Three proliferating cell nuclear antigen-like proteins found in the hyperthermophilic archaeon Aeropyrum pernix: interactions with the two DNA polymerases

X-ray crystal structure of proliferating cell nuclear antigen 1 from Aeropyrum pernix

Proliferating cell nuclear antigen (PCNA) plays a critical role in DNA replication by enhancing the activity of various proteins involved in replication. In this study, the crystal structure of ApePCNA1, one of three PCNAs from the thermophilic archaeon Aeropyrum pernix, was elucidated. ApePCNA1 was cloned and expressed in Escherichia coli and the protein was purified and crystallized. The resulting crystal structure determined at 2.00 Å resolution revealed that ApePCNA1 does not form a trimeric ring, unlike PCNAs from other domains of life. It has unique structural features, including a long interdomain-connecting loop and a PIP-box-like sequence at the N-terminus, indicating potential interactions with other proteins. These findings provide insights into the functional mechanisms of PCNAs in archaea and their evolutionary conservation across different domains of life. A modified medium and protocol were used to express recombinant protein containing the lac operon. The expression of the target protein increased and the total incubation time decreased when using this system compared with those of previous expression protocols.

Coevolutionary and Phylogenetic Analysis of Mimiviral Replication Machinery Suggest the Cellular Origin of Mimiviruses

Mimivirus is one of the most complex and largest viruses known. The origin and evolution of Mimivirus and other giant viruses have been a subject of intense study in the last two decades. The two prevailing hypotheses on the origin of Mimivirus and other viruses are the reduction hypothesis, which posits that viruses emerged from modern unicellular organisms; whereas the virus-first hypothesis proposes viruses as relics of precellular forms of life. In this study, to gain insights into the origin of Mimivirus, we have carried out extensive phylogenetic, correlation, and multidimensional scaling analyses of the putative proteins involved in the replication of its 1.2-Mb large genome. Correlation analysis and multidimensional scaling methods were validated using bacteriophage, bacteria, archaea, and eukaryotic replication proteins before applying to Mimivirus. We show that a large fraction of mimiviral replication proteins, including polymerase B, clamp, and clamp loaders are of eukaryotic origin and are coevolving. Although phylogenetic analysis places some components along the lineages of phage and bacteria, we show that all the replication-related genes have been homogenized and are under purifying selection. Collectively our analysis supports the idea that Mimivirus originated from a complex cellular ancestor. We hypothesize that Mimivirus has largely retained complex replication machinery reminiscent of its progenitor while losing most of the other genes related to processes such as metabolism and translation.

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 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.

Evolution of the archaeal and mammalian information processing systems: towards an archaeal model for human disease

Current evolutionary models suggest that Eukaryotes originated from within Archaea instead of being a sister lineage. To test this model of ancient evolution, we review recent studies and compare the three major information processing subsystems of replication, transcription and translation in the Archaea and Eukaryotes. Our hypothesis is that if the Eukaryotes arose within the archaeal radiation, their information processing systems will appear to be one of kind and not wholly original. Within the Eukaryotes, the mammalian or human systems are emphasized because of their importance in understanding health. Biochemical as well as genetic studies provide strong evidence for the functional similarity of archaeal homologs to the mammalian information processing system and their dissimilarity to the bacterial systems. In many independent instances, a simple archaeal system is functionally equivalent to more elaborate eukaryotic homologs, suggesting that evolution of complexity is likely an central feature of the eukaryotic information processing system. Because fewer components are often involved, biochemical characterizations of the archaeal systems are often easier to interpret. Similarly, the archaeal cell provides a genetically and metabolically simpler background, enabling convenient studies on the complex information processing system. Therefore, Archaea could serve as a parsimonious and tractable host for studying human diseases that arise in the information processing systems.

Diversity of the DNA replication system in the Archaea domain

The precise and timely duplication of the genome is essential for cellular life. It is achieved by DNA replication, a complex process that is conserved among the three domains of life. Even though the cellular structure of archaea closely resembles that of bacteria, the information processing machinery of archaea is evolutionarily more closely related to the eukaryotic system, especially for the proteins involved in the DNA replication process. While the general DNA replication mechanism is conserved among the different domains of life, modifications in functionality and in some of the specialized replication proteins are observed. Indeed, Archaea possess specific features unique to this domain. Moreover, even though the general pattern of the replicative system is the same in all archaea, a great deal of variation exists between specific groups.

Comparative proteomic analysis of aluminum tolerance in tibetan wild and cultivated barleys

Aluminum (Al) toxicity is a major limiting factor for plant production in acid soils. Wild barley germplasm is rich in genetic diversity and may provide elite genes for crop Al tolerance improvement. The hydroponic-experiments were performed to compare proteomic and transcriptional characteristics of two contrasting Tibetan wild barley genotypes Al- resistant/tolerant XZ16 and Al-sensitive XZ61 as well as Al-resistant cv. Dayton. Results showed that XZ16 had less Al uptake and translocation than XZ61 and Dayton under Al stress. Thirty-five Al-tolerance/resistance-associated proteins were identified and categorized mainly in metabolism, energy, cell growth/division, protein biosynthesis, protein destination/storage, transporter, signal transduction, disease/defense, etc. Among them, 30 were mapped on barley genome, with 16 proteins being exclusively up-regulated by Al stress in XZ16, including 4 proteins (S-adenosylmethionine-synthase 3, ATP synthase beta subunit, triosephosphate isomerase, Bp2A) specifically expressed in XZ16 but not Dayton. The findings highlighted the significance of specific-proteins associated with Al tolerance, and verified Tibetan wild barley as a novel genetic resource for Al tolerance.

Replication clamps and clamp loaders

To achieve the high degree of processivity required for DNA replication, DNA polymerases associate with ring-shaped sliding clamps that encircle the template DNA and slide freely along it. The closed circular structure of sliding clamps necessitates an enzyme-catalyzed mechanism, which not only opens them for assembly and closes them around DNA, but specifically targets them to sites where DNA synthesis is initiated and orients them correctly for replication. Such a feat is performed by multisubunit complexes known as clamp loaders, which use ATP to open sliding clamp rings and place them around the 3' end of primer-template (PT) junctions. Here we discuss the structure and composition of sliding clamps and clamp loaders from the three domains of life as well as T4 bacteriophage, and provide our current understanding of the clamp-loading process.

In vitro reconstitution of RNA primer removal in Archaea reveals the existence of two pathways

Using model DNA substrates and purified recombinant proteins from Pyrococcus abyssi, I have reconstituted the enzymatic reactions involved in RNA primer elimination in vitro. In my dual-labelled system, polymerase D performed efficient strand displacement DNA synthesis, generating 5'-RNA flaps which were subsequently released by Fen1, before ligation by Lig1. In this pathway, the initial cleavage event by RNase HII facilitated RNA primer removal of Okazaki fragments. In addition, I have shown that polymerase B was able to displace downstream DNA strands with a single ribonucleotide at the 5'-end, a product resulting from a single cut in the RNA initiator by RNase HII. After RNA elimination, the combined activities of strand displacement DNA synthesis by polymerase B and flap cleavage by Fen1 provided a nicked substrate for ligation by Lig1. The unique specificities of Okazaki fragment maturation enzymes and replicative DNA polymerases strongly support the existence of two pathways in the resolution of RNA fragments.

Comparative analyses of the two proliferating cell nuclear antigens from the hyperthermophilic archaeon, Thermococcus kodakarensis

The DNA sliding clamp is a multifunctional protein involved in cellular DNA transactions. In Archaea and Eukaryota, proliferating cell nuclear antigen (PCNA) is the sliding clamp. The ring-shaped PCNA encircles double-stranded DNA within its central hole and tethers other proteins on DNA. The majority of Crenarchaeota, a subdomain of Archaea, have multiple PCNA homologues, and they are capable of forming heterotrimeric rings for their functions. In contrast, most organisms in Euryarchaeota, the other major subdomain, have a single PCNA forming a homotrimeric ring structure. Among the Euryarchaeota whose genome is sequenced, Thermococcus kodakarensis is the only species with two genes encoding PCNA homologues on its genome. We cloned the two genes from the T. kodakarensis genome, and the gene products, PCNA1 and PCNA2, were characterized. PCNA1 stimulated the DNA synthesis reactions of the two DNA polymerases, PolB and PolD, from T. kodakarensis in vitro. PCNA2, however, only had an effect on PolB. We were able to disrupt the gene for PCNA2, whereas gene disruption for PCNA1 was not possible, suggesting that PCNA1 is essential for DNA replication. The sensitivities of the Δpcna2 mutant strain to ultraviolet irradiation (UV), methyl methanesulfonate (MMS) and mitomycin C (MMC) were indistinguishable from those of the wild-type strain.

Rapid progress of DNA replication studies in Archaea, the third domain of life

Archaea, the third domain of life, are interesting organisms to study from the aspects of molecular and evolutionary biology. Archaeal cells have a unicellular ultrastructure without a nucleus, resembling bacterial cells, but the proteins involved in genetic information processing pathways, including DNA replication, transcription, and translation, share strong similarities with those of Eukaryota. Therefore, archaea provide useful model systems to understand the more complex mechanisms of genetic information processing in eukaryotic cells. Moreover, the hyperthermophilic archaea provide very stable proteins, which are especially useful for the isolation of replisomal multicomplexes, to analyze their structures and functions. This review focuses on the history, current status, and future directions of archaeal DNA replication studies.

The archaeal PCNA proteins

PCNA (proliferating-cell nuclear antigen) is a ring-shaped protein that encircles duplex DNA and plays an essential role in many DNA metabolic processes. The PCNA protein interacts with a large number of cellular factors and modulates their enzymatic activities. In the present paper, we summarize the structures, functions and interactions of the archaeal PCNA proteins.

The information transfer system of halophilic archaea

Information transfer is fundamental to all life forms. In the third domain of life, the archaea, many of the genes functioning in these processes are similar to their eukaryotic counterparts, including DNA replication and repair, basal transcription, and translation genes, while many transcriptional regulators and the overall genome structure are more bacterial-like. Among halophilic (salt-loving) archaea, the genomes commonly include extrachromosomal elements, many of which are large megaplasmids or minichromosomes. With the sequencing of genomes representing ten different genera of halophilic archaea and the availability of genetic systems in two diverse models, Halobacterium sp. NRC-1 and Haloferax volcanii, a large number of genes have now been annotated, classified, and studied. Here, we review the comparative genomic, genetic, and biochemical work primarily aimed at the information transfer system of halophilic archaea, highlighting gene conservation and differences in the chromosomes and the large extrachromosomal elements among these organisms.

Revealing the essentiality of multiple archaeal pcna genes using a mutant propagation assay based on an improved knockout method

Organisms belonging to the Crenarchaeota lineage contain three proliferating cell nuclear antigen (PCNA) subunits, while those in the Euryarchaeota have only one, as for Eukarya. To study the mechanism of archaeal sliding clamps, we sought to generate knockouts for each pcna gene in Sulfolobus islandicus, a hyperthermophilic crenarchaeon, but failed with two conventional knockout methods. Then, a new knockout scheme, known as marker insertion and target gene deletion (MID), was developed, with which transformants were obtained for each pMID-pcna plasmid. We found that mutant cells persisted in transformant cultures during incubation of pMID-pcna3 and pMID-araS-pcna1 transformants under counter selection. Studying the propagation of mutant cells by semiquantitative PCR analysis of the deleted target gene allele (Δpcna1 or Δpcna3) revealed that mutant cells could no longer be propagated, demonstrating that these pcna genes are absolutely required for host cell viability. Because the only prerequisite for this assay is the generation of a MID transformant, this approach can be applied generally to any micro-organisms proficient in homologous recombination.

Binding to PCNA in Euryarchaeal DNA Replication requires two PIP motifs for DNA polymerase D and one PIP motif for DNA polymerase B

Replicative DNA polymerases possess a canonical C-terminal proliferating cell nuclear antigen (PCNA)-binding motif termed the PCNA-interacting protein (PIP) box. We investigated the role of the PIP box on the functional interactions of the two DNA polymerases, PabPol B (family B) and PabPol D (family D), from the hyperthermophilic euryarchaeon Pyrococcus abyssi, with its cognate PCNA. The PIP box was essential for interactions of PabPol B with PCNA, as shown by surface plasmon resonance and primer extension studies. In contrast, binding of PabPol D to PCNA was affected only partially by removing the PIP motif. We identified a second palindromic PIP box motif at the N-terminus of the large subunit of PabPol D that was required for the interactions of PabPol D with PCNA. Thus, two PIP motifs were needed for PabPol D for binding to PabPCNA. Moreover, the C-terminus of PabPCNA was essential for stimulation of PabPol D activity but not for stimulation of PabPol B activity. Neither DNA polymerase interacted with the PabPCNA interdomain connecting loop. Our data suggest that distinct processes are involved in PabPol D and PabPol B binding to PCNA, raising the possibility that Archaea require two mechanisms for recruiting replicative DNA polymerases at the replication fork.

Plasmodium falciparum origin recognition complex subunit 5: functional characterization and role in DNA replication foci formation

The mechanism of DNA replication initiation and progression is poorly understood in the parasites, including human malaria parasite Plasmodium falciparum. Using bioinformatics tools and yeast complementation assay, we identified a putative homologue of Saccharomyces cerevisiaeorigin recognition complex subunit 5 in P. falciparum (PfORC5). PfORC5 forms distinct nuclear foci colocalized with the replication foci marker proliferating cell nuclear antigen (PfPCNA) and co-immunoprecipitates with PCNA during early-to-mid trophozoite stage replicating parasites. Interestingly, these proteins separate from each other at the non-replicating late schizont stage, citing the evidence of the presence of both PCNA and ORC components in replication foci during eukaryotic DNA replication. PfORC1, another ORC subunit, colocalizes with PfPCNA and PfORC5 at the beginning of DNA replication, but gets degraded at the late schizont stage, ensuring the regulation of DNA replication in the parasites. Further, we have identified putative PCNA-interacting protein box in PfORC1 that may explain in part the colocalization of PfORC and PfPCNA. Additionally, use of specific DNA replication inhibitor hydroxyurea affects ORC5/PCNA foci formation and parasitic growth. These results strongly favour replication factory model in the parasites and confer great potential to understand the co-ordination between ORC and PCNA during eukaryotic DNA replication in general.

Essential and non-essential DNA replication genes in the model halophilic Archaeon, Halobacterium sp. NRC-1

Background

Information transfer systems in Archaea, including many components of the DNA replication machinery, are similar to those found in eukaryotes. Functional assignments of archaeal DNA replication genes have been primarily based upon sequence homology and biochemical studies of replisome components, but few genetic studies have been conducted thus far. We have developed a tractable genetic system for knockout analysis of genes in the model halophilic archaeon, Halobacterium sp. NRC-1, and used it to determine which DNA replication genes are essential.

Results

Using a directed in-frame gene knockout method in Halobacterium sp. NRC-1, we examined nineteen genes predicted to be involved in DNA replication. Preliminary bioinformatic analysis of the large haloarchaeal Orc/Cdc6 family, related to eukaryotic Orc1 and Cdc6, showed five distinct clades of Orc/Cdc6 proteins conserved in all sequenced haloarchaea. Of ten orc/cdc6 genes in Halobacterium sp. NRC-1, only two were found to be essential, orc10, on the large chromosome, and orc2, on the minichromosome, pNRC200. Of the three replicative-type DNA polymerase genes, two were essential: the chromosomally encoded B family, polB1, and the chromosomally encoded euryarchaeal-specific D family, polD1/D2 (formerly called polA1/polA2 in the Halobacterium sp. NRC-1 genome sequence). The pNRC200-encoded B family polymerase, polB2, was non-essential. Accessory genes for DNA replication initiation and elongation factors, including the putative replicative helicase, mcm, the eukaryotic-type DNA primase, pri1/pri2, the DNA polymerase sliding clamp, pcn, and the flap endonuclease, rad2, were all essential. Targeted genes were classified as non-essential if knockouts were obtained and essential based on statistical analysis and/or by demonstrating the inability to isolate chromosomal knockouts except in the presence of a complementing plasmid copy of the gene.

Conclusion

The results showed that ten out of nineteen eukaryotic-type DNA replication genes are essential for Halobacterium sp. NRC-1, consistent with their requirement for DNA replication. The essential genes code for two of ten Orc/Cdc6 proteins, two out of three DNA polymerases, the MCM helicase, two DNA primase subunits, the DNA polymerase sliding clamp, and the flap endonuclease.

DNA Polymerase Switching on Homotrimeric PCNA at the Replication Fork of the Euryarchaea Pyrococcus abyssi

DNA replication in Archaea, as in other organisms, involves large protein complexes called replisomes. In the Euryarchaeota subdomain, only two putative replicases have been identified, and their roles in leading and lagging strand DNA synthesis are still poorly understood. In this study, we focused on the coupling of proliferating cell nuclear antigen (PCNA)-loading mechanisms with DNA polymerase function in the Euryarchaea Pyrococcus abyssi. PCNA spontaneously loaded onto primed DNA, and replication factor C dramatically increased this loading. Surprisingly, the family B DNA polymerase (Pol B) also increased PCNA loading, probably by stabilizing the clamp on primed DNA via an essential motif. In contrast, on an RNA-primed DNA template, the PCNA/Pol B complex was destabilized in the presence of dNTPs, allowing the family D DNA polymerase (Pol D) to perform RNA-primed DNA synthesis. Then, Pol D is displaced by Pol B to perform processive DNA synthesis, at least on the leading strand.

Specific interactions of three proliferating cell nuclear antigens with replication-related proteins in Aeropyrum pernix

Proliferating cell nuclear antigen (PCNA) is a well-known multifunctional protein involved in eukaryotic and archaeal DNA transactions. The homotrimeric PCNA ring encircles double-stranded DNA within its central hole and tethers many proteins on DNA. Plural genes encoding PCNA-like proteins have been found in the genome sequence of crenarchaeal organisms. We describe here the biochemical properties of the three PCNAs, PCNA1, PCNA2 and PCNA3, from the hyperthermophilic archaeon, Aeropyrum pernix. PCNA2 can form a trimeric structure by itself, and it also forms heterotrimeric structures with PCNA1 and PCNA3. However, neither PCNA1 nor PCNA3 can form homotrimers. The DNA synthesis activity of DNA polymerase I and II, the endonuclease activity of FEN1, and the nick-sealing activity of DNA ligase were stimulated by the complex of PCNA2 and 3 or PCNA1, 2 and 3. These results suggest that the heterotrimeric PCNA at least including PCNA2 and 3 function as the clamp in the replisome. However, PCNA2 is the most abundant in the cells throughout the growth stages among the three PCNAs, and therefore, PCNA2 may perform multitasks by changing complex composition.

A single amino acid substitution in the DNA-binding domain of Aeropyrum pernix DNA ligase impairs its interaction with proliferating cell nuclear antigen

Proliferating cell nuclear antigen (PCNA) is known as a DNA sliding clamp that acts as a platform for the assembly of enzymes involved in DNA replication and repair. Previously, it was reported that a crenarchaeal PCNA formed a heterotrimeric structure, and that each PCNA subunit has distinct binding specificity to PCNA-binding proteins. Here we describe the PCNA-binding properties of a DNA ligase from the hyperthermophilic crenarchaeon Aeropyrum pernix K1. Based on our findings on the Pyrococcus furiosus DNA ligase-PCNA interaction, we predicted that the aromatic residue, Phe132, in the DNA-binding domain of A. pernix DNA ligase (ApeLig) would play a critical role in binding to A. pernix PCNA (ApePCNA). Surface plasmon resonance analyses revealed that the ApeLig F132A mutant does not interact with an immobilized subunit of ApePCNA. Furthermore, we could not detect any stimulation of the ligation activity of the ApeLig F132A protein by ApePCNA in vitro. These results indicated that the phenylalanine, which is located in our predicted PCNA-binding region in ApeLig, has a critical role for the physical and functional interaction with ApePCNA.

DNA replication in the archaea

The archaeal DNA replication machinery bears striking similarity to that of eukaryotes and is clearly distinct from the bacterial apparatus. In recent years, considerable advances have been made in understanding the biochemistry of the archaeal replication proteins. Furthermore, a number of structures have now been obtained for individual components and higher-order assemblies of archaeal replication factors, yielding important insights into the mechanisms of DNA replication in both archaea and eukaryotes.

Characterization of a second proliferating cell nuclear antigen (PCNA2) from Drosophila melanogaster

The eukaryotic DNA polymerase processivity factor, proliferating cell nuclear antigen, is an essential component in the DNA replication and repair machinery. In Drosophila melanogaster, we cloned a second PCNA cDNA that differs from that encoded by the gene mus209 (for convenience called DmPCNA1 in this article). The second PCNA cDNA (DmPCNA2) encoded a 255 amino acid protein with 51.7% identity to DmPCNA1, and was ubiquitously expressed during Drosophila development. DmPCNA2 was localized in nuclei as a homotrimeric complex and associated with Drosophila DNA polymerase delta and epsilonin vivo. Treatment of cells with methyl methanesulfonate or hydrogen peroxide increased the amount of both DmPCNA2 and DmPCNA1 associating with chromatin, whereas exposure to UV light increased the level of association of only DmPCNA1. Our observations suggest that DmPCNA2 may function as an independent sliding clamp of DmPCNA1 when DNA repair occurs.

Characterization of an archaeal family 4 uracil DNA glycosylase and its interaction with PCNA and chromatin proteins

We describe the characterization of a family 4 UDG1 (uracil DNA glycosylase) from the crenarchaeote Sulfolobus solfataricus. UDG1 is found to have a marked preference for substrates containing a G:U base pair over either A:U or single-stranded uracil-containing DNA substrates. UDG1 is found to interact with the sliding clamp PCNA (proliferating cell nuclear antigen), and does so by a conserved motif in the C-terminus of the protein. S. solfataricus has a heterotrimeric PCNA, and only one of the subunits, PCNA3, interacts with UDG1. We have been unable to detect any stimulation of UDG activity by PCNA, in contrast with the observed effects of PCNA on a number of DNA metabolic enzymes. However, analysis of the effects of Sulfolobus chromatin proteins on UDG1 leads us to propose a mechanistic basis for coupling UDG1 to the replication fork.

Biochemical and genetic analysis of the distinct proliferating cell nuclear antigens of Toxoplasma gondii

The apicomplexa parasite Toxoplasma gondii expresses two distinct proliferating cell nuclear antigens (PCNA) that exhibit distinct patterns of subcellular localization during tachyzoite growth. In all cell cycle phases, TgPCNA1 is concentrated in the nucleus, while TgPCNA2 is only concentrated in the nucleus during S-phase and uniformly distributed throughout the cell during mitosis and early G1-phase. TgPCNA1-GFP and native TgPCNA2 display a punctate staining pattern that is consistent with assembly into replication foci during S-phase; however, TgPCNA2 disassociates from replication foci before TgPCNA1-GFP. Consistent with the distinct pattern of TgPCNA2 cellular localization, homotypic TgPCNA2 interactions were primarily observed by yeast two-hybrid or co-immunoprecipitation analysis. Transgenic parasites in which the TgPCNA2 gene was disrupted displayed a slower growth rate in vitro; however, no difference in DNA polymerase activity, response to chemical mutagens, or recombinational frequency was observed in these mutant clones demonstrating that TgPCNA2 is non-essential in the tachyzoite developmental stage. Heterologous expression of TgPCNA1, but not TgPCNA2, was able to complement a POL30 cold-sensitive yeast strain suggesting that this isoform may serve as a major replisomal factor in T. gondii and is consistent with the failure to disrupt this gene in tachyzoites.

Structure of an XPF endonuclease with and without DNA suggests a model for substrate recognition

The XPF/Mus81 structure-specific endonucleases cleave double-stranded DNA (dsDNA) within asymmetric branched DNA substrates and play an essential role in nucleotide excision repair, recombination and genome integrity. We report the structure of an archaeal XPF homodimer alone and bound to dsDNA. Superposition of these structures reveals a large domain movement upon binding DNA, indicating how the (HhH)(2) domain and the nuclease domain are coupled to allow the recognition of double-stranded/single-stranded DNA junctions. We identify two nonequivalent DNA-binding sites and propose a model in which XPF distorts the 3' flap substrate in order to engage both binding sites and promote strand cleavage. The model rationalises published biochemical data and implies a novel role for the ERCC1 subunit of eukaryotic XPF complexes.

Aeropyrum camini sp. nov., a strictly aerobic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney

A novel hyperthermophilic archaeon, designated strain SY1T, was isolated from a deep-sea hydrothermal vent chimney sample collected from the Suiyo Seamount in the Izu-Bonin Arc, Japan, at a depth of 1385 m. The cells were irregular cocci (1·2 to 2·1 μm in diameter), occurring singly or in pairs, and stained Gram-negative. Growth was observed between 70 and 97 °C (optimum, 85 °C; 220 min doubling time), pH 6·5 and 8·8 (optimum, pH 8·0), and salinity of 2·2 and 5·3 % (optimum, 3·5 %). It was a strictly aerobic heterotroph capable of growing on complex proteinaceous substrates such as yeast extract and tryptone. The G+C content of the genomic DNA was 54·4 mol%. Phylogenetic analysis based on the 16S rDNA sequence of the isolate indicated that the isolate was closely related toAeropyrum pernixstrain K1T. However, no significant genetic relatedness was observed between them by DNA–DNA hybridization. On the basis of the molecular and physiological traits of the new isolate, the nameAeropyrum caminisp. nov. is proposed, with the type strain SY1T(=JCM 12091T=ATCC BAA-758T).

Archeal DNA replication: eukaryal proteins in a bacterial context

Genome sequences of a number of archaea have revealed an apparent paradox in the phylogenies of the bacteria, archaea, and eukarya, as well as an intriguing set of problems to be resolved in the study of DNA replication. The archaea, long thought to be bacteria, are not only different enough to merit their own domain but also appear to be an interesting mosaic of bacterial, eukaryal, and unique features. Most archaeal proteins participating in DNA replication are more similar in sequence to those found in eukarya than to analogous replication proteins in bacteria. However, archaea have only a subset of the eukaryal replication machinery, apparently needing fewer polypeptides and structurally simpler complexes. The archaeal replication apparatus also contains features not found in other organisms owing, in part, to the broad range of environmental conditions, some extreme, in which members of this domain thrive. In this review the current knowledge of the mechanisms governing DNA replication in archaea is summarized and the similarities and differences of those of bacteria and eukarya are highlighted.

Enzymology of base excision repair in the hyperthermophilic archaeon Pyrobaculum aerophilum

DNA of all living organisms is constantly modified by exogenous and endogenous reagents. The mutagenic threat of modifications such as methylation, oxidation, and hydrolytic deamination of DNA bases is counteracted by base excision repair (BER). This process is initiated by the action of one of several DNA glycosylases, which removes the aberrant base and thus initiates a cascade of events that involves scission of the DNA backbone, removal of the baseless sugar-phosphate residue, filling in of the resulting single nucleotide gap, and ligation of the remaining nick. We were interested to find out how the BER process functions in hyperthermophiles, organisms growing at temperatures around 100 degrees C, where the rates of these spontaneous reactions are greatly accelerated. In our previous studies, we could show that the crenarchaeon Pyrobaculum aerophilum has at least three uracil-DNA glycosylases, Pa-UDGa, Pa-UDGb, and Pa-MIG, that can initiate the BER process by catalyzing the removal of uracil residues arising through the spontaneous deamination of cytosines. We now report that the genome of P. aerophilum encodes also the remaining functions necessary for BER and show that a system consisting of four P. aerophilum encoded enzymes, Pa-UDGb, AP endonuclease IV, DNA polymerase B2, and DNA ligase, can efficiently repair a G.U mispair in an oligonucleotide substrate to a G.C pair. Interestingly, the efficiency of the in vitro repair reaction was stimulated by Pa-PCNA1, the processivity clamp of DNA polymerases.

A heterotrimeric PCNA in the hyperthermophilic archaeon Sulfolobus solfataricus

The sliding clamp, PCNA, of the archaeon Sulfolobus solfataricus P2 is a heterotrimer of three distinct subunits (PCNA1, 2, and 3) that assembles in a defined manner. The PCNA heterotrimer, but not individual subunits, stimulates the activities of the DNA polymerase, DNA ligase I, and the flap endonuclease (FEN1) of S. solfataricus. Distinct PCNA subunits contact DNA polymerase, DNA ligase, or FEN1, imposing a defined architecture at the lagging strand fork and suggesting the existence of a preformed scanning complex at the fork. This provides a mechanism to tightly couple DNA synthesis and Okazaki fragment maturation. Additionally, unique subunit-specific interactions between components of the clamp loader, RFC, suggest a model for clamp loading of PCNA.

Molecular characterization and expression of an alternate proliferating cell nuclear antigen homologue, PfPCNA2, in Plasmodium falciparum

The malaria parasite Plasmodium falciparum genome sequencing has revealed the existence of a second gene for proliferating cell nuclear antigen (PCNA), a key factor in a variety of DNA metabolic events. The alternate copy of PCNA (PfPCNA2) shows only 23% identity to an earlier reported P. falciparum PCNA homologue (PfPCNA1). Our analysis indicated structural conservation of PfPCNA2 compared to eukaryotic PCNAs. PfPCNA1 and 2 polypeptides showed differential expression in the intraerythrocytic cell cycle of the malaria parasite. PfPCNA1 expression slowly increases about threefold from the ring to the late schizont stage. In contrast PfPCNA2 showed robust expression in trophozoites and early schizonts with a sudden drop in expression in the late schizont stage, suggesting that the two PfPCNAs may function under different physiological conditions. Chemical cross-linking indicated the presence of a trimeric PfPCNA2 protein, indicating the possible existence of a functional ring-like PfPCNA2 structure.