Structural insight into translesion synthesis by DNA Pol II

Structural basis of error-prone DNA synthesis by DNA polymerase θ

DNA polymerase θ (Pol θ) is an A-family DNA polymerase specialized in DNA double-strand breaks repair and translesion synthesis. Distinct from its high-fidelity homologs in DNA replication, Pol θ catalyzes template-dependent DNA synthesis with an inherent propensity for error incorporation. However, the structural basis of Pol θ's low-fidelity DNA synthesis is not clear. Here, we present cryo-electron microscopy structures detailing the polymerase domain of human Pol θ in complex with a cognate C:G base pair (bp), a mismatched T:G bp, or a mismatched T:T bp. Our structures illustrate that Pol θ snugly accommodates the mismatched nascent base pairs within its active site with the finger domain well-closed, consistent with our in-solution fluorescence measurement but in contrast to its high-fidelity homologs. In addition, structural examination and mutagenesis study show that unique residues surrounding the active site contribute to the stabilization of the mismatched nascent base pair. Furthermore, Pol θ can efficiently extend from the misincorporated T:G or T:T mismatches, yet with a preference for template or primer looping-out, resulting in insertions and deletions. Collectively, our results elucidate how an A-family polymerase is adapted for error-prone DNA synthesis.

Common themes in architecture and interactions of prokaryotic PolB2 and Pol V mutasomes inferred from in silico studies

Translesion DNA synthesis (TLS) is typically performed by inherently error-prone Y-family DNA polymerases. Extensively studied Escherichia coli Pol V mutasome, composed of UmuC, an UmuD' dimer and RecA is an example of a multimeric Y-family TLS polymerase. Less commonly TLS is performed by DNA polymerases of other families. One of the most intriguing such cases in B-family is represented by archaeal PolB2 and its bacterial homologs. Previously thought to be catalytically inactive, PolB2 was recently shown to be absolutely required for targeted mutagenesis in Sulfolobus islandicus. However, the composition and structure of the PolB2 holoenzyme remain unknown. We used highly accurate AlphaFold structural models, coupled with protein sequence and genome context analysis to comprehensively characterize PolB2 and its associated proteins, PPB2, a small helical protein, and iRadA, a catalytically inactive Rad51 homolog. We showed that these three proteins can form a heteropentameric PolB2 complex featuring high confidence modeling scores. Unexpectedly, we found that PolB2 binds iRadA through a structural motif reminiscent of RadA/Rad51 oligomerization motif. In some mutasomes we identified clamp binding motifs, present in either iRadA or PolB2, but rarely in both. We also used AlphaFold to derive a three-dimensional structure of Pol V, for which the experimental structure remains unsolved thus precluding comprehensive understanding of its molecular mechanism. Our analysis showed that the structural features of Pol V explain many of the puzzling previous experimental results. Even though models of PolB2 and Pol V mutasomes are structurally different, we found striking similarities in their architectural organization and interactions.

The path to the G protein-coupled receptor structural landscape: Major milestones and future directions

G protein-coupled receptors (GPCRs) play a crucial role in cell function by transducing signals from the extracellular environment to the inside of the cell. They mediate the effects of various stimuli, including hormones, neurotransmitters, ions, photons, food tastants and odorants, and are renowned drug targets. Advancements in structural biology techniques, including X-ray crystallography and cryo-electron microscopy (cryo-EM), have driven the elucidation of an increasing number of GPCR structures. These structures reveal novel features that shed light on receptor activation, dimerization and oligomerization, dichotomy between orthosteric and allosteric modulation, and the intricate interactions underlying signal transduction, providing insights into diverse ligand-binding modes and signalling pathways. However, a substantial portion of the GPCR repertoire and their activation states remain structurally unexplored. Future efforts should prioritize capturing the full structural diversity of GPCRs across multiple dimensions. To do so, the integration of structural biology with biophysical and computational techniques will be essential. We describe in this review the progress of nuclear magnetic resonance (NMR) to examine GPCR plasticity and conformational dynamics, of atomic force microscopy (AFM) to explore the spatial-temporal dynamics and kinetic aspects of GPCRs, and the recent breakthroughs in artificial intelligence for protein structure prediction to characterize the structures of the entire GPCRome. In summary, the journey through GPCR structural biology provided in this review illustrates how far we have come in decoding these essential proteins architecture and function. Looking ahead, integrating cutting-edge biophysics and computational tools offers a path to navigating the GPCR structural landscape, ultimately advancing GPCR-based applications.

8-OxodGuo and Fapy•dG Mutagenicity in Escherichia coli Increases Significantly when They Are Part of a Tandem Lesion with 5-Formyl-2'-deoxyuridine

Tandem lesions, which are defined by two or more contiguously damaged nucleotides, are a hallmark of ionizing radiation. Recently, tandem lesions containing 5-formyl-2'-deoxyuridine (5-fdU) flanked by a 5'-8-OxodGuo or Fapy•dG were discovered, and they are more mutagenic in human cells than the isolated lesions. In the current study, we examined replication of these tandem lesions in Escherichia coli. Bypass efficiency of both tandem lesions was reduced by 30-40% compared to the isolated lesions. Mutation frequencies (MFs) of isolated 8-OxodGuo and Fapy•dG were low, and no mutants were isolated from replication of a 5-fdU construct. The types of mutations from 8-OxodGuo were targeted G → T transversion, whereas Fapy•dG predominantly gave G → T and G deletion. 5'-8-OxodGuo-5-fdU also gave exclusively G → T mutation, which was 3-fold and 11-fold greater, without and with SOS induction, respectively, compared to that of an isolated 8-OxodGuo. In mutY/mutM cells, the MF of 8-OxodGuo and 5'-8-OxodGuo-5-fdU increased 13-fold and 7-fold, respectively. The MF of 5'-8-OxodGuo-5-fdU increased 2-fold and 3-fold in Pol II- and Pol IV-deficient cells, respectively, suggesting that these polymerases carry out largely error-free bypass. The MF of 5'- Fapy•dG-5-fdU was similar without (13 ± 1%) and with (16 ± 2%) SOS induction. Unlike the complex mutation spectrum reported earlier in human cells for 5'- Fapy•dG-5-fdU, with G → T as the major type of errors, in E. coli, the mutations were predominantly from deletion of 5-fdU. We postulate that removal of adenine-incorporated opposite 8-OxodGuo by Fpg and MutY repair proteins is partially impaired in the tandem 5'-8-OxodGuo-5-fdU, resulting in an increase in the G → T mutations, whereas a slippage mechanism may be operating in the 5'- Fapy•dG-5-fdU mutagenesis. This study showed that not only are these tandem lesions more mutagenic than the isolated lesions but they may also exhibit different types of mutations in different organisms.

Replication Bypass of the N-(2-Deoxy-d-erythro-pentofuranosyl)-urea DNA Lesion by Human DNA Polymerase η

Urea lesions in DNA arise from thymine glycol (Tg) or 8-oxo-dG; their genotoxicity is thought to arise in part due to their potential to accommodate the insertion of all four dNTPs during error-prone replication. Replication bypass with human DNA polymerase η (hPol η) confirmed that all four dNTPs were inserted opposite urea lesions but with purines exhibiting greater incorporation efficiency. X-ray crystal structures of ternary replication bypass complexes in the presence of Mg2+ ions with incoming dNTP analogs dAMPnPP, dCMPnPP, dGMPnPP, and dTMPnPP bound opposite urea lesions (hPol η·DNA·dNMPnPP complexes) revealed all were accommodated by hPol η. In each, the Watson-Crick face of the dNMPnPP was paired with the urea lesion, exploiting the ability of the amine and carbonyl groups of the urea to act as H-bond donors or acceptors, respectively. With incoming dAMPnPP or dGMPnPP, the distance between the imino nitrogen of urea and the N9 atoms of incoming dNMPnPP approximated the canonical distance of 9 Å in B-DNA. With incoming dCMPnPP or dTMPnPP, the corresponding distance of about 7 Å was less ideal. Improved base-stacking interactions were also observed with incoming purines vs pyrimidines. Nevertheless, in each instance, the α-phosphate of incoming dNMPnPPs was close to the 3'-hydroxyl group of the primer terminus, consistent with the catalysis of nucleotidyl transfer and the observation that all four nucleotides could be inserted opposite urea lesions. Preferential insertion of purines by hPol η may explain, in part, why the urea-directed spectrum of mutations arising from Tg vs 8-oxo-dG lesions differs.

Structure and function of extreme TLS DNA polymerase TTEDbh from Thermoanaerobacter tengcongensis

Translesion synthesis (TLS) is a kind of DNA repair that maintains the stability of the genome and ensures the normal growth of life in cells under emergencies. Y-family DNA polymerases, as a kind of error-prone DNA polymerase, mainly perform TLS. Previous studies have suggested that the occurrence of tumors is associated with the overexpression of human DNA polymerase of the Y family. And the combination of Y-family DNA polymerase inhibitors is promising for cancer therapy. Here we report the functional and structural characterization of a member of the Y-family DNA polymerases, TTEDbh. We determine TTEDbh is an extreme TLS polymerase that can cross oxidative damage sites, and further identify the amino acids and novel structures that are critical for DNA binding, synthesis, fidelity, and oxidative damage bypass. Moreover, previously unnoticed structural elements with important functions have been discovered and analyzed. These studies provide a more experimental basis for further elucidating the molecular mechanisms of DNA polymerase in the Y family. It could also shed light on the design of drugs to target tumors.

In crystallo observation of active site dynamics and transient metal ion binding within DNA polymerases

DNA polymerases are the enzymatic catalysts that synthesize DNA during DNA replication and repair. Kinetic studies and x-ray crystallography have uncovered the overall kinetic pathway and led to a two-metal-ion dependent catalytic mechanism. Diffusion-based time-resolved crystallography has permitted the visualization of the catalytic reaction at atomic resolution and made it possible to capture transient events and metal ion binding that have eluded static polymerase structures. This review discusses past static structures and recent time-resolved structures that emphasize the crucial importance of primer alignment and different metal ions binding during catalysis and substrate discrimination.

Primer terminal ribonucleotide alters the active site dynamics of DNA polymerase η and reduces DNA synthesis fidelity

DNA polymerases catalyze DNA synthesis with high efficiency, which is essential for all life. Extensive kinetic and structural efforts have been executed in exploring mechanisms of DNA polymerases, surrounding their kinetic pathway, catalytic mechanisms, and factors that dictate polymerase fidelity. Recent time-resolved crystallography studies on DNA polymerase η (Pol η) and β have revealed essential transient events during the DNA synthesis reaction, such as mechanisms of primer deprotonation, separated roles of the three metal ions, and conformational changes that disfavor incorporation of the incorrect substrate. DNA-embedded ribonucleotides (rNs) are the most common lesion on DNA and a major threat to genome integrity. While kinetics of rN incorporation has been explored and structural studies have revealed that DNA polymerases have a steric gate that destabilizes ribonucleotide triphosphate binding, the mechanism of extension upon rN addition remains poorly characterized. Using steady-state kinetics, static and time-resolved X-ray crystallography with Pol η as a model system, we showed that the extra hydroxyl group on the primer terminus does alter the dynamics of the polymerase active site as well as the catalysis and fidelity of DNA synthesis. During rN extension, Pol η error incorporation efficiency increases significantly across different sequence contexts. Finally, our systematic structural studies suggest that the rN at the primer end improves primer alignment and reduces barriers in C2'-endo to C3'-endo sugar conformational change. Overall, our work provides further mechanistic insights into the effects of rN incorporation on DNA synthesis.

The thumb subdomain of Pyrococcus furiosus DNA polymerase is responsible for deoxyuracil binding, hydrolysis and polymerization of nucleotides

B-family DNA polymerases, which are found in eukaryotes, archaea, viruses, and some bacteria, participate in DNA replication and repair. Starting from the N-terminus of archaeal and bacterial B-family DNA polymerases, three domains include the N-terminal, exonuclease, and polymerase domains. The N-terminal domain of the archaeal B-family DNA polymerase has a conserved deoxyuracil-binding pocket for specially binding the deoxyuracil base on DNA. The exonuclease domain is responsible for removing the mismatched base pair. The polymerase domain is the core functional domain and takes a highly conserved structure composed of fingers, palm and thumb subdomains. Previous studies have demonstrated that the thumb subdomain mainly functions as a DNA-binding element and has coordination with the exonuclease domain and palm subdomain. To further elucidate the possible functions of the thumb subdomain of archaeal B-family DNA polymerases, the thumb subdomain of Pyrococcus furiosus DNA polymerase was mutated, and the effects on three activities were characterized. Our results demonstrate that the thumb subdomain participates in the three activities of archaeal B-family DNA polymerases as a common structural element. Both the N-terminal deoxyuracil-binding pocket and thumb subdomain are critical for deoxyuracil binding. Moreover, the thumb subdomain assists DNA polymerization and hydrolysis reactions, but it does not contribute to the fidelity of DNA polymerization.

Flexibility of telomerase in binding the RNA template and DNA telomeric repeat

Telomerase reverse transcriptase (TERT) has a conserved central cavity near the active site. Using enzymatic and mutagenesis analyses, we provide experimental evidence that an artificially looped-out telomeric repeat in the DNA primer can be transiently accommodated in the cavity of Tribolium castaneum (tcTERT). Kinetically, tcTERT requires a minimum of 4 bp between the RNA template and DNA primer for efficient DNA synthesis. An RNA duplex downstream of the RNA-template region after a flexible linker enhances the efficiency of primer extension by tcTERT. In addition to the peripheral cavities that accommodate looped-out RNA during each telomeric repeat synthesis, the central cavity that can accommodate the looped-out DNA may aid RNA-template translocation between cycles of telomeric repeat synthesis.

Telomerase synthesizes telomeres at the ends of linear chromosomes by repeated reverse transcription from a short RNA template. Crystal structures of Tribolium castaneum telomerase reverse transcriptase (tcTERT) and cryoelectron microscopy (cryo-EM) structures of human and Tetrahymena telomerase have revealed conserved features in the reverse-transcriptase domain, including a cavity near the DNA 3′ end and snug interactions with the RNA template. For the RNA template to translocate, it needs to be unpaired and separated from the DNA product. Here we investigate the potential of the structural cavity to accommodate a looped-out DNA bulge and enable the separation of the RNA/DNA hybrid. Using tcTERT as a model system, we show that a looped-out telomeric repeat in the DNA primer can be accommodated and extended by tcTERT but not by retroviral reverse transcriptase. Mutations that reduce the cavity size reduce the ability of tcTERT to extend the looped-out DNA substrate. In agreement with cryo-EM structures of telomerases, we find that tcTERT requires a minimum of 4 bp between the RNA template and DNA primer for efficient DNA synthesis. We also have determined the ternary-complex structure of tcTERT including a downstream RNA/DNA hybrid at 2.0-Å resolution and shown that a downstream RNA duplex, equivalent to the 5′ template-boundary element in telomerase RNA, enhances the efficiency of telomere synthesis by tcTERT. Although TERT has a preformed active site without the open-and-closed conformational changes, it contains cavities to accommodate looped-out RNA and DNA. The flexible RNA–DNA binding likely underlies the processivity of telomeric repeat addition.

The mechanism of action of the SSB interactome reveals it is the first OB-fold family of genome guardians in prokaryotes

The single-stranded DNA binding protein (SSB) is essential to all aspects of DNA metabolism in bacteria. This protein performs two distinct, but closely intertwined and indispensable functions in the cell. SSB binds to single-stranded DNA (ssDNA) and at least 20 partner proteins resulting in their regulation. These partners comprise a family of genome guardians known as the SSB interactome. Essential to interactome regulation is the linker/OB-fold network of interactions. This network of interactions forms when one or more PXXP motifs in the linker of SSB bind to an OB-fold in a partner, with interactome members involved in competitive binding between the linker and ssDNA to their OB-fold. Consequently, when linker-binding occurs to an OB-fold in an interactome partner, proteins are loaded onto the DNA. When linker/OB-fold interactions occur between SSB tetramers, cooperative ssDNA-binding results, producing a multi-tetrameric complex that rapidly protects the ssDNA. Within this SSB-ssDNA complex, there is an extensive and dynamic network of linker/OB-fold interactions that involves multiple tetramers bound contiguously along the ssDNA lattice. The dynamic behavior of these tetramers which includes binding mode changes, sliding as well as DNA wrapping/unwrapping events, are likely coupled to the formation and disruption of linker/OB-fold interactions. This behavior is essential to facilitating downstream DNA processing events. As OB-folds are critical to the essence of the linker/OB-fold network of interactions, and they are found in multiple interactome partners, the SSB interactome is classified as the first family of prokaryotic, oligosaccharide/oligonucleotide binding fold (OB-fold) genome guardians.

Etheno adducts: from tRNA modifications to DNA adducts and back to miscoding ribonucleotides

Etheno (and ethano) derivatives of nucleic acid bases have an extra 5-membered ring attached. These were first noted as wyosine bases in tRNAs. Some were fluorescent, and the development of etheno derivatives of adenosine, cytosine, and guanosine led to the synthesis of fluorescent analogs of ATP, NAD+, and other cofactors for use in biochemical studies. Early studies with the carcinogen vinyl chloride revealed that these modified bases were being formed in DNA and RNA and might be responsible for mutations and cancer. The etheno bases are also derived from other carcinogenic vinyl monomers. Further work showed that endogenous etheno DNA adducts were present in animals and humans and are derived from lipid peroxidation. The chemical mechanisms of etheno adduct formation involve reactions with bis-electrophiles generated by cytochrome P450 enzymes or lipid peroxidation, which have been established in isotopic labeling studies. The mechanisms by which etheno DNA adducts miscode have been studied with several DNA polymerases, aided by the X-ray crystal structures of these polymerases in mispairing situations and in extension beyond mispairs. Repair of etheno DNA adduct damage is done primarily by glycosylases and also by the direct action of dioxygenases. Some human DNA polymerases (η, κ) can insert bases opposite etheno adducts in DNA and RNA, and the reverse transcriptase activity may be of relevance with the RNA etheno adducts. Further questions involve the extent that the etheno adducts contribute to human cancer.

The three-component helicase/primase complex of herpes simplex virus-1

Herpes simplex virus type 1 (HSV-1) is one of the nine herpesviruses that infect humans. HSV-1 encodes seven proteins to replicate its genome in the hijacked human cell. Among these are the herpes virus DNA helicase and primase that are essential components of its replication machinery. In the HSV-1 replisome, the helicase-primase complex is composed of three components including UL5 (helicase), UL52 (primase) and UL8 (non-catalytic subunit). UL5 and UL52 subunits are functionally interdependent, and the UL8 component is required for the coordination of UL5 and UL52 activities proceeding in opposite directions with respect to the viral replication fork. Anti-viral compounds currently under development target the functions of UL5 and UL52. Here, we review the structural and functional properties of the UL5/UL8/UL52 complex and highlight the gaps in knowledge to be filled to facilitate molecular characterization of the structure and function of the helicase-primase complex for development of alternative anti-viral treatments.

Multiple deprotonation paths of the nucleophile 3'-OH in the DNA synthesis reaction

DNA synthesis by polymerases is essential for life. Deprotonation of the nucleophile 3'-OH is thought to be the obligatory first step in the DNA synthesis reaction. We have examined each entity surrounding the nucleophile 3'-OH in the reaction catalyzed by human DNA polymerase (Pol) η and delineated the deprotonation process by combining mutagenesis with steady-state kinetics, high-resolution structures of in crystallo reactions, and molecular dynamics simulations. The conserved S113 residue, which forms a hydrogen bond with the primer 3'-OH in the ground state, stabilizes the primer end in the active site. Mutation of S113 to alanine destabilizes primer binding and reduces the catalytic efficiency. Displacement of a water molecule that is hydrogen bonded to the 3'-OH using the 2'-OH of a ribonucleotide or 2'-F has little effect on catalysis. Moreover, combining the S113A mutation with 2'-F replacement, which removes two potential hydrogen acceptors of the 3'-OH, does not reduce the catalytic efficiency. We conclude that the proton can leave the O3' via alternative paths, supporting the hypothesis that binding of the third Mg²⁺ initiates the reaction by breaking the α-β phosphodiester bond of an incoming deoxyribonucleoside triphosphate (dNTP).

Mechanisms of feedback inhibition and sequential firing of active sites in plant aspartate transcarbamoylase

Aspartate transcarbamoylase (ATC), an essential enzyme for de novo pyrimidine biosynthesis, is uniquely regulated in plants by feedback inhibition of uridine 5-monophosphate (UMP). Despite its importance in plant growth, the structure of this UMP-controlled ATC and the regulatory mechanism remain unknown. Here, we report the crystal structures of Arabidopsis ATC trimer free and bound to UMP, complexed to a transition-state analog or bearing a mutation that turns the enzyme insensitive to UMP. We found that UMP binds and blocks the ATC active site, directly competing with the binding of the substrates. We also prove that UMP recognition relies on a loop exclusively conserved in plants that is also responsible for the sequential firing of the active sites. In this work, we describe unique regulatory and catalytic properties of plant ATCs that could be exploited to modulate de novo pyrimidine synthesis and plant growth.

DNA Polymerase II Supports the Replicative Bypass of N2-Alkyl-2'-deoxyguanosine Lesions in Escherichia coli Cells

Alkylation represents a main form of DNA damage. The N² position of guanine is frequently alkylated in DNA. The SOS-induced polymerases have been shown to be capable of bypassing various DNA damage products in Escherichia coli. Herein, we explored the influences of four N²-alkyl-dG lesions (alkyl = ethyl, n-butyl, isobutyl, or sec-butyl) on DNA replication in AB1157 E. coli cells and the corresponding strains with polymerases (Pol) II, IV, and V being individually or simultaneously knocked out. We found that N²-Et-dG is slightly less blocking to DNA replication than the N²-Bu-dG lesions, which display very similar replication bypass efficiencies. Additionally, Pol II and, to a lesser degree, Pol IV and Pol V are required for the efficient bypass of the N²-alkyl-dG adducts, and none of these lesions was mutagenic. Together, our results support that the efficient replication across small N²-alkyl-dG DNA adducts in E. coli depends mainly on Pol II.

Diversity and evolution of B-family DNA polymerases

B-family DNA polymerases (PolBs) represent the most common replicases. PolB enzymes that require RNA (or DNA) primed templates for DNA synthesis are found in all domains of life and many DNA viruses. Despite extensive research on PolBs, their origins and evolution remain enigmatic. Massive accumulation of new genomic and metagenomic data from diverse habitats as well as availability of new structural information prompted us to conduct a comprehensive analysis of the PolB sequences, structures, domain organizations, taxonomic distribution and co-occurrence in genomes. Based on phylogenetic analysis, we identified a new, widespread group of bacterial PolBs that are more closely related to the catalytically active N-terminal half of the eukaryotic PolEpsilon (PolEpsilonN) than to Escherichia coli Pol II. In Archaea, we characterized six new groups of PolBs. Two of them show close relationships with eukaryotic PolBs, the first one with PolEpsilonN, and the second one with PolAlpha, PolDelta and PolZeta. In addition, structure comparisons suggested common origin of the catalytically inactive C-terminal half of PolEpsilon (PolEpsilonC) and PolAlpha. Finally, in certain archaeal PolBs we discovered C-terminal Zn-binding domains closely related to those of PolAlpha and PolEpsilonC. Collectively, the obtained results allowed us to propose a scenario for the evolution of eukaryotic PolBs.

Structure and mechanism of B-family DNA polymerase ζ specialized for translesion DNA synthesis

DNA polymerase ζ (Polζ) belongs to the same B-family as high-fidelity replicative polymerases, yet is specialized for the extension reaction in translesion DNA synthesis (TLS). Despite its importance in TLS, the structure of Polζ is unknown. We present cryo-EM structures of the Saccharomyces cerevisiae Polζ holoenzyme in the act of DNA synthesis (3.1 Å) and without DNA (4.1 Å). Polζ displays a pentameric ring-like architecture, with catalytic Rev3, accessory Pol31' Pol32 and two Rev7 subunits forming an uninterrupted daisy chain of protein-protein interactions. We also uncover the features that impose high fidelity during the nucleotide-incorporation step and those that accommodate mismatches and lesions during the extension reaction. Collectively, we decrypt the molecular underpinnings of Polζ's role in TLS and provide a framework for new cancer therapeutics.

How a B family DNA polymerase has been evolved to copy RNA

We report here crystal structures of a reverse transcriptase RTX, which was evolved in vitro from the B family polymerase KOD, in complex with either a DNA duplex or an RNA-DNA hybrid. Compared with the apo, binary, and ternary complex structures of the original KOD polymerase, the 16 substitutions that result in the function of copying RNA to DNA do not change the overall protein structure. Only six substitutions occur at the substrate-binding surface, and the others change domain-domain interfaces in the polymerase to enable RNA-DNA hybrid binding and reverse transcription. Most notably, F587L at the Palm and Thumb interface stabilizes the open and apo conformation of the Thumb. The intrinsically flexible Thumb domain seems to play a major role in accommodating the RNA-DNA hybrid product distal to the active site. This is reminiscent of naturally occurring RNA-dependent DNA polymerases, including telomerase, which have a dramatically augmented Thumb domain, and of reverse transcriptase, which extends its Thumb with the RNase H domain.

A Unique B-Family DNA Polymerase Facilitating Error-Prone DNA Damage Tolerance in Crenarchaeota

Sulfolobus islandicus codes for four DNA polymerases: three are of the B-family (Dpo1, Dpo2, and Dpo3), and one is of the Y-family (Dpo4). Western analysis revealed that among the four polymerases, only Dpo2 exhibited DNA damage-inducible expression. To investigate how these DNA polymerases could contribute to DNA damage tolerance in S. islandicus, we conducted genetic analysis of their encoding genes in this archaeon. Plasmid-borne gene expression revealed that Dpo2 increases cell survival upon DNA damage at the expense of mutagenesis. Gene deletion studies showed although dpo1 is essential, the remaining three genes are dispensable. Furthermore, although Dpo4 functions in housekeeping translesion DNA synthesis (TLS), Dpo2, a B-family DNA polymerase once predicted to be inactive, functions as a damage-inducible TLS enzyme solely responsible for targeted mutagenesis, facilitating GC to AT/TA conversions in the process. Together, our data indicate that Dpo2 is the main DNA polymerase responsible for DNA damage tolerance and is the primary source of targeted mutagenesis. Given that crenarchaea encoding a Dpo2 also have a low-GC composition genome, the Dpo2-dependent DNA repair pathway may be conserved in this archaeal lineage.

A polar filter in DNA polymerases prevents ribonucleotide incorporation

The presence of ribonucleotides in DNA can lead to genomic instability and cellular lethality. To prevent adventitious rNTP incorporation, the majority of the DNA polymerases (dPols) possess a steric filter. The dPol named MsDpo4 (Mycobacterium smegmatis) naturally lacks this steric filter and hence is capable of rNTP addition. The introduction of the steric filter in MsDpo4 did not result in complete abrogation of the ability of this enzyme to incorporate ribonucleotides. In comparison, DNA polymerase IV (PolIV) from Escherichia coli exhibited stringent selection for deoxyribonucleotides. A comparison of MsDpo4 and PolIV led to the discovery of an additional polar filter responsible for sugar selectivity. Thr43 represents the filter in PolIV and this residue forms interactions with the incoming nucleotide to draw it closer to the enzyme surface. As a result, the 2’-OH in rNTPs will clash with the enzyme surface, and therefore ribonucleotides cannot be accommodated in the active site in a conformation compatible with productive catalysis. The substitution of the equivalent residue in MsDpo4–Cys47, with Thr led to a drastic reduction in the ability of the mycobacterial enzyme to incorporate rNTPs. Overall, our studies evince that the polar filter serves to prevent ribonucleotide incorporation by dPols.

The mechanism of Single strand binding protein-RecG binding: Implications for SSB interactome function

The Escherichia coli single-strand DNA binding protein (SSB) is essential to viability where it functions to regulate SSB interactome function. Here it binds to single-stranded DNA and to target proteins that comprise the interactome. The region of SSB that links these two essential protein functions is the intrinsically disordered linker. Key to linker function is the presence of three, conserved PXXP motifs that mediate binding to oligosaccharide-oligonucleotide binding folds (OB-fold) present in SSB and its interactome partners. Not surprisingly, partner OB-fold deletions eliminate SSB binding. Furthermore, single point mutations in either the PXXP motifs or, in the RecG OB-fold, obliterate SSB binding. The data also demonstrate that, and in contrast to the view currently held in the field, the C-terminal acidic tip of SSB is not required for interactome partner binding. Instead, we propose the tip has two roles. First, and consistent with the proposal of Dixon, to regulate the structure of the C-terminal domain in a biologically active conformation that prevents linkers from binding to SSB OB-folds until this interaction is required. Second, as a secondary binding domain. Finally, as OB-folds are present in SSB and many of its partners, we present the SSB interactome as the first family of OB-fold genome guardians identified in prokaryotes.

CRL4 Ubiquitin Pathway and DNA Damage Response

DNA damage occurs in a human cell at an average frequency of 10,000 incidences per day by means of external and internal culprits, damage that triggers sequential cellular responses and stalls the cell cycle while activating specific DNA repair pathways. Failure to remove DNA lesions would compromise genomic integrity, leading to human diseases such as cancer and premature aging. If DNA damage is extensive and cannot be repaired, cells undergo apoptosis. DNA damage response (DDR) often entails posttranslational modifications of key DNA repair and DNA damage checkpoint proteins, including phosphorylation and ubiquitination. Cullin-RING ligase 4 (CRL4) enzyme has been found to target multiple DDR proteins for ubiquitination. In this chapter, we will discuss key repair and checkpoint proteins that are subject to ubiquitin-dependent regulation by members of the CRL4 family during ultraviolet light (UV)-induced DNA damage.

A gatekeeping function of the replicative polymerase controls pathway choice in the resolution of lesion-stalled replisomes

DNA lesions stall the replisome and proper resolution of these obstructions is critical for genome stability. Replisomes can directly replicate past a lesion by error-prone translesion synthesis. Alternatively, replisomes can reprime DNA synthesis downstream of the lesion, creating a single-stranded DNA gap that is repaired primarily in an error-free, homology-directed manner. Here we demonstrate how structural changes within the Escherichia coli replisome determine the resolution pathway of lesion-stalled replisomes. This pathway selection is controlled by a dynamic interaction between the proofreading subunit of the replicative polymerase and the processivity clamp, which sets a kinetic barrier to restrict access of translesion synthesis (TLS) polymerases to the primer/template junction. Failure of TLS polymerases to overcome this barrier leads to repriming, which competes kinetically with TLS. Our results demonstrate that independent of its exonuclease activity, the proofreading subunit of the replisome acts as a gatekeeper and influences replication fidelity during the resolution of lesion-stalled replisomes.

Evolution of Inosine-Specific Endonuclease V from Bacterial DNase to Eukaryotic RNase

Endonuclease V (EndoV) cleaves the second phosphodiester bond 3' to a deaminated adenosine (inosine). Although highly conserved, EndoV homologs change substrate preference from DNA in bacteria to RNA in eukaryotes. We have characterized EndoV from six different species and determined crystal structures of human EndoV and three EndoV homologs from bacteria to mouse in complex with inosine-containing DNA/RNA hybrid or double-stranded RNA (dsRNA). Inosine recognition is conserved, but changes in several connecting loops in eukaryotic EndoV confer recognition of 3 ribonucleotides upstream and 7 or 8 bp of dsRNA downstream of the cleavage site, and bacterial EndoV binds only 2 or 3 nt flanking the scissile phosphate. In addition to the two canonical metal ions in the active site, a third Mn²⁺ that coordinates the nucleophilic water appears necessary for product formation. Comparison of EndoV with its homologs RNase H1 and Argonaute reveals the principles by which these enzymes recognize RNA versus DNA.

5-Formylcytosine mediated DNA-protein cross-links block DNA replication and induce mutations in human cells

5-Formylcytosine (5fC) is an epigenetic DNA modification introduced via TET protein-mediated oxidation of 5-methyl-dC. We recently reported that 5fC form reversible DNA–protein conjugates (DPCs) with histone proteins in living cells (Ji et al. (2017) Angew. Chem. Int. Ed., 56:14130–14134). We now examined the effects of 5fC mediated DPCs on DNA replication. Synthetic DNA duplexes containing site-specific DPCs between 5fC and lysine-containing proteins and peptides were subjected to primer extension experiments in the presence of human translesion synthesis DNA polymerases η and κ. We found that DPCs containing histones H2A or H4 completely inhibited DNA replication, but the replication block was removed when the proteins were subjected to proteolytic digestion. Cross-links to 11-mer or 31-mer peptides were bypassed by both polymerases in an error-prone manner, inducing targeted C→T transitions and –1 deletions. Similar types of mutations were observed when plasmids containing 5fC-peptide cross-links were replicated in human embryonic kidney (HEK) 293T cells. Molecular simulations of the 11-mer peptide-dC cross-links bound to human polymerases η and κ revealed that the peptide fits well on the DNA major groove side, and the modified dC forms a stable mismatch with incoming dATP via wobble base pairing in the polymerase active site.

The SOS system: A complex and tightly regulated response to DNA damage

Genomes of all living organisms are constantly threatened by endogenous and exogenous agents that challenge the chemical integrity of DNA. Most bacteria have evolved a coordinated response to DNA damage. In Escherichia coli, this inducible system is termed the SOS response. The SOS global regulatory network consists of multiple factors promoting the integrity of DNA as well as error-prone factors allowing for survival and continuous replication upon extensive DNA damage at the cost of elevated mutagenesis. Due to its mutagenic potential, the SOS response is subject to elaborate regulatory control involving not only transcriptional derepression, but also post-translational activation, and inhibition. This review summarizes current knowledge about the molecular mechanism of the SOS response induction and progression and its consequences for genome stability. Environ. Mol. Mutagen. 60:368-384, 2019. © 2018 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Hydrogen-Bonding Interactions at the DNA Terminus Promote Extension from Methylguanine Lesions by Human Extender DNA Polymerase ζ

Chemically induced DNA lesions can become DNA replication substrates that are bypassed by low-fidelity DNA polymerases. Following nucleotide misinsertion opposite a DNA lesion, the extension step can contribute to preserving such errors and lead to genomic instability and cancer. DNA polymerase ζ, a B-family polymerase, is proficient as an extender polymerase that catalyzes elongation; however, the chemical factors that impact its DNA replication are not understood. This study addresses the question of how DNA polymerase ζ achieves extension by examining the ability of recombinant human DNA polymerase ζ to extend from a series of methylated guanine lesions. The influence of H-bonding was examined by placing structurally altered nucleoside analogues and canonical bases opposite G, O⁶-MeG, N¹-MeG, and N²-MeG. We determined that terminal base pairs with the highest proclivity for H-bonding were most efficiently extended in both primer extension assays and steady-state kinetic analysis. In contrast, when no H-bonding was possible at the DNA terminus, the least efficient steady-state kinetics were observed. To evaluate H-bonding protein minor groove interactions that may underlie this phenomenon, we performed computational modeling with Escherichia coli DNA polymerase II, a homologue for DNA polymerase ζ. The modeling data together with the primer extension assays demonstrate the importance of having a carbonyl group on the primer strand that can interact with a lysine residue found to be conserved in many B-family polymerases, including human Pol ζ. These data provide a model whereby interbase H-bonding interactions at the DNA terminus promote lesion bypass and extension by human DNA polymerase ζ.

2.0 Å resolution crystal structure of human polκ reveals a new catalytic function of N-clasp in DNA replication

Human polymerase kappa (polκ) is a distinct Y-family DNA polymerase with a unique N-terminal N-clasp domain. The N-clasp renders polκ’s high efficiency and accuracy in DNA replication and lesion bypass. How N-clasp empowers polκ in replication remains unclear due to the disordering of N-clasp. Here, we present a 2.0-Å resolution crystal structure of a polκ ternary complex with DNA and an incoming nucleotide. The structure-function study reveals an ordered N-clasp domain that brings conserved and functionally important residues in contact with the replicating basepair in the active site and contributes to the nucleotidyl transfer reaction. Particularly, a fully ordered Lys25 from the N-clasp domain is in H-bonding with the α- and γ-phosphates of the incoming nucleotide. K25A mutation reduces the polymerase activity of polκ significantly. This lysine is structurally analogous to a conserved lysine in the A-family DNA polymerases in the closed form. In contrast, Lys25 in the previous structures of polκ does not have any contacts with the incoming nucleotide, resembling an open form of a DNA polymerase. Based on structural and functional similarity, we propose a local open/closed mechanism for polκ in DNA replication catalysis, which mimics the common mechanism for all DNA polymerases.

Bypassing a 8,5'-cyclo-2'-deoxyadenosine lesion by human DNA polymerase η at atomic resolution

Oxidatively induced DNA lesions 8,5'-cyclopurine-2'-deoxynucleosides (cdPus) are prevalent and cytotoxic by impeding DNA replication and transcription. Both the 5'R- and 5'S-diastereomers of cdPu can be removed by nucleotide excision repair; however, the 5'S-cdPu is more resistant to repair than the 5'R counterpart. Here, we report the crystal structures of human polymerase (Pol) η bypassing 5'S-8,5'-cyclo-2'-deoxyadenosine (cdA) in insertion and the following two extension steps. The cdA-containing DNA structures vary in response to the protein environment. Supported by the "molecular splint" of Pol η, the structure of 5'S-cdA at 1.75-Å resolution reveals that the backbone is pinched toward the minor groove and the adenine base is tilted. In the templating position, the cdA takes up the extra space usually reserved for the thymine dimer, and dTTP is efficiently incorporated by Pol η in the presence of Mn²⁺ Rigid distortions of the DNA duplex by cdA, however, prevent normal base pairing and hinder immediate primer extension by Pol η. Our results provide structural insights into the strong replication blockage effect and the mutagenic property of the cdPu lesions in cells.

Directed evolution of Escherichia coli with lower-than-natural plasmid mutation rates

Unwanted evolution of designed DNA sequences limits metabolic and genome engineering efforts. Engineered functions that are burdensome to host cells and slow their replication are rapidly inactivated by mutations, and unplanned mutations with unpredictable effects often accumulate alongside designed changes in large-scale genome editing projects. We developed a directed evolution strategy, Periodic Reselection for Evolutionarily Reliable Variants (PResERV), to discover mutations that prolong the function of a burdensome DNA sequence in an engineered organism. Here, we used PResERV to isolate Escherichia coli cells that replicate ColE1-type plasmids with higher fidelity. We found mutations in DNA polymerase I and in RNase E that reduce plasmid mutation rates by 6- to 30-fold. The PResERV method implicitly selects to maintain the growth rate of host cells, and high plasmid copy numbers and gene expression levels are maintained in some of the evolved E. coli strains, indicating that it is possible to improve the genetic stability of cellular chassis without encountering trade-offs in other desirable performance characteristics. Utilizing these new antimutator E. coli and applying PResERV to other organisms in the future promises to prevent evolutionary failures and unpredictability to provide a more stable genetic foundation for synthetic biology.

Structural basis for polymerase η-promoted resistance to the anticancer nucleoside analog cytarabine

Cytarabine (AraC) is an essential chemotherapeutic for acute myeloid leukemia (AML) and resistance to this drug is a major cause of treatment failure. AraC is a nucleoside analog that differs from 2′-deoxycytidine only by the presence of an additional hydroxyl group at the C2′ position of the 2′-deoxyribose. The active form of the drug AraC 5′-triphosphate (AraCTP) is utilized by human replicative DNA polymerases to insert AraC at the 3′ terminus of a growing DNA chain. This impedes further primer extension and is a primary basis for the drug action. The Y-family translesion synthesis (TLS) DNA polymerase η (Polη) counteracts this barrier to DNA replication by efficient extension from AraC-terminated primers. Here, we provide high-resolution structures of human Polη with AraC incorporated at the 3′-primer terminus. We show that Polη can accommodate AraC at different stages of the catalytic cycle, and that it can manipulate the conformation of the AraC sugar via specific hydrogen bonding and stacking interactions. Taken together, the structures provide a basis for the ability of Polη to extend DNA synthesis from AraC terminated primers.

Specialised DNA polymerases in Escherichia coli: roles within multiple pathways

In many bacterial species, DNA damage triggers the SOS response; a pathway that regulates the production of DNA repair and damage tolerance proteins, including error-prone DNA polymerases. These specialised polymerases are capable of bypassing lesions in the template DNA, a process known as translesion synthesis (TLS). Specificity for lesion types varies considerably between the different types of TLS polymerases. TLS polymerases are mainly described as working in the context of replisomes that are stalled at lesions or in lesion-containing gaps left behind the replisome. Recently, a series of single-molecule fluorescence microscopy studies have revealed that two TLS polymerases, pol IV and pol V, rarely colocalise with replisomes in Escherichia coli cells, suggesting that most TLS activity happens in a non-replisomal context. In this review, we re-visit the evidence for the involvement of TLS polymerases in other pathways. A series of genetic and biochemical studies indicates that TLS polymerases could participate in nucleotide excision repair, homologous recombination and transcription. In addition, oxidation of the nucleotide pool, which is known to be induced by multiple stressors, including many antibiotics, appears to favour TLS polymerase activity and thus increases mutation rates. Ultimately, participation of TLS polymerases within non-replisomal pathways may represent a major source of mutations in bacterial cells and calls for more extensive investigation.

Fidelity of DNA replication-a matter of proofreading

DNA that is transmitted to daughter cells must be accurately duplicated to maintain genetic integrity and to promote genetic continuity. A major function of replicative DNA polymerases is to replicate DNA with the very high accuracy. The fidelity of DNA replication relies on nucleotide selectivity of replicative DNA polymerase, exonucleolytic proofreading, and postreplicative DNA mismatch repair (MMR). Proofreading activity that assists most of the replicative polymerases is responsible for removal of incorrectly incorporated nucleotides from the primer terminus before further primer extension. It is estimated that proofreading improves the fidelity by a 2–3 orders of magnitude. The primer with the incorrect terminal nucleotide has to be moved to exonuclease active site, and after removal of the wrong nucleotide must be transferred back to polymerase active site. The mechanism that allows the transfer of the primer between pol and exo site is not well understood. While defects in MMR are well known to be linked with increased cancer incidence only recently, the replicative polymerases that have alterations in the exonuclease domain have been associated with some sporadic and hereditary human cancers. In this review, we would like to emphasize the importance of proofreading (3′-5′ exonuclease activity) in the fidelity of DNA replication and to highlight what is known about switching from polymerase to exonuclease active site.

Translesion and Repair DNA Polymerases: Diverse Structure and Mechanism

The number of DNA polymerases identified in each organism has mushroomed in the past two decades. Most newly found DNA polymerases specialize in translesion synthesis and DNA repair instead of replication. Although intrinsic error rates are higher for translesion and repair polymerases than for replicative polymerases, the specialized polymerases increase genome stability and reduce tumorigenesis. Reflecting the numerous types of DNA lesions and variations of broken DNA ends, translesion and repair polymerases differ in structure, mechanism, and function. Here, we review the unique and general features of polymerases specialized in lesion bypass, as well as in gap-filling and end-joining synthesis.

A Strategically Located Arg/Lys Residue Promotes Correct Base Paring During Nucleic Acid Biosynthesis in Polymerases

Polymerases (Pols) synthesize the double-stranded nucleic acids in the Watson-Crick (W-C) conformation, which is critical for DNA and RNA functioning. Yet, the molecular basis to catalyze the W-C base pairing during Pol-mediated nucleic acids biosynthesis remains unclear. Here, through bioinformatics analyses on a large data set of Pol/DNA structures, we first describe the conserved presence of one positively charged residue (Lys or Arg), which is similarly located near the enzymatic two-metal active site, always interacting directly with the incoming substrate (d)NTP. Incidentally, we noted that some Pol/DNA structures showing the alternative Hoogsteen base pairing were often solved with this specific residue either mutated, displaced, or missing. We then used quantum and classical simulations coupled to free-energy calculations to illustrate how, in human DNA Pol-η, the conserved Arg61 favors W-C base pairing through defined interactions with the incoming nucleotide. Taken together, these structural observations and computational results suggest a structural framework in which this specific residue is critical for stabilizing the incoming (d)NTP nucleotide and base pairing during Pol-mediated nucleic acid biosynthesis. These results may benefit enzyme engineering for nucleic acid processing and encourage new drug discovery strategies to modulate Pols function.

Crystal structures of ternary complexes of archaeal B-family DNA polymerases

Archaeal B-family polymerases drive biotechnology by accepting a wide substrate range of chemically modified nucleotides. By now no structural data for archaeal B-family DNA polymerases in a closed, ternary complex are available, which would be the basis for developing next generation nucleotides. We present the ternary crystal structures of KOD and 9°N DNA polymerases complexed with DNA and the incoming dATP. The structures reveal a third metal ion in the active site, which was so far only observed for the eukaryotic B-family DNA polymerase δ and no other B-family DNA polymerase. The structures reveal a wide inner channel and numerous interactions with the template strand that provide space for modifications within the enzyme and may account for the high processivity, respectively. The crystal structures provide insights into the superiority over other DNA polymerases concerning the acceptance of modified nucleotides.

The vaccinia virus DNA polymerase structure provides insights into the mode of processivity factor binding

Vaccinia virus (VACV), the prototype member of the Poxviridae, replicates in the cytoplasm of an infected cell. The catalytic subunit of the DNA polymerase E9 binds the heterodimeric processivity factor A20/D4 to form the functional polymerase holoenzyme. Here we present the crystal structure of full-length E9 at 2.7 Å resolution that permits identification of important poxvirus-specific structural insertions. One insertion in the palm domain interacts with C-terminal residues of A20 and thus serves as the processivity factor-binding site. This is in strong contrast to all other family B polymerases that bind their co-factors at the C terminus of the thumb domain. The VACV E9 structure also permits rationalization of polymerase inhibitor resistance mutations when compared with the closely related eukaryotic polymerase delta–DNA complex.

The catalytic subunit E9 of the vaccinia virus DNA polymerase forms a functional polymerase holoenzyme by interacting with the heterodimeric processivity factor A20/D4. Here the authors present the structure of full-length E9 and show that an insertion within its palm domain binds A20, in a mode different from other family B polymerases.

Plant organellar DNA polymerases are replicative and translesion DNA synthesis polymerases

Genomes acquire lesions that can block the replication fork and some lesions must be bypassed to allow survival. The nuclear genome of flowering plants encodes two family-A DNA polymerases (DNAPs), the result of a duplication event, that are the sole DNAPs in plant organelles. These DNAPs, dubbed Plant Organellar Polymerases (POPs), resemble the Klenow fragment of bacterial DNAP I and are not related to metazoan and fungal mitochondrial DNAPs. Herein we report that replicative POPs from the plant model Arabidopsis thaliana (AtPolI) efficiently bypass one the most insidious DNA lesions, an apurinic/apyrimidinic (AP) site. AtPolIs accomplish lesion bypass with high catalytic efficiency during nucleotide insertion and extension. Lesion bypass depends on two unique polymerization domain insertions evolutionarily unrelated to the insertions responsible for lesion bypass by DNAP θ, an analogous lesion bypass polymerase. AtPolIs exhibit an insertion fidelity that ranks between the fidelity of replicative and lesion bypass DNAPs, moderate 3′-5′ exonuclease activity and strong strand-displacement. AtPolIs are the first known example of a family-A DNAP evolved to function in both DNA replication and lesion bypass. The lesion bypass capabilities of POPs may be required to prevent replication fork collapse in plant organelles.

The A-Rule and Deletion Formation During Abasic and Oxidized Abasic Site Bypass by DNA Polymerase θ

DNA polymerase θ (Pol θ) is implicated in various cellular processes including double-strand break repair and apurinic/apyrimidinic site bypass. Because Pol θ expression correlates with poor cancer prognosis, the ability of Pol θ to bypass the C4'-oxidized abasic site (C4-AP) and 2-deoxyribonolactone (L), which are generated by cytotoxic agents, is of interest. Translesion synthesis and subsequent extension by Pol θ past C4-AP or L and an abasic site (AP) or its tetrahydrofuran analogue (F) was examined. Pol θ conducts translesion synthesis on templates containing AP and F with similar efficiencies and follows the "A-rule," inserting nucleotides in the order A > G > T. Translesion synthesis on templates containing C4-AP and L is less efficient than AP and F, and the preference for A insertion is reduced for L and absent for C4-AP. Extension past all abasic lesions (AP, F, C4-AP, and L) was significantly less efficient than translesion synthesis and yielded deletions caused by the base one or two nucleotides downstream from the lesion being used as a template, with the latter being favored. These results suggest that bypass of abasic lesions by Pol θ is highly mutagenic.

Rev7, the regulatory subunit of Polζ, undergoes UV-induced and Cul4-dependent degradation

In eukaryotic cells, Rev7 interacts with Rev3 and functions as a regulatory subunit of Polζ, a translesion DNA synthesis (TLS) polymerase. In addition to its role in TLS, mammalian Rev7, also known as Mad2B/Mad2L2, participates in multiple cellular activities including cell cycle progression and double-strand break repair through its interaction with several proteins. Here we show that in mammalian cells, Rev7 undergoes ubiquitin/proteasome-mediated degradation upon UV irradiation in a time-dependent manner. We identified the Rev7 N-terminal destruction box as the degron and Cul4A/B as putative E3 ligases in this process. We also show that the nucleotide excision repair (NER) pathway protein HR23B physically interacts and colocalizes with Rev7 in the nuclear foci after UV irradiation and protects Rev7 from accelerated degradation. Furthermore, a similar Rev7 degradation profile was observed in cells treated with the UV-mimetic agent 4-nitroquinoline 1-oxide but not with cisplatin or camptothecin, suggesting a role of the NER pathway protein(s) in UV-induced Rev7 degradation. These data and the observation that cells deficient in Rev7 are sensitized to UV irradiation while excessive Rev7 protects cells from UV-induced DNA damage provide a new insight into the potential interplay between TLS and NER.

Polymorphism of apyrimidinic DNA structures in the nucleosome

Huge amounts (>10,000/day) of apurinic/apyrimidinic (AP) sites are produced in genomes, but their structures in chromatin remain undetermined. We determined the crystal structure of the nucleosome containing AP-site analogs at two symmetric sites, which revealed structural polymorphism: one forms an inchworm configuration without an empty space at the AP site, and the other forms a B-form-like structure with an empty space and the orphan base. This unexpected inchworm configuration of the AP site is important to understand the AP DNA repair mechanism, because it may not be recognized by the major AP-binding protein, APE1, during the base excision repair process.

Self-correcting mismatches during high-fidelity DNA replication

Faithful DNA replication is essential to all forms of life and depends on the action of 3'-5' exonucleases that remove misincorporated nucleotides from the newly synthesized strand. However, how the DNA is transferred from the polymerase to the exonuclease active site is not known. Here we present the cryo-EM structure of the editing mode of the catalytic core of the Escherichia coli replisome, revealing a dramatic distortion of the DNA whereby the polymerase thumb domain acts as a wedge that separates the two DNA strands. Importantly, NMR analysis of the DNA substrate shows that the presence of a mismatch increases the fraying of the DNA, thus enabling it to reach the exonuclease active site. Therefore the mismatch corrects itself, whereas the exonuclease subunit plays a passive role. Hence, our work provides unique insights into high-fidelity replication and establishes a new paradigm for the correction of misincorporated nucleotides.

Phosphoribosyl Diphosphate (PRPP): Biosynthesis, Enzymology, Utilization, and Metabolic Significance

Phosphoribosyl diphosphate (PRPP) is an important intermediate in cellular metabolism. PRPP is synthesized by PRPP synthase, as follows: ribose 5-phosphate + ATP → PRPP + AMP. PRPP is ubiquitously found in living organisms and is used in substitution reactions with the formation of glycosidic bonds. PRPP is utilized in the biosynthesis of purine and pyrimidine nucleotides, the amino acids histidine and tryptophan, the cofactors NAD and tetrahydromethanopterin, arabinosyl monophosphodecaprenol, and certain aminoglycoside antibiotics. The participation of PRPP in each of these metabolic pathways is reviewed. Central to the metabolism of PRPP is PRPP synthase, which has been studied from all kingdoms of life by classical mechanistic procedures. The results of these analyses are unified with recent progress in molecular enzymology and the elucidation of the three-dimensional structures of PRPP synthases from eubacteria, archaea, and humans. The structures and mechanisms of catalysis of the five diphosphoryltransferases are compared, as are those of selected enzymes of diphosphoryl transfer, phosphoryl transfer, and nucleotidyl transfer reactions. PRPP is used as a substrate by a large number phosphoribosyltransferases. The protein structures and reaction mechanisms of these phosphoribosyltransferases vary and demonstrate the versatility of PRPP as an intermediate in cellular physiology. PRPP synthases appear to have originated from a phosphoribosyltransferase during evolution, as demonstrated by phylogenetic analysis. PRPP, furthermore, is an effector molecule of purine and pyrimidine nucleotide biosynthesis, either by binding to PurR or PyrR regulatory proteins or as an allosteric activator of carbamoylphosphate synthetase. Genetic analyses have disclosed a number of mutants altered in the PRPP synthase-specifying genes in humans as well as bacterial species.

Capture of a third Mg²⁺ is essential for catalyzing DNA synthesis

It is generally assumed that an enzyme-substrate (ES) complex contains all components necessary for catalysis and that conversion to products occurs by rearrangement of atoms, protons, and electrons. However, we find that DNA synthesis does not occur in a fully assembled DNA polymerase-DNA-deoxynucleoside triphosphate complex with two canonical metal ions bound. Using time-resolved x-ray crystallography, we show that the phosphoryltransfer reaction takes place only after the ES complex captures a third divalent cation that is not coordinated by the enzyme. Binding of the third cation is incompatible with the basal ES complex and requires thermal activation of the ES for entry. It is likely that the third cation provides the ultimate boost over the energy barrier to catalysis of DNA synthesis.

Exchange between Escherichia coli polymerases II and III on a processivity clamp

Escherichia coli has three DNA polymerases implicated in the bypass of DNA damage, a process called translesion synthesis (TLS) that alleviates replication stalling. Although these polymerases are specialized for different DNA lesions, it is unclear if they interact differently with the replication machinery. Of the three, DNA polymerase (Pol) II remains the most enigmatic. Here we report a stable ternary complex of Pol II, the replicative polymerase Pol III core complex and the dimeric processivity clamp, β. Single-molecule experiments reveal that the interactions of Pol II and Pol III with β allow for rapid exchange during DNA synthesis. As with another TLS polymerase, Pol IV, increasing concentrations of Pol II displace the Pol III core during DNA synthesis in a minimal reconstitution of primer extension. However, in contrast to Pol IV, Pol II is inefficient at disrupting rolling-circle synthesis by the fully reconstituted Pol III replisome. Together, these data suggest a β-mediated mechanism of exchange between Pol II and Pol III that occurs outside the replication fork.

Comparative Molecular Dynamics Studies of Human DNA Polymerase η

High-energy ultraviolet radiation damages DNA through the formation of cyclobutane pyrimidine dimers, which stall replication. When the lesion is a thymine–thymine dimer (TTD), human DNA polymerase η (Pol η) assists in resuming the replication process by inserting nucleotides opposite the damaged site. We performed extensive molecular dynamics (MD) simulations to investigate the structural and dynamical effects of four different Pol η complexes with or without a TTD and with either dATP or dGTP as the incoming base. No major differences in the overall structures and equilibrium dynamics were detected among the four systems, suggesting that the specificity of this enzyme is due predominantly to differences in local interactions in the binding regions. Analysis of the hydrogen-bonding interactions between the enzyme and the DNA and dNTP provided molecular-level insights. Specifically, the TTD was observed to engage in more hydrogen-bonding interactions with the enzyme than its undamaged counterpart of two normal thymines. The resulting greater rigidity and specific orientation of the TTD are consistent with the experimental observation of higher processivity and overall efficiency at TTD sites than at analogous sites with two normal thymines. The similarities between the systems containing dATP and dGTP are consistent with the experimental observation of relatively low fidelity with respect to the incoming base. Moreover, Q38 and R61, two strictly conserved amino acids across the Pol η family, were found to exhibit persistent hydrogen-bonding interactions with the TTD and cation-π interactions with the free base, respectively. Thus, these simulations provide molecular level insights into the basis for the selectivity and efficiency of this enzyme, as well as the roles of the two most strictly conserved residues.

cryo-EM structures of the E. coli replicative DNA polymerase reveal its dynamic interactions with the DNA sliding clamp, exonuclease and

The replicative DNA polymerase PolIIIα from Escherichia coli is a uniquely fast and processive enzyme. For its activity it relies on the DNA sliding clamp β, the proofreading exonuclease ε and the C-terminal domain of the clamp loader subunit τ. Due to the dynamic nature of the four-protein complex it has long been refractory to structural characterization. Here we present the 8 Å resolution cryo-electron microscopy structures of DNA-bound and DNA-free states of the PolIII-clamp-exonuclease-τ_c complex. The structures show how the polymerase is tethered to the DNA through multiple contacts with the clamp and exonuclease. A novel contact between the polymerase and clamp is made in the DNA bound state, facilitated by a large movement of the polymerase tail domain and τ_c. These structures provide crucial insights into the organization of the catalytic core of the replisome and form an important step towards determining the structure of the complete holoenzyme.