Genome Replication in Thermococcus kodakarensis Independent of Cdc6 and an Origin of Replication

Regulation of DNA Topology in Archaea: State of the Art and Perspectives

DNA topology is a direct consequence of the double helical nature of DNA and is defined by how the two complementary DNA strands are intertwined. Virtually every reaction involving DNA is influenced by DNA topology or has topological effects. It is therefore of fundamental importance to understand how this phenomenon is controlled in living cells. DNA topoisomerases are the key actors dedicated to the regulation of DNA topology in cells from all domains of life. While significant progress has been made in the last two decades in understanding how these enzymes operate in vivo in Bacteria and Eukaryotes, studies in Archaea have been lagging behind. This review article aims to summarize what is currently known about DNA topology regulation by DNA topoisomerases in main archaeal model organisms. These model archaea exhibit markedly different lifestyles, genome organization and topoisomerase content, thus highlighting the diversity and the complexity of DNA topology regulation mechanisms and their evolution in this domain of life. The recent development of functional genomic assays supported by next-generation sequencing now allows to delve deeper into this timely and exciting, yet still understudied topic.

The Archaeal Cell Cycle

Since first identified as a separate domain of life in the 1970s, it has become clear that archaea differ profoundly from both eukaryotes and bacteria. In this review, we look across the archaeal domain and discuss the diverse mechanisms by which archaea control cell cycle progression, DNA replication, and cell division. While the molecular and cellular processes archaea use to govern these critical cell biological processes often differ markedly from those described in bacteria and eukaryotes, there are also striking similarities that highlight both unique and common principles of cell cycle control across the different domains of life. Since much of the eukaryotic cell cycle machinery has its origins in archaea, exploration of the mechanisms of archaeal cell division also promises to illuminate the evolution of the eukaryotic cell cycle.

Intein splicing efficiency and RadA levels can control the mode of archaeal DNA replication

Inteins (intervening proteins), mobile genetic elements removed through protein splicing, often interrupt proteins required for DNA replication, recombination, and repair. An abundance of in vitro evidence implies that inteins may act as regulatory elements, whereby reduced splicing inhibits production of the mature protein lacking the intein, but in vivo evidence of regulatory intein excision in the native host is absent. The model archaeon Thermococcus kodakarensis encodes 15 inteins, and we establish the impacts of intein splicing inhibition on host physiology and replication in vivo. We report that a decrease in intein splicing efficiency of the recombinase RadA, a Rad51/RecA homolog, has widespread physiological consequences, including a general growth defect, increased sensitivity to DNA damage, and a switch in the mode of DNA replication from recombination-dependent replication toward origin-dependent replication.

Cooperation between two modes for DNA replication initiation in the archaeon Thermococcus barophilus

The mechanisms underpinning the replication of genomic DNA have recently been challenged in Archaea. Indeed, the lack of origin of replication has no deleterious effect on growth, suggesting that replication initiation relies on homologous recombination. Recombination-dependent replication (RDR) appears to be based on the recombinase RadA, which is of absolute requirement when no initiation origins are detected. The origin of this flexibility in the initiation of replication and the extent to which it is used in nature are yet to be understood. Here, we followed the process of DNA replication throughout the growth stages of Thermococcus barophilus. We combined deep sequencing and genetics to elucidate the dynamics of oriC utilization according to growth phases. We discovered that in T. barophilus, the use of oriC diminishes from the lag to the middle of the log phase, and subsequently increases gradually upon entering the stationary phase. Although oriC demonstrates no indispensability, RadA does exhibit essentiality. Notably, a knockdown mutant strain provides confirmation of the pivotal role of RadA in RDR for the first time. Thus, we demonstrate the existence of a tight combination between oriC utilization and homologous recombination to initiate DNA replication along the growth phases. Overall, this study demonstrates how diverse physiological states can influence the initiation of DNA replication, offering insights into how environmental sensing might impact this fundamental mechanism of life.

IMPORTANCE

Replication of DNA is highly important in all organisms. It initiates at a specific locus called ori, which serves as the binding site for scaffold proteins-either Cdc6 or DnaA-depending on the domain of life. However, recent studies have shown that the Archaea, Haloferax volcanii and Thermococcus kodakarensis could subsist without ori. Recombination-dependent replication (RDR), via the recombinase RadA, is the mechanism that uses homologous recombination to initiate DNA replication. The extent to which ori's use is necessary in natural growth remains to be characterized. In this study, using Thermococcus barophilus, we demonstrated that DNA replication initiation relies on both oriC and RDR throughout its physiological growth, each to varying degrees depending on the phase. Notably, a knockdown RadA mutant confirmed the prominent use of RDR during the log phase. Moreover, the study of ploidy in oriC and radA mutant strains showed that the number of chromosomes per cell is a critical proxy for ensuring proper growth and cell survival.

A self-transmissible plasmid from a hyperthermophile that facilitates genetic modification of diverse Archaea

Conjugative plasmids are self-transmissible mobile genetic elements that transfer DNA between host cells via type IV secretion systems (T4SS). While T4SS-mediated conjugation has been well-studied in bacteria, information is sparse in Archaea and known representatives exist only in the Sulfolobales order of Crenarchaeota. Here we present the first self-transmissible plasmid identified in a Euryarchaeon, Thermococcus sp. 33-3. The 103 kbp plasmid, pT33-3, is seen in CRISPR spacers throughout the Thermococcales order. We demonstrate that pT33-3 is a bona fide conjugative plasmid that requires cell-to-cell contact and is dependent on canonical, plasmid-encoded T4SS-like genes. Under laboratory conditions, pT33-3 transfers to various Thermococcales and transconjugants propagate at 100 °C. Using pT33-3, we developed a genetic toolkit that allows modification of phylogenetically diverse Archaeal genomes. We demonstrate pT33-3-mediated plasmid mobilization and subsequent targeted genome modification in previously untransformable Thermococcales species, and extend this process to interphylum transfer to a Crenarchaeon.

Archaea as a Model System for Molecular Biology and Biotechnology

Archaea represents the third domain of life, displaying a closer relationship with eukaryotes than bacteria. These microorganisms are valuable model systems for molecular biology and biotechnology. In fact, nowadays, methanogens, halophiles, thermophilic euryarchaeota, and crenarchaeota are the four groups of archaea for which genetic systems have been well established, making them suitable as model systems and allowing for the increasing study of archaeal genes' functions. Furthermore, thermophiles are used to explore several aspects of archaeal biology, such as stress responses, DNA replication and repair, transcription, translation and its regulation mechanisms, CRISPR systems, and carbon and energy metabolism. Extremophilic archaea also represent a valuable source of new biomolecules for biological and biotechnological applications, and there is growing interest in the development of engineered strains. In this review, we report on some of the most important aspects of the use of archaea as a model system for genetic evolution, the development of genetic tools, and their application for the elucidation of the basal molecular mechanisms in this domain of life. Furthermore, an overview on the discovery of new enzymes of biotechnological interest from archaea thriving in extreme environments is reported.

Form and function of archaeal genomes

A key maxim in modernist architecture is that 'form follows function'. While modernist buildings are hopefully the product of intelligent design, the architectures of chromosomes have been sculpted by the forces of evolution over many thousands of generations. In the following, I will describe recent advances in our understanding of chromosome architecture in the archaeal domain of life. Although much remains to be learned about the mechanistic details of archaeal chromosome organization, some general principles have emerged. At the 10-100 kb level, archaeal chromosomes have a conserved local organization reminiscent of bacterial genomes. In contrast, lineage-specific innovations appear to have imposed distinct large-scale architectural features. The ultimate functions of genomes are to store and to express genetic information. Gene expression profiles have been shown to influence chromosome architecture, thus their form follows function. However, local changes to chromosome conformation can also influence gene expression and therefore, in these instances, function follows form.

Archaeal tyrosine recombinases

The integration of mobile genetic elements into their host chromosome influences the immediate fate of cellular organisms and gradually shapes their evolution. Site-specific recombinases catalyzing this integration have been extensively characterized both in bacteria and eukarya. More recently, a number of reports provided the in-depth characterization of archaeal tyrosine recombinases and highlighted new particular features not observed in the other two domains. In addition to being active in extreme environments, archaeal integrases catalyze reactions beyond site-specific recombination. Some of these integrases can catalyze low-sequence specificity recombination reactions with the same outcome as homologous recombination events generating deep rearrangements of their host genome. A large proportion of archaeal integrases are termed suicidal due to the presence of a specific recombination target within their own gene. The paradoxical maintenance of integrases that disrupt their gene upon integration implies novel mechanisms for their evolution. In this review, we assess the diversity of the archaeal tyrosine recombinases using a phylogenomic analysis based on an exhaustive similarity network. We outline the biochemical, ecological and evolutionary properties of these enzymes in the context of the families we identified and emphasize similarities and differences between archaeal recombinases and their bacterial and eukaryal counterparts.

Thermococcus kodakarensis provides a versatile hyperthermophilic archaeal platform for protein expression

Hyperthermophiles, typically defined as organisms with growth optima ≥80°C, are dominated by the Archaea. Proteins that support life at the extremes of temperatures often retain substantial biotechnological and commercial value, but the recombinant expression of individual hyperthermophilic proteins is commonly complicated in non-native mesophilic hosts due to differences in codon bias, intracellular solutes and the requirement for accessory factors that aid in folding or deposition of metal centers within archaeal proteins. The development of versatile protein expression and facilitated protein purification systems in the model, genetically tractable, hyperthermophilic marine archaeon Thermococcus kodakarensis provides an attractive platform for protein expression within the hyperthermophiles. The assortment of T. kodakarensis genetic backgrounds and compatible selection markers allow iterative genetic manipulations that facilitate protein overexpression and expedite protein purifications. Expression vectors that stably replicate both in T. kodakarensis and Escherichia coli have been validated and permit high-level ectopic gene expression from a variety of controlled and constitutive promoters. Biologically relevant protein associations can be maintained during protein purifications to identify native protein partnerships and define protein interaction networks. T. kodakarensis thus provides a versatile platform for the expression and purification of thermostable proteins.

Pervasive Suicidal Integrases in Deep-Sea Archaea

Mobile genetic elements (MGEs) often encode integrases which catalyze the site-specific insertion of their genetic information into the host genome and the reverse reaction of excision. Hyperthermophilic archaea harbor integrases belonging to the SSV-family which carry the MGE recombination site within their open reading frame. Upon integration into the host genome, SSV integrases disrupt their own gene into two inactive pseudogenes and are termed suicidal for this reason. The evolutionary maintenance of suicidal integrases, concurring with the high prevalence and multiples recruitments of these recombinases by archaeal MGEs, is highly paradoxical. To elucidate this phenomenon, we analyzed the wide phylogenomic distribution of a prominent class of suicidal integrases which revealed a highly variable integration site specificity. Our results highlighted the remarkable hybrid nature of these enzymes encoded from the assembly of inactive pseudogenes of different origins. The characterization of the biological properties of one of these integrases, IntpT26-2 showed that this enzyme was active over a wide range of temperatures up to 99 °C and displayed a less-stringent site specificity requirement than comparable integrases. These observations concurred in explaining the pervasiveness of these suicidal integrases in the most hyperthermophilic organisms. The biochemical and phylogenomic data presented here revealed a target site switching system operating on highly thermostable integrases and suggested a new model for split gene reconstitution. By generating fast-evolving pseudogenes at high frequency, suicidal integrases constitute a powerful model to approach the molecular mechanisms involved in the generation of active genes variants by the recombination of proto-genes.

Haloferax volcanii-a model archaeon for studying DNA replication and repair

The tree of life shows the relationship between all organisms based on their common ancestry. Until 1977, it comprised two major branches: prokaryotes and eukaryotes. Work by Carl Woese and other microbiologists led to the recategorization of prokaryotes and the proposal of three primary domains: Eukarya, Bacteria and Archaea. Microbiological, genetic and biochemical techniques were then needed to study the third domain of life. Haloferax volcanii, a halophilic species belonging to the phylum Euryarchaeota, has provided many useful tools to study Archaea, including easy culturing methods, genetic manipulation and phenotypic screening. This review will focus on DNA replication and DNA repair pathways in H. volcanii, how this work has advanced our knowledge of archaeal cellular biology, and how it may deepen our understanding of bacterial and eukaryotic processes.

Archaeal DNA Repair Mechanisms

Archaea often thrive in environmental extremes, enduring levels of heat, pressure, salinity, pH, and radiation that prove intolerable to most life. Many environmental extremes raise the propensity for DNA damaging events and thus, impact DNA stability, placing greater reliance on molecular mechanisms that recognize DNA damage and initiate accurate repair. Archaea can presumably prosper in harsh and DNA-damaging environments in part due to robust DNA repair pathways but surprisingly, no DNA repair pathways unique to Archaea have been described. Here, we review the most recent advances in our understanding of archaeal DNA repair. We summarize DNA damage types and their consequences, their recognition by host enzymes, and how the collective activities of many DNA repair pathways maintain archaeal genomic integrity.

Role of RadA and DNA Polymerases in Recombination-Associated DNA Synthesis in Hyperthermophilic Archaea

Among the three domains of life, the process of homologous recombination (HR) plays a central role in the repair of double-strand DNA breaks and the restart of stalled replication forks. Curiously, main protein actors involved in the HR process appear to be essential for hyperthermophilic Archaea raising interesting questions about the role of HR in replication and repair strategies of those Archaea living in extreme conditions. One key actor of this process is the recombinase RadA, which allows the homologous strand search and provides a DNA substrate required for following DNA synthesis and restoring genetic information. DNA polymerase operation after the strand exchange step is unclear in Archaea. Working with Pyrococcus abyssi proteins, here we show that both DNA polymerases, family-B polymerase (PolB) and family-D polymerase (PolD), can take charge of processing the RadA-mediated recombination intermediates. Our results also indicate that PolD is far less efficient, as compared with PolB, to extend the invaded DNA at the displacement-loop (D-loop) substrate. These observations coincide with previous genetic analyses obtained on Thermococcus species showing that PolB is mainly involved in DNA repair without being essential probably because PolD could take over combined with additional partners.

Archaeal DNA Replication

It is now well recognized that the information processing machineries of archaea are far more closely related to those of eukaryotes than to those of their prokaryotic cousins, the bacteria. Extensive studies have been performed on the structure and function of the archaeal DNA replication origins, the proteins that define them, and the macromolecular assemblies that drive DNA unwinding and nascent strand synthesis. The results from various archaeal organisms across the archaeal domain of life show surprising levels of diversity at many levels-ranging from cell cycle organization to chromosome ploidy to replication mode and nature of the replicative polymerases. In the following, we describe recent advances in the field, highlighting conserved features and lineage-specific innovations.

Emerging views of genome organization in Archaea

Over the past decade, advances in methodologies for the determination of chromosome conformation have provided remarkable insight into the local and higher-order organization of bacterial and eukaryotic chromosomes. Locally folded domains are found in both bacterial and eukaryotic genomes, although they vary in size. Importantly, genomes of metazoans also possess higher-order organization into A- and B-type compartments, regions of transcriptionally active and inactive chromatin, respectively. Until recently, nothing was known about the organization of genomes of organisms in the third domain of life - the archaea. However, despite archaea possessing simple circular genomes that are morphologically reminiscent of those seen in many bacteria, a recent study of archaea of the genus Sulfolobus has revealed that it organizes its genome into large-scale domains. These domains further interact to form defined A- and B-type compartments. The interplay of transcription and localization of a novel structural maintenance of chromosomes (SMC) superfamily protein, termed coalescin, defines compartment identity. In this Review, we discuss the mechanistic and evolutionary implications of these findings.

Microbe Profile: Thermococcus kodakarensis: the model hyperthermophilic archaeon

Thermococcus kodakarensis is a hyperthermophilic Euryarchaeon that grows well under laboratory conditions and, being naturally competent for genetic transformation, it has become a widely studied experimental model species. With the genome sequence available since 2004, combining genetic, enzymological and structural biochemical approaches has revealed previously unknown and unanticipated features of archaeal molecular biology and metabolism. T. kodakarensis DNA polymerase is already commercialized and with the details of metabolism and hydrogenase available, generating H2 from biopolymers solubilized at high temperatures, most notably chitin, now seems a very attractive possibility as a renewable energy bioprocess.

The global distribution and evolutionary history of the pT26-2 archaeal plasmid family

Although plasmids play an important role in biological evolution, the number of plasmid families well‐characterized in terms of geographical distribution and evolution remains limited, especially in archaea. Here, we describe the first systematic study of an archaeal plasmid family, the pT26‐2 plasmid family. The in‐depth analysis of the distribution, biogeography and host–plasmid co‐evolution patterns of 26 integrated and 3 extrachromosomal plasmids of this plasmid family shows that they are widespread in Thermococcales and Methanococcales isolated from around the globe but are restricted to these two orders. All members of the family share seven core genes but employ different integration and replication strategies. Phylogenetic analysis of the core genes and CRISPR spacer distribution suggests that plasmids of the pT26‐2 family evolved with their hosts independently in Thermococcales and Methanococcales, despite these hosts exhibiting similar geographic distribution. Remarkably, core genes are conserved even in integrated plasmids that have lost replication genes and/or replication origins suggesting that they may be beneficial for their hosts. We hypothesize that the core proteins encode for a novel type of DNA/protein transfer mechanism, explaining the widespread oceanic distribution of the pT26‐2 plasmid family.

Polyploidy in halophilic archaea: regulation, evolutionary advantages, and gene conversion

All analyzed haloarachea are polyploid. In addition, haloarchaea contain more than one type of chromosome, and thus the gene dosage can be regulated independently on different replicons. Haloarchaea and several additional archaea have more than one replication origin on their major chromosome, in stark contrast with bacteria, which have a single replication origin. Two of these replication origins of Haloferax volcanii have been studied in detail and turned out to have very different properties. The chromosome copy number appears to be regulated in response to growth phases and environmental factors. Archaea typically contain about two Origin Recognition Complex (ORC) proteins, which are homologous to eukaryotic ORC proteins. However, haloarchaea are the only archaeal group that contains a multitude of ORC proteins. All 16 ORC protein paralogs from H. volcanii are involved in chromosome copy number regulation. Polyploidy has many evolutionary advantages for haloarchaea, e.g. a high resistance to desiccation, survival over geological times, and the relaxation of cell cycle-specific replication control. A further advantage is the ability to grow in the absence of external phosphate while using the many genome copies as internal phosphate storage polymers. Very efficient gene conversion operates in haloarchaea and results in the unification of genome copies. Taken together, haloarchaea are excellent models to study many aspects of genome biology in prokaryotes, exhibiting properties that have not been found in bacteria.

Biotechnology of extremely thermophilic archaea

Although the extremely thermophilic archaea (Topt ≥ 70°C) may be the most primitive extant forms of life, they have been studied to a limited extent relative to mesophilic microorganisms. Many of these organisms have unique biochemical and physiological characteristics with important biotechnological implications. These include methanogens that generate methane, fermentative anaerobes that produce hydrogen gas with high efficiency, and acidophiles that can mobilize base, precious and strategic metals from mineral ores. Extremely thermophilic archaea have also been a valuable source of thermoactive, thermostable biocatalysts, but their use as cellular systems has been limited because of the general lack of facile genetics tools. This situation has changed recently, however, thereby providing an important avenue for understanding their metabolic and physiological details and also opening up opportunities for metabolic engineering efforts. Along these lines, extremely thermophilic archaea have recently been engineered to produce a variety of alcohols and industrial chemicals, in some cases incorporating CO2 into the final product. There are barriers and challenges to these organisms reaching their full potential as industrial microorganisms but, if these can be overcome, a new dimension for biotechnology will be forthcoming that strategically exploits biology at high temperatures.

Archaeal DNA replication and repair: new genetic, biophysical and molecular tools for discovering and characterizing enzymes, pathways and mechanisms

DNA replication and repair are essential biological processes needed for the survival of all organisms. Although these processes are fundamentally conserved in the three domains, archaea, bacteria and eukarya, the proteins and complexes involved differ. The genetic and biophysical tools developed for archaea in the last several years have accelerated the study of DNA replication and repair in this domain. In this review, the current knowledge of DNA replication and repair processes in archaea will be summarized, with emphasis on the contribution of genetics and other recently developed biophysical and molecular tools, including capillary gel electrophoresis, next-generation sequencing and single-molecule approaches. How these new tools will continue to drive archaeal DNA replication and repair research will also be discussed.

Do Archaea Need an Origin of Replication?

Chromosomal DNA replication starts at a specific region called an origin of replication. Until recently, all organisms were thought to require origins to replicate their chromosomes. It was recently discovered that some archaeal species do not utilize origins of replication under laboratory growth conditions.

DNA repair in the archaea-an emerging picture

There has long been a fascination in the DNA repair pathways of archaea, for two main reasons. Firstly, many archaea inhabit extreme environments where the rate of physical damage to DNA is accelerated. These archaea might reasonably be expected to have particularly robust or novel DNA repair pathways to cope with this. Secondly, the archaea have long been understood to be a lineage distinct from the bacteria, and to share a close relationship with the eukarya, particularly in their information processing systems. Recent discoveries suggest the eukarya arose from within the archaeal domain, and in particular from lineages related to the TACK superphylum and Lokiarchaea. Thus, archaeal DNA repair proteins and pathways can represent a useful model system. This review focuses on recent advances in our understanding of archaeal DNA repair processes including base excision repair, nucleotide excision repair, mismatch repair and double-strand break repair. These advances are discussed in the context of the emerging picture of the evolution and relationship of the three domains of life.