Solution structure of the N-terminal domain of the archaeal D-family DNA polymerase small subunit reveals evolutionary relationship to eukaryotic B-family polymerases

Family D DNA polymerase interacts with GINS to promote CMG-helicase in the archaeal replisome

Genomic DNA replication requires replisome assembly. We show here the molecular mechanism by which CMG (GAN-MCM-GINS)-like helicase cooperates with the family D DNA polymerase (PolD) in Thermococcus kodakarensis. The archaeal GINS contains two Gins51 subunits, the C-terminal domain of which (Gins51C) interacts with GAN. We discovered that Gins51C also interacts with the N-terminal domain of PolD's DP1 subunit (DP1N) to connect two PolDs in GINS. The two replicases in the replisome should be responsible for leading- and lagging-strand synthesis, respectively. Crystal structure analysis of the DP1N-Gins51C-GAN ternary complex was provided to understand the structural basis of the connection between the helicase and DNA polymerase. Site-directed mutagenesis analysis supported the interaction mode obtained from the crystal structure. Furthermore, the assembly of helicase and replicase identified in this study is also conserved in Eukarya. PolD enhances the parental strand unwinding via stimulation of ATPase activity of the CMG-complex. This is the first evidence of the functional connection between replicase and helicase in Archaea. These results suggest that the direct interaction of PolD with CMG-helicase is critical for synchronizing strand unwinding and nascent strand synthesis and possibly provide a functional machinery for the effective progression of the replication fork.

An updated structural classification of replicative DNA polymerases

Replicative DNA polymerases are nano-machines essential to life, which have evolved the ability to copy the genome with high fidelity and high processivity. In contrast with cellular transcriptases and ribosome machines, which evolved by accretion of complexity from a conserved catalytic core, no replicative DNA polymerase is universally conserved. Strikingly, four different families of DNA polymerases have evolved to perform DNA replication in the three domains of life. In Bacteria, the genome is replicated by DNA polymerases belonging to the A- and C-families. In Eukarya, genomic DNA is copied mainly by three distinct replicative DNA polymerases, Polα, Polδ, and Polε, which all belong to the B-family. Matters are more complicated in Archaea, which contain an unusual D-family DNA polymerase (PolD) in addition to PolB, a B-family replicative DNA polymerase that is homologous to the eukaryotic ones. PolD is a heterodimeric DNA polymerase present in all Archaea discovered so far, except Crenarchaea. While PolD is an essential replicative DNA polymerase, it is often underrepresented in the literature when the diversity of DNA polymerases is discussed. Recent structural studies have shown that the structures of both polymerase and proofreading active sites of PolD differ from other structurally characterized DNA polymerases, thereby extending the repertoire of folds known to perform DNA replication. This review aims to provide an updated structural classification of all replicative DNAPs and discuss their evolutionary relationships, both regarding the DNA polymerase and proofreading active sites.

Elucidating functions of DP1 and DP2 subunits from the Thermococcus kodakarensis family D DNA polymerase

DNA polymerase D (PolD), originally discovered in Pyrococcus furiosus, has no sequence homology with any other DNA polymerase family. Genes encoding PolD are found in most of archaea, except for those archaea in the Crenarchaeota phylum. PolD is composed of two proteins: DP1 and DP2. To date, the 3D structure of the PolD heteromeric complex is yet to be determined. In this study, we established a method that prepared highly purified PolD from Thermococcus kodakarensis, and purified DP1 and DP2 proteins formed a stable complex in solution. An intrinsically disordered region was identified in the N-terminal region of DP1, but the static light scattering analysis provided a reasonable molecular weight of DP1. In addition, PolD forms as a complex of DP1 and DP2 in a 1:1 ratio. Electron microscope single particle analysis supported this composition of PolD. Both proteins play an important role in DNA synthesis activity and in 3'-5' degradation activity. DP1 has extremely low affinity for DNA, while DP2 is mainly responsible for DNA binding. Our work will provide insight and the means to further understand PolD structure and the molecular mechanism of this archaea-specific DNA polymerase.

Foreword to 'Multiscale structural biology: biophysical principles and mechanisms underlying the action of bio-nanomachines', a special issue in Honour of Fumio Arisaka's 70th birthday

This issue of Biophysical Reviews, titled 'Multiscale structural biology: biophysical principles and mechanisms underlying the action of bio-nanomachines', is a collection of articles dedicated in honour of Professor Fumio Arisaka's 70th birthday. Initially, working in the fields of haemocyanin and actin filament assembly, Fumio went on to publish important work on the elucidation of structural and functional aspects of T4 phage biology. As his career has transitioned levels of complexity from proteins (hemocyanin) to large protein complexes (actin) to even more massive bio-nanomachinery (phage), it is fitting that the subject of this special issue is similarly reflective of his multiscale approach to structural biology. This festschrift contains articles spanning biophysical structure and function from the bio-molecular through to the bio-nanomachine level.

Shared active site architecture between archaeal PolD and multi-subunit RNA polymerases revealed by X-ray crystallography

Archaeal replicative DNA polymerase D (PolD) constitute an atypical class of DNA polymerases made of a proofreading exonuclease subunit (DP1) and a larger polymerase catalytic subunit (DP2), both with unknown structures. We have determined the crystal structures of Pyrococcus abyssi DP1 and DP2 at 2.5 and 2.2 Å resolution, respectively, revealing a catalytic core strikingly different from all other known DNA polymerases (DNAPs). Rather, the PolD DP2 catalytic core has the same ‘double-psi β-barrel' architecture seen in the RNA polymerase (RNAP) superfamily, which includes multi-subunit transcriptases of all domains of life, homodimeric RNA-silencing pathway RNAPs and atypical viral RNAPs. This finding bridges together, in non-viral world, DNA transcription and DNA replication within the same protein superfamily. This study documents further the complex evolutionary history of the DNA replication apparatus in different domains of life and proposes a classification of all extant DNAPs.

The structures of many DNA polymerases is known, but PolD was a missing piece. Here, the authors report the crystal structure of this protein and use it to connect the DNA replication machinery with the transcription machinery in the same protein family.

Archaeal DNA replication

DNA replication is essential for all life forms. Although the process is fundamentally conserved in the three domains of life, bioinformatic, biochemical, structural, and genetic studies have demonstrated that the process and the proteins involved in archaeal DNA replication are more similar to those in eukaryal DNA replication than in bacterial DNA replication, but have some archaeal-specific features. The archaeal replication system, however, is not monolithic, and there are some differences in the replication process between different species. In this review, the current knowledge of the mechanisms governing DNA replication in Archaea is summarized. The general features of the replication process as well as some of the differences are discussed.

Evolution of replicative DNA polymerases in archaea and their contributions to the eukaryotic replication machinery

The elaborate eukaryotic DNA replication machinery evolved from the archaeal ancestors that themselves show considerable complexity. Here we discuss the comparative genomic and phylogenetic analysis of the core replication enzymes, the DNA polymerases, in archaea and their relationships with the eukaryotic polymerases. In archaea, there are three groups of family B DNA polymerases, historically known as PolB1, PolB2 and PolB3. All three groups appear to descend from the last common ancestors of the extant archaea but their subsequent evolutionary trajectories seem to have been widely different. Although PolB3 is present in all archaea, with the exception of Thaumarchaeota, and appears to be directly involved in lagging strand replication, the evolution of this gene does not follow the archaeal phylogeny, conceivably due to multiple horizontal transfers and/or dramatic differences in evolutionary rates. In contrast, PolB1 is missing in Euryarchaeota but otherwise seems to have evolved vertically. The third archaeal group of family B polymerases, PolB2, includes primarily proteins in which the catalytic centers of the polymerase and exonuclease domains are disrupted and accordingly the enzymes appear to be inactivated. The members of the PolB2 group are scattered across archaea and might be involved in repair or regulation of replication along with inactivated members of the RadA family ATPases and an additional, uncharacterized protein that are encoded within the same predicted operon. In addition to the family B polymerases, all archaea, with the exception of the Crenarchaeota, encode enzymes of a distinct family D the origin of which is unclear. We examine multiple considerations that appear compatible with the possibility that family D polymerases are highly derived homologs of family B. The eukaryotic DNA polymerases show a highly complex relationship with their archaeal ancestors including contributions of proteins and domains from both the family B and the family D archaeal polymerases.

PCR performance of a thermostable heterodimeric archaeal DNA polymerase

DNA polymerases are versatile tools used in numerous important molecular biological core technologies like the ubiquitous polymerase chain reaction (PCR), cDNA cloning, genome sequencing, and nucleic acid based diagnostics. Taking into account the multiple DNA amplification techniques in use, different DNA polymerases must be optimized for each type of application. One of the current tendencies is to reengineer or to discover new DNA polymerases with increased performance and broadened substrate spectra. At present, there is a great demand for such enzymes in applications, e.g., forensics or paleogenomics. Current major limitations hinge on the inability of conventional PCR enzymes, such as Taq, to amplify degraded or low amounts of template DNA. Besides, a wide range of PCR inhibitors can also impede reactions of nucleic acid amplification. Here we looked at the PCR performances of the proof-reading D-type DNA polymerase from P. abyssi, Pab-polD. Fragments, 3 kilobases in length, were specifically PCR-amplified in its optimized reaction buffer. Pab-polD showed not only a greater resistance to high denaturation temperatures than Taq during cycling, but also a superior tolerance to the presence of potential inhibitors. Proficient proof-reading Pab-polD enzyme could also extend a primer containing up to two mismatches at the 3' primer termini. Overall, we found valuable biochemical properties in Pab-polD compared to the conventional Taq, which makes the enzyme ideally suited for cutting-edge PCR-applications.

Characterization of family D DNA polymerase from Thermococcus sp. 9°N

Accurate DNA replication is essential for maintenance of every genome. All archaeal genomes except Crenarchaea, encode for a member of Family B (polB) and Family D (polD) DNA polymerases. Gene deletion studies in Thermococcus kodakaraensis and Methanococcus maripaludis show that polD is the only essential DNA polymerase in these organisms. Thus, polD may be the primary replicative DNA polymerase for both leading and lagging strand synthesis. To understand this unique archaeal enzyme, we report the biochemical characterization of a heterodimeric polD from Thermococcus. PolD contains both DNA polymerase and proofreading 3′–5′ exonuclease activities to ensure efficient and accurate genome duplication. The polD incorporation fidelity was determined for the first time. Despite containing 3′–5′ exonuclease proofreading activity, polD has a relatively high error rate (95 × 10⁻⁵) compared to polB (19 × 10⁻⁵) and at least 10-fold higher than the polB DNA polymerases from yeast (polε and polδ) or Escherichia coli DNA polIII holoenzyme. The implications of polD fidelity and biochemical properties in leading and lagging strand synthesis are discussed.

Domain structures and inter-domain interactions defining the holoenzyme architecture of archaeal d-family DNA polymerase

Archaea-specific D-family DNA polymerase (PolD) forms a dimeric heterodimer consisting of two large polymerase subunits and two small exonuclease subunits. According to the protein-protein interactions identified among the domains of large and small subunits of PolD, a symmetrical model for the domain topology of the PolD holoenzyme is proposed. The experimental evidence supports various aspects of the model. The conserved amphipathic nature of the N-terminal putative α-helix of the large subunit plays a key role in the homodimeric assembly and the self-cyclization of the large subunit and is deeply involved in the archaeal PolD stability and activity. We also discuss the evolutional transformation from archaeal D-family to eukaryotic B-family polymerase on the basis of the structural information.

Novel inhibition of archaeal family-D DNA polymerase by uracil

Archaeal family-D DNA polymerase is inhibited by the presence of uracil in DNA template strands. When the enzyme encounters uracil, following three parameters change: DNA binding increases roughly 2-fold, the rate of polymerization slows by a factor of ≈ 5 and 3'-5' proof-reading exonuclease activity is stimulated by a factor of ≈ 2. Together these changes result in a significant decrease in polymerization activity and a reduction in net DNA synthesis. Pol D appears to interact with template strand uracil irrespective of its distance ahead of the replication fork. Polymerization does not stop at a defined location relative to uracil, rather a general decrease in DNA synthesis is observed. 'Trans' inhibition, the slowing of Pol D by uracil on a DNA strand not being replicated is also observed. It is proposed that Pol D is able to interact with uracil by looping out the single-stranded template, allowing simultaneous contact of both the base and the primer-template junction to give a polymerase-DNA complex with diminished extension ability.

Novel structure of an N-terminal domain that is crucial for the dimeric assembly and DNA-binding of an archaeal DNA polymerase D large subunit from Pyrococcus horikoshii

Archaea-specific D-family DNA polymerase forms a heterotetramer consisting of two large polymerase subunits and two small exonuclease subunits. The N-terminal (1-300) domain structure of the large subunit was determined by X-ray crystallography, although ∼50 N-terminal residues were disordered. The determined structure consists of nine alpha helices and three beta strands. We also identified the DNA-binding ability of the domain by SPR measurement. The N-terminal (1-100) region plays crucial roles in the folding of the large subunit dimer by connecting the ∼50 N-terminal residues with their own catalytic region (792-1163).