generates cysteine from serine via O-phosphoserine, in addition to the classical route from 3-phosphoglycerate. The protein responsible for serine phosphorylation is encoded by TK0378, annotated as a chromosome partitioning protein ParB. The TK0378 protein utilizes ADP as the phosphate donor, but in contrast to previously reported ADP-dependent kinases, recognizes a non-sugar substrate. Activity is specific towards free serine, and not observed with threonine, homoserine and serine residues within a peptide. Genetic analyses suggest that TK0378 is involved in serine assimilation and clearly responsible for cysteine biosynthesis from serine. TK0378 homologs, present in Thermococcales and Desulfurococcales, are most likely not ParB proteins and constitute a group of kinases involved in serine utilization.

Archaea metabolism has unique adaptations to hostile environments. Here Makino et al. describe an unusual ADP-dependent kinase that phosphorylates free serine to O-phosphoserine and participates in an additional cysteine biosynthetic pathway in the archaeon Thermococcus kodakarensis.

Archaeal Mo-Containing Glyceraldehyde Oxidoreductase Isozymes Exhibit Diverse Substrate Specificities through Unique Subunit Assemblies

Archaea use glycolytic pathways distinct from those found in bacteria and eukaryotes, where unique enzymes catalyze each reaction step. In this study, we isolated three isozymes of glyceraldehyde oxidoreductase (GAOR1, GAOR2 and GAOR3) from the thermoacidophilic archaeon Sulfolobus tokodaii. GAOR1-3 belong to the xanthine oxidoreductase superfamily, and are composed of a molybdo-pyranopterin subunit (L), a flavin subunit (M), and an iron-sulfur subunit (S), forming an LMS hetero-trimer unit. We found that GAOR1 is a tetramer of the STK17810/STK17830/STK17820 hetero-trimer, GAOR2 is a dimer of the STK23390/STK05620/STK05610 hetero-trimer, and GAOR3 is the STK24840/STK05620/STK05610 hetero-trimer. GAOR1-3 exhibited diverse substrate specificities for their electron donors and acceptors, due to their different L-subunits, and probably participate in the non-phosphorylative Entner-Doudoroff glycolytic pathway. We determined the crystal structure of GAOR2, as the first three-dimensional structure of an archaeal molybdenum-containing hydroxylase, to obtain structural insights into their substrate specificities and subunit assemblies. The gene arrangement and the crystal structure suggested that the M/S-complex serves as a structural scaffold for the binding of the L-subunit, to construct the three enzymes with different specificities. Collectively, our findings illustrate a novel principle of a prokaryotic multicomponent isozyme system.

Possibilities for extremophilic microorganisms in microbial electrochemical systems

Microbial electrochemical systems exploit the metabolism of microorganisms to generate electrical energy or a useful product. In the past couple of decades, the application of microbial electrochemical systems has increased from the use of wastewaters to produce electricity to a versatile technology that can use numerous sources for the extraction of electrons on the one hand, while on the other hand these electrons can be used to serve an ever increasing number of functions. Extremophilic microorganisms grow in environments that are hostile to most forms of life and their utilization in microbial electrochemical systems has opened new possibilities to oxidize substrates in the anode and produce novel products in the cathode. For example, extremophiles can be used to oxidize sulfur compounds in acidic pH to remediate wastewaters, generate electrical energy from marine sediment microbial fuel cells at low temperatures, desalinate wastewaters and act as biosensors of low amounts of organic carbon. In this review, we will discuss the recent advances that have been made in using microbial catalysts under extreme conditions and show possible new routes that extremophilic microorganisms open for microbial electrochemical systems.

NADPH-generating systems in bacteria and archaea

Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is an essential electron donor in all organisms. It provides the reducing power that drives numerous anabolic reactions, including those responsible for the biosynthesis of all major cell components and many products in biotechnology. The efficient synthesis of many of these products, however, is limited by the rate of NADPH regeneration. Hence, a thorough understanding of the reactions involved in the generation of NADPH is required to increase its turnover through rational strain improvement. Traditionally, the main engineering targets for increasing NADPH availability have included the dehydrogenase reactions of the oxidative pentose phosphate pathway and the isocitrate dehydrogenase step of the tricarboxylic acid (TCA) cycle. However, the importance of alternative NADPH-generating reactions has recently become evident. In the current review, the major canonical and non-canonical reactions involved in the production and regeneration of NADPH in prokaryotes are described, and their key enzymes are discussed. In addition, an overview of how different enzymes have been applied to increase NADPH availability and thereby enhance productivity is provided.

Carbohydrate metabolism in Archaea: current insights into unusual enzymes and pathways and their regulation

The metabolism of Archaea, the third domain of life, resembles in its complexity those of Bacteria and lower Eukarya. However, this metabolic complexity in Archaea is accompanied by the absence of many "classical" pathways, particularly in central carbohydrate metabolism. Instead, Archaea are characterized by the presence of unique, modified variants of classical pathways such as the Embden-Meyerhof-Parnas (EMP) pathway and the Entner-Doudoroff (ED) pathway. The pentose phosphate pathway is only partly present (if at all), and pentose degradation also significantly differs from that known for bacterial model organisms. These modifications are accompanied by the invention of "new," unusual enzymes which cause fundamental consequences for the underlying regulatory principles, and classical allosteric regulation sites well established in Bacteria and Eukarya are lost. The aim of this review is to present the current understanding of central carbohydrate metabolic pathways and their regulation in Archaea. In order to give an overview of their complexity, pathway modifications are discussed with respect to unusual archaeal biocatalysts, their structural and mechanistic characteristics, and their regulatory properties in comparison to their classic counterparts from Bacteria and Eukarya. Furthermore, an overview focusing on hexose metabolic, i.e., glycolytic as well as gluconeogenic, pathways identified in archaeal model organisms is given. Their energy gain is discussed, and new insights into different levels of regulation that have been observed so far, including the transcript and protein levels (e.g., gene regulation, known transcription regulators, and posttranslational modification via reversible protein phosphorylation), are presented.

Bifunctional ADP-dependent phosphofructokinase/glucokinase activity in the order Methanococcales--biochemical characterization of the mesophilic enzyme from Methanococcus maripaludis

In some archaea, the phosphorylation of glucose and fructose 6-phosphate (fructose 6P) is carried out by enzymes that are specific for either substrate and that use ADP as phosphoryl donor. In the hyperthermophilic archaeon Methanocaldococcus jannaschii, a bifunctional enzyme able to phosphorylate glucose and fructose 6P has been described. To determine whether the ability to phosphorylate both glucose and fructose 6P is a common feature for all enzymes of the order Methanococcales, we expressed, purified and characterized the unique homologous protein of the mesophilic archaea Methanococcus maripaludis. Assay of the enzyme activity with different sugars, metals and nucleotides allows us to conclude that the enzyme is able to phosphorylate both fructose 6P and glucose in the presence of ADP and a divalent metal cation. Kinetic characterization of the enzyme revealed complex regulation by the free Mg(2+) concentration and AMP, with the latter appearing to be a key metabolite. To determine whether this enzyme could have a role in gluconeogenesis, we evaluated the reversibility of both reactions and found that glucokinase activity is reversible, whereas phosphofructokinase activity is not. To determine the important residues for glucose and fructose 6P binding, we modeled the bifunctional phosphofructokinase/glucokinase enzyme from M. maripaludis and its interactions with both sugar substrates using protein–ligand docking. Comparison of the active site of the phosphofructokinase/glucokinase enzyme from M. maripaludis with the structural models constructed for all the homology sequences present in the order Methanococcales shows that all of the ADP-dependent kinases from this order would be able to phosphorylate glucose and fructose 6P, which rules out the current annotation of these enzymes as specific phosphofructokinases.

Haloferax volcanii N-glycosylation: delineating the pathway of dTDP-rhamnose biosynthesis

In the halophilic archaea Haloferax volcanii, the surface (S)-layer glycoprotein can be modified by two distinct N-linked glycans. The tetrasaccharide attached to S-layer glycoprotein Asn-498 comprises a sulfated hexose, two hexoses and a rhamnose. While Agl11-14 have been implicated in the appearance of the terminal rhamnose subunit, the precise roles of these proteins have yet to be defined. Accordingly, a series of in vitro assays conducted with purified Agl11-Agl14 showed these proteins to catalyze the stepwise conversion of glucose-1-phosphate to dTDP-rhamnose, the final sugar of the tetrasaccharide glycan. Specifically, Agl11 is a glucose-1-phosphate thymidylyltransferase, Agl12 is a dTDP-glucose-4,6-dehydratase and Agl13 is a dTDP-4-dehydro-6-deoxy-glucose-3,5-epimerase, while Agl14 is a dTDP-4-dehydrorhamnose reductase. Archaea thus synthesize nucleotide-activated rhamnose by a pathway similar to that employed by Bacteria and distinct from that used by Eukarya and viruses. Moreover, a bioinformatics screen identified homologues of agl11-14 clustered in other archaeal genomes, often as part of an extended gene cluster also containing aglB, encoding the archaeal oligosaccharyltransferase. This points to rhamnose as being a component of N-linked glycans in Archaea other than Hfx. volcanii.

Molecular paleontology and complexity in the last eukaryotic common ancestor

Eukaryogenesis, the origin of the eukaryotic cell, represents one of the fundamental evolutionary transitions in the history of life on earth. This event, which is estimated to have occurred over one billion years ago, remains rather poorly understood. While some well-validated examples of fossil microbial eukaryotes for this time frame have been described, these can provide only basic morphology and the molecular machinery present in these organisms has remained unknown. Complete and partial genomic information has begun to fill this gap, and is being used to trace proteins and cellular traits to their roots and to provide unprecedented levels of resolution of structures, metabolic pathways and capabilities of organisms at these earliest points within the eukaryotic lineage. This is essentially allowing a molecular paleontology. What has emerged from these studies is spectacular cellular complexity prior to expansion of the eukaryotic lineages. Multiple reconstructed cellular systems indicate a very sophisticated biology, which by implication arose following the initial eukaryogenesis event but prior to eukaryotic radiation and provides a challenge in terms of explaining how these early eukaryotes arose and in understanding how they lived. Here, we provide brief overviews of several cellular systems and the major emerging conclusions, together with predictions for subsequent directions in evolution leading to extant taxa. We also consider what these reconstructions suggest about the life styles and capabilities of these earliest eukaryotes and the period of evolution between the radiation of eukaryotes and the eukaryogenesis event itself.

The common ancestor of archaea and eukarya was not an archaeon

It is often assumed that eukarya originated from archaea. This view has been recently supported by phylogenetic analyses in which eukarya are nested within archaea. Here, I argue that these analyses are not reliable, and I critically discuss archaeal ancestor scenarios, as well as fusion scenarios for the origin of eukaryotes. Based on recognized evolutionary trends toward reduction in archaea and toward complexity in eukarya, I suggest that their last common ancestor was more complex than modern archaea but simpler than modern eukaryotes (the bug in-between scenario). I propose that the ancestors of archaea (and bacteria) escaped protoeukaryotic predators by invading high temperature biotopes, triggering their reductive evolution toward the "prokaryotic" phenotype (the thermoreduction hypothesis). Intriguingly, whereas archaea and eukarya share many basic features at the molecular level, the archaeal mobilome resembles more the bacterial than the eukaryotic one. I suggest that selection of different parts of the ancestral virosphere at the onset of the three domains played a critical role in shaping their respective biology. Eukarya probably evolved toward complexity with the help of retroviruses and large DNA viruses, whereas similar selection pressure (thermoreduction) could explain why the archaeal and bacterial mobilomes somehow resemble each other.

The divergence and natural selection of autocatalytic primordial metabolic systems

The diversity of the central metabolism of modern organisms is caused by the existence of a few metabolic modules, combination of which produces multiple metabolic pathways. This paper analyzes biomimetically reconstructed coupled autocatalytic cycles as the basis of ancestral metabolic systems. The mechanism for natural selection and evolution in autocatalytic chemical systems may be affected by natural homeostatic parameters such as ambient chemical potentials, temperature, and pressure. Competition between separate parts of an autocatalytic network with positive-plus-negative feedback resulted in the formation of primordial autotrophic, mixotrophic, and heterotrophic metabolic systems. This work examined the last common ancestor of a set of coupled metabolic cycles in a population of protocells. Physical-chemical properties of these cycles determined the main principles of natural selection for the ancestral Bacteria and Archaea taxa.