Archaeal replicative primases can perform translesion DNA synthesis

Primase-polymerases: how to make a primer from scratch

To pass on genetic information to the next generation, cells must faithfully replicate their genomes to provide copies for each daughter cell. To synthesise these duplicates, cells employ specialised enzymes called DNA polymerases, which rapidly and accurately replicate nucleic acid polymers. However, most polymerases lack the ability to directly initiate DNA synthesis and required specialised replicases called primases to make short polynucleotide primers, from which they then extend. Replicative primases (eukaryotes and archaea) belong to a functionally diverse enzyme superfamily known as Primase-Polymerases (Prim-Pols), with orthologues present throughout all domains of life. Characterised by a conserved catalytic Prim-Pol domain, these enzymes have evolved various roles in DNA metabolism, including DNA replication, repair, and damage tolerance. Many of these biological roles are fundamentally underpinned by the ability of Prim-Pols to generate primers de novo. This review examines our current understanding of the catalytic mechanisms utilised by Prim-Pols to initiate primer synthesis.

The monomeric archaeal primase from Nanoarchaeum equitans harbours the features of heterodimeric archaeoeukaryotic primases and primes sequence-specifically

The marine thermophilic archaeon Nanoarchaeum equitans possesses a monomeric primase encompassing the conserved domains of the small catalytic and the large regulatory subunits of archaeoeukaryotic heterodimeric primases in one protein chain. The recombinant protein primes on templates containing a triplet with a central thymidine, thus displaying a pronounced sequence specificity typically observed with bacterial type primases only. The N. equitans primase (NEQ395) is a highly active primase enzyme synthesizing short RNA primers. Termination occurs preferentially at about nine nucleotides, as determined by HPLC analysis and confirmed with mass spectrometry. Possibly, the compact monomeric primase NEQ395 represents the minimal archaeoeukaryotic primase and could serve as a functional and structural model of the heterodimeric archaeoeukaryotic primases, whose study is hindered by engagement in protein assemblies and rather low activity.

Repriming DNA synthesis: an intrinsic restart pathway that maintains efficient genome replication

To bypass a diverse range of fork stalling impediments encountered during genome replication, cells possess a variety of DNA damage tolerance (DDT) mechanisms including translesion synthesis, template switching, and fork reversal. These pathways function to bypass obstacles and allow efficient DNA synthesis to be maintained. In addition, lagging strand obstacles can also be circumvented by downstream priming during Okazaki fragment generation, leaving gaps to be filled post-replication. Whether repriming occurs on the leading strand has been intensely debated over the past half-century. Early studies indicated that both DNA strands were synthesised discontinuously. Although later studies suggested that leading strand synthesis was continuous, leading to the preferred semi-discontinuous replication model. However, more recently it has been established that replicative primases can perform leading strand repriming in prokaryotes. An analogous fork restart mechanism has also been identified in most eukaryotes, which possess a specialist primase called PrimPol that conducts repriming downstream of stalling lesions and structures. PrimPol also plays a more general role in maintaining efficient fork progression. Here, we review and discuss the historical evidence and recent discoveries that substantiate repriming as an intrinsic replication restart pathway for maintaining efficient genome duplication across all domains of life.

PolB1 Is Sufficient for DNA Replication and Repair Under Normal Growth Conditions in the Extremely Thermophilic Crenarchaeon Sulfolobus acidocaldarius

The thermophilic crenarchaeon Sulfolobus acidocaldarius has four DNA polymerases (DNAPs): PolB1, PolB2, PolB3, and Dbh (PolY). Previous in vitro studies suggested that PolB1 is the main replicative DNAP of Sulfolobales whereas PolB2 and Y-family polymerases Dpo4 (Saccharolobus solfataricus) or Dbh are involved in DNA repair and translesion DNA synthesis. On the other hand, there are various opinions about the role of PolB3, which remains to be clearly resolved. In order to examine the roles of the DNAPs of S. acidocaldarius through in vivo experiments, we constructed polB2, polB3, and dbh deletion strains and characterized their phenotypes. Efforts to construct a polB1 deletion strain were not successful; in contrast, it was possible to isolate triple gene-deletion strains lacking polB2, polB3, and dbh. The growth of these strains was nearly the same as that of the parent strains under normal growth conditions. The polB2, polB3, and dbh single-deletion strains were sensitive to some types of DNA-damaging treatments, but exhibited normal sensitivity to UV irradiation and several other damaging treatments. Overall, the genotype which exhibited the greatest sensitivity to the DNA-damaging treatments we tested was the ΔpolB2 ΔpolB3 combination, providing the first evidence of overlapping function for these two DNAPs in vivo. The results of our study strongly suggest that PolB1 is responsible for the DNA replication of both the leading and lagging strands and is sufficient to complete the repair of most DNA damage under normal growth conditions in S. acidocaldarius.

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.

Genetic Control of Oxidative Mutagenesis in Sulfolobus acidocaldarius

To identify DNA-oxidation defenses of hyperthermophilic archaea, we deleted genes encoding the putative 7,8-dihydro-8-oxoguanine (oxoG)-targeted N-glycosylase of S. acidocaldarius (ogg; Saci_01367), the Y-family DNA polymerase (dbh; Saci_0554), or both, and measured the effects on cellular survival, replication accuracy, and oxoG bypass in vivo Spontaneous G:C to T:A transversions were elevated in all Δogg and Δdbh constructs, and the Δogg Δdbh double mutant lost viability at a faster rate than isogenic WT and ogg strains. The distribution of G:C to T:A transversions within mutation-detector genes suggested that reactivity of G toward oxidation and the effect on translation contribute heavily to the pattern of mutations that are recovered. An impact of the Ogg protein on overall efficiency of bypassing oxoG in transforming DNA was evident only in the absence of Dbh, and Ogg status did not affect the accuracy of bypass. Dbh function, in contrast, dramatically influenced both the efficiency and accuracy of oxoG bypass. Thus, Ogg and Dbh were found to work independently to avoid mutagenesis by oxoG, and inactivating this simple but effective defense system by deleting both genes imposed a severe mutational burden on S. acidocaldarius cells.IMPORTANCE Hyperthermophilic archaea are expected to have effective (and perhaps atypical) mechanisms to limit the genetic consequences of DNA damage, but few gene products have been demonstrated to have genome-preserving functions in vivo This study confirmed by genetic criteria that the S. acidocaldarius Ogg protein avoids the characteristic mutagenesis of G oxidation. This enzyme and the bypass polymerase Dbh have similar impacts on genome stability but work independently, and may comprise most of the DNA-oxidation defense of S. acidocaldarius The critical dependence of accurate oxoG bypass on the accessory DNA polymerase Dbh further argues that some form of polymerase exchange is important for accurate genome replication in Sulfolobus, and perhaps in related hyperthermophilic archaea.

Structural studies reveal a ring-shaped architecture of deep-sea vent phage NrS-1 polymerase

NrS-1 is the first known phage that can infect Epsilonproteobacteria, one of the predominant primary producers in the deep-sea hydrothermal vent ecosystems. NrS-1 polymerase is a multidomain enzyme and is one key component of the phage replisome. The N-terminal Prim/Pol and HBD domains are responsible for DNA polymerization and de novo primer synthesis activities of NrS-1 polymerase. However, the structure and function of the C-terminus (CTR) of NrS-1 polymerase are poorly understood. Here, we report two crystal structures, showing that NrS-1 CTR adopts one unique hexameric ring-shaped conformation. Although the central helicase domain of NrS-1 CTR shares structural similarity with the superfamily III helicases, the folds of the Head and Tail domains are completely novel. Via mutagenesis and in vitro biochemical analysis, we identified many residues important for the helicase and polymerization activities of NrS-1 polymerase. In addition to NrS-1 polymerase, our study may also help us identify and understand the functions of multidomain polymerases expressed by many NrS-1 related phages.

Mimivirus encodes a multifunctional primase with DNA/RNA polymerase, terminal transferase and translesion synthesis activities

Acanthamoeba polyphaga mimivirus is an amoeba-infecting giant virus with over 1000 genes including several involved in DNA replication and repair. Here, we report the biochemical characterization of gene product 577 (gp577), a hypothetical protein (product of L537 gene) encoded by mimivirus. Sequence analysis and phylogeny suggested gp577 to be a primase-polymerase (PrimPol)-the first PrimPol to be identified in a nucleocytoplasmic large DNA virus (NCLDV). Recombinant gp577 protein purified as a homodimer and exhibited de novo RNA as well as DNA synthesis on circular and linear single-stranded DNA templates. Further, gp577 extends a DNA/RNA primer annealed to a DNA or RNA template using deoxyribonucleoties (dNTPs) or ribonucleotides (NTPs) demonstrating its DNA/RNA polymerase and reverse transcriptase activity. We also show that gp577 possesses terminal transferase activity and is capable of extending ssDNA and dsDNA with NTPs and dNTPs. Mutation of the conserved primase motif residues of gp577 resulted in the loss of primase, polymerase, reverse transcriptase and terminal transferase activities. Additionally, we show that gp577 possesses translesion synthesis (TLS) activity. Mimiviral gp577 represents the first protein from an NCLDV endowed with primase, polymerase, reverse transcriptase, terminal transferase and TLS activities.

The interplay at the replisome mitigates the impact of oxidative damage on the genetic integrity of hyperthermophilic Archaea

8-oxodeoxyguanosine (8-oxodG), a major oxidised base modification, has been investigated to study its impact on DNA replication in hyperthermophilic Archaea. Here we show that 8-oxodG is formed in the genome of growing cells, with elevated levels following exposure to oxidative stress. Functional characterisation of cell-free extracts and the DNA polymerisation enzymes, PolB, PolD, and the p41/p46 complex, alone or in the presence of accessory factors (PCNA and RPA) indicates that translesion synthesis occurs under replicative conditions. One of the major polymerisation effects was stalling, but each of the individual proteins could insert and extend past 8-oxodG with differing efficiencies. The introduction of RPA and PCNA influenced PolB and PolD in similar ways, yet provided a cumulative enhancement to the polymerisation performance of p41/p46. Overall, 8-oxodG translesion synthesis was seen to be potentially mutagenic leading to errors that are reminiscent of dA:8-oxodG base pairing.

Divalent Cations Alter the Rate-Limiting Step of PrimPol-Catalyzed DNA Elongation

PrimPol is the most recently discovered human DNA polymerase/primase and plays an emerging role in nuclear and mitochondrial genomic maintenance. As a member of archaeo-eukaryotic primase superfamily enzymes, PrimPol possesses DNA polymerase and primase activities that are important for replication fork progression in vitro and in cellulo. The enzymatic activities of PrimPol are critically dependent on the nucleotidyl-transfer reaction to incorporate deoxyribonucleotides successively; however, our knowledge concerning the kinetic mechanism of the reaction remains incomplete. Using enzyme kinetic analyses and computer simulations, we dissected the mechanism by which PrimPol transfers a nucleotide to a primer-template DNA, which comprises DNA binding, conformational transition, nucleotide binding, phosphoester bond formation, and dissociation steps. We obtained the rate constants of the steps by steady-state and pre-steady-state kinetic analyses and simulations. Our data demonstrate that the rate-limiting step of PrimPol-catalyzed DNA elongation depends on the metal cofactor involved. In the presence of Mn²⁺, a conformational transition step from non-productive to productive PrimPol:DNA complexes limits the enzymatic turnover, whereas in the presence of Mg²⁺, the chemical step becomes rate limiting. As evidenced from our kinetic and simulation data, PrimPol maintains the same kinetic mechanism under either millimolar or physiological micromolar Mn²⁺ concentration. Our study revealed the underlying mechanism by which PrimPol catalyzes nucleotide incorporation with two common metal cofactors and provides a kinetic basis for further understanding the regulatory mechanism of this functionally diverse primase-polymerase.

Snapshots of archaeal DNA replication and repair in living cells using super-resolution imaging

Using the haloarchaeon Haloferax volcanii as a model, we developed nascent DNA labeling and the functional GFP-labeled single-stranded binding protein RPA2 as novel tools to gain new insight into DNA replication and repair in live haloarchaeal cells. Our quantitative fluorescence microscopy data revealed that RPA2 forms distinct replication structures that dynamically responded to replication stress and DNA damaging agents. The number of the RPA2 foci per cell followed a probabilistic Poisson distribution, implying hitherto unnoticed stochastic cell-to-cell variation in haloarchaeal DNA replication and repair processes. The size range of haloarchaeal replication structures is very similar to those observed earlier in eukaryotic cells. The improved lateral resolution of 3D-SIM fluorescence microscopy allowed proposing that inhibition of DNA synthesis results in localized replication foci clustering and facilitated observation of RPA2 complexes brought about by chemical agents creating DNA double-strand breaks. Altogether our in vivo observations are compatible with earlier in vitro studies on archaeal single-stranded DNA binding proteins. Our work thus underlines the great potential of live cell imaging for unraveling the dynamic nature of transient molecular interactions that underpin fundamental molecular processes in the Third domain of life.

Archaeal DNA polymerases: new frontiers in DNA replication and repair

Archaeal DNA polymerases have long been studied due to their superior properties for DNA amplification in the polymerase chain reaction and DNA sequencing technologies. However, a full comprehension of their functions, recruitment and regulation as part of the replisome during genome replication and DNA repair lags behind well-established bacterial and eukaryotic model systems. The archaea are evolutionarily very broad, but many studies in the major model systems of both Crenarchaeota and Euryarchaeota are starting to yield significant increases in understanding of the functions of DNA polymerases in the respective phyla. Recent advances in biochemical approaches and in archaeal genetic models allowing knockout and epitope tagging have led to significant increases in our understanding, including DNA polymerase roles in Okazaki fragment maturation on the lagging strand, towards reconstitution of the replisome itself. Furthermore, poorly characterised DNA polymerase paralogues are finding roles in DNA repair and CRISPR immunity. This review attempts to provide a current update on the roles of archaeal DNA polymerases in both DNA replication and repair, addressing significant questions that remain for this field.

The Polyphyletic Origins of Primase-Helicase Bifunctional Proteins

We studied the evolutionary relationships of different primase-helicase bifunctional proteins, found mostly in viruses, virophages, plasmids, and organellar genomes, by phylogeny and correlation analysis. Our study suggests independent origins of primase-helicase bifunctional proteins resulting from multiple fusion events between genes encoding primase and helicase domains of different families. The correlation analysis further indicated strong functional dependencies of domains in the bifunctional proteins that are part of smaller genomes and plasmids. Bifunctional proteins found in some bacterial genomes exhibited weak coevolution probably suggesting that these are the non-functional remnants of the proteins acquired via horizontal transfer. We have put forward possible scenarios for the origin of primase-helicase bifunctional proteins in large eukaryotic DNA viruses and virophages.

Coordination and Substitution of DNA Polymerases in Response to Genomic Obstacles

The ability for DNA polymerases (Pols) to overcome a variety of obstacles in its path to maintain genomic stability during replication is a complex endeavor. It requires the coordination of multiple Pols with differing specificities through molecular control and access to the replisome. Although a number of contacts directly between Pols and accessory proteins have been identified, forming the basis of a variety of holoenzyme complexes, the dynamics of Pol active site substitutions remain uncharacterized. Substitutions can occur externally by recruiting new Pols to replisome complexes through an "exchange" of enzyme binding or internally through a "switch" in the engagement of DNA from preformed associated enzymes contained within supraholoenzyme complexes. Models for how high fidelity (HiFi) replication Pols can be substituted by translesion synthesis (TLS) Pols at sites of damage during active replication will be discussed. These substitution mechanisms may be as diverse as the number of Pol families and types of damage; however, common themes can be recognized across species. Overall, Pol substitutions will be controlled by explicit protein contacts, complex multiequilibrium processes, and specific kinetic activities. Insight into how these dynamic processes take place and are regulated will be of utmost importance for our greater understanding of the specifics of TLS as well as providing for future novel chemotherapeutic and antimicrobial strategies.

Elaborated Action of the Human Primosome

The human primosome is a 340-kilodalton complex of primase (DNA-dependent RNA polymerase) and DNA polymerase α, which initiates genome replication by synthesizing chimeric RNA-DNA primers for DNA polymerases δ and ϵ. Accumulated biochemical and structural data reveal the complex mechanism of concerted primer synthesis by two catalytic centers. First, primase generates an RNA primer through three steps: initiation, consisting of dinucleotide synthesis from two nucleotide triphosphates; elongation, resulting in dinucleotide extension; and termination, owing to primase inhibition by a mature 9-mer primer. Then Polα, which works equally well on DNA:RNA and DNA:DNA double helices, intramolecularly catches the template primed by a 9mer RNA and extends the primer with dNTPs. All primosome transactions are highly coordinated by autoregulation through the alternating activation/inhibition of the catalytic centers. This coordination is mediated by the small C-terminal domain of the primase accessory subunit, which forms a tight complex with the template:primer, shuttles between the primase and DNA polymerase active sites, and determines their access to the substrate.

PrimPol-Prime Time to Reprime

The complex molecular machines responsible for genome replication encounter many obstacles during their progression along DNA. Tolerance of these obstructions is critical for efficient and timely genome duplication. In recent years, primase-polymerase (PrimPol) has emerged as a new player involved in maintaining eukaryotic replication fork progression. This versatile replicative enzyme, a member of the archaeo-eukaryotic primase (AEP) superfamily, has the capacity to perform a range of template-dependent and independent synthesis activities. Here, we discuss the emerging roles of PrimPol as a leading strand repriming enzyme and describe the mechanisms responsible for recruiting and regulating the enzyme during this process. This review provides an overview and update of the current PrimPol literature, as well as highlighting unanswered questions and potential future avenues of investigation.

PolDIP2 interacts with human PrimPol and enhances its DNA polymerase activities

Translesion synthesis (TLS) employs specialized DNA polymerases to bypass replication fork stalling lesions. PrimPol was recently identified as a TLS primase and polymerase involved in DNA damage tolerance. Here, we identify a novel PrimPol binding partner, PolDIP2, and describe how it regulates PrimPol's enzymatic activities. PolDIP2 stimulates the polymerase activity of PrimPol, enhancing both its capacity to bind DNA and the processivity of the catalytic domain. In addition, PolDIP2 stimulates both the efficiency and error-free bypass of 8-oxo-7,8-dihydrodeoxyguanosine (8-oxoG) lesions by PrimPol. We show that PolDIP2 binds to PrimPol's catalytic domain and identify potential binding sites. Finally, we demonstrate that depletion of PolDIP2 in human cells causes a decrease in replication fork rates, similar to that observed in PrimPol^−/− cells. However, depletion of PolDIP2 in PrimPol^−/− cells does not produce a further decrease in replication fork rates. Together, these findings establish that PolDIP2 can regulate the TLS polymerase and primer extension activities of PrimPol, further enhancing our understanding of the roles of PolDIP2 and PrimPol in eukaryotic DNA damage tolerance.

Mechanism of Concerted RNA-DNA Primer Synthesis by the Human Primosome

The human primosome, a 340-kilodalton complex of primase and DNA polymerase α (Polα), synthesizes chimeric RNA-DNA primers to be extended by replicative DNA polymerases δ and ϵ. The intricate mechanism of concerted primer synthesis by two catalytic centers was an enigma for over three decades. Here we report the crystal structures of two key complexes, the human primosome and the C-terminal domain of the primase large subunit (p58C) with bound DNA/RNA duplex. These structures, along with analysis of primase/polymerase activities, provide a plausible mechanism for all transactions of the primosome including initiation, elongation, accurate counting of RNA primer length, primer transfer to Polα, and concerted autoregulation of alternate activation/inhibition of the catalytic centers. Our findings reveal a central role of p58C in the coordinated actions of two catalytic domains in the primosome and ultimately could impact the design of anticancer drugs.

Insight into the Human DNA Primase Interaction with Template-Primer

DNA replication in almost all organisms depends on the activity of DNA primase, a DNA-dependent RNA polymerase that synthesizes short RNA primers of defined size for DNA polymerases. Eukaryotic and archaeal primases are heterodimers consisting of small catalytic and large accessory subunits, both of which are necessary for the activity. The mode of interaction of primase subunits with substrates during the various steps of primer synthesis that results in the counting of primer length is not clear. Here we show that the C-terminal domain of the large subunit (p58C) plays a major role in template-primer binding and also defines the elements of the DNA template and the RNA primer that interact with p58C. The specific mode of interaction with a template-primer involving the terminal 5'-triphosphate of RNA and the 3'-overhang of DNA results in a stable complex between p58C and the DNA/RNA duplex. Our results explain how p58C participates in RNA synthesis and primer length counting and also indicate that the binding site for initiating NTP is located on p58C. These findings provide notable insight into the mechanism of primase function and are applicable for DNA primases from other species.

Lesion-Induced Mutation in the Hyperthermophilic Archaeon Sulfolobus acidocaldarius and Its Avoidance by the Y-Family DNA Polymerase Dbh

Hyperthermophilic archaea offer certain advantages as models of genome replication, and Sulfolobus Y-family polymerases Dpo4 (S. solfataricus) and Dbh (S. acidocaldarius) have been studied intensively in vitro as biochemical and structural models of trans-lesion DNA synthesis (TLS). However, the genetic functions of these enzymes have not been determined in the native context of living cells. We developed the first quantitative genetic assays of replication past defined DNA lesions and error-prone motifs in Sulfolobus chromosomes and used them to measure the efficiency and accuracy of bypass in normal and dbh(-) strains of Sulfolobus acidocaldarius. Oligonucleotide-mediated transformation allowed low levels of abasic-site bypass to be observed in S. acidocaldarius and demonstrated that the local sequence context affected bypass specificity; in addition, most erroneous TLS did not require Dbh function. Applying the technique to another common lesion, 7,8-dihydro-8-oxo-deoxyguanosine (8-oxo-dG), revealed an antimutagenic role of Dbh. The efficiency and accuracy of replication past 8-oxo-dG was higher in the presence of Dbh, and up to 90% of the Dbh-dependent events inserted dC. A third set of assays, based on phenotypic reversion, showed no effect of Dbh function on spontaneous -1 frameshifts in mononucleotide tracts in vivo, despite the extremely frequent slippage at these motifs documented in vitro. Taken together, the results indicate that a primary genetic role of Dbh is to avoid mutations at 8-oxo-dG that occur when other Sulfolobus enzymes replicate past this lesion. The genetic evidence that Dbh is recruited to 8-oxo-dG raises questions regarding the mechanism of recruitment, since Sulfolobus spp. have eukaryotic-like replisomes but no ubiquitin.

Primase-polymerases are a functionally diverse superfamily of replication and repair enzymes

Until relatively recently, DNA primases were viewed simply as a class of proteins that synthesize short RNA primers requisite for the initiation of DNA replication. However, recent studies have shown that this perception of the limited activities associated with these diverse enzymes can no longer be justified. Numerous examples can now be cited demonstrating how the term ‘DNA primase’ only describes a very narrow subset of these nucleotidyltransferases, with the vast majority fulfilling multifunctional roles from DNA replication to damage tolerance and repair. This article focuses on the archaeo-eukaryotic primase (AEP) superfamily, drawing on recently characterized examples from all domains of life to highlight the functionally diverse pathways in which these enzymes are employed. The broad origins, functionalities and enzymatic capabilities of AEPs emphasizes their previous functional misannotation and supports the necessity for a reclassification of these enzymes under a category called primase-polymerases within the wider functional grouping of polymerases. Importantly, the repositioning of AEPs in this way better recognizes their broader roles in DNA metabolism and encourages the discovery of additional functions for these enzymes, aside from those highlighted here.

Understanding DNA Repair in Hyperthermophilic Archaea: Persistent Gaps and Other Reasons to Focus on the Fork

Although hyperthermophilic archaea arguably have a great need for efficient DNA repair, they lack members of several DNA repair protein families broadly conserved among bacteria and eukaryotes. Conversely, the putative DNA repair genes that do occur in these archaea often do not generate the expected phenotype when deleted. The prospect that hyperthermophilic archaea have some unique strategies for coping with DNA damage and replication errors has intellectual and technological appeal, but resolving this question will require alternative coping mechanisms to be proposed and tested experimentally. This review evaluates a combination of four enigmatic properties that distinguishes the hyperthermophilic archaea from all other organisms: DNA polymerase stalling at dU, apparent lack of conventional NER, lack of MutSL homologs, and apparent essentiality of homologous recombination proteins. Hypothetical damage-coping strategies that could explain this set of properties may provide new starting points for efforts to define how archaea differ from conventional models of DNA repair and replication fidelity.