The structure of a high fidelity DNA polymerase bound to a mismatched nucleotide reveals an "ajar" intermediate conformation in the nucleotide selection mechanism

Structural basis for processive daughter-strand synthesis and proofreading by the human leading-strand DNA polymerase Pol ε

During chromosome replication, the nascent leading strand is synthesized by DNA polymerase epsilon (Pol ε), which associates with the sliding clamp processivity factor proliferating cell nuclear antigen (PCNA) to form a processive holoenzyme. For high-fidelity DNA synthesis, Pol ε relies on nucleotide selectivity and its proofreading ability to detect and excise a misincorporated nucleotide. Here, we present cryo-electron microscopy (cryo-EM) structures of human Pol ε in complex with PCNA, DNA and an incoming nucleotide, revealing how Pol ε associates with PCNA through its PCNA-interacting peptide box and additional unique features of its catalytic domain. Furthermore, by solving a series of cryo-EM structures of Pol ε at a mismatch-containing DNA, we elucidate how Pol ε senses and edits a misincorporated nucleotide. Our structures delineate steps along an intramolecular switching mechanism between polymerase and exonuclease activities, providing the basis for a proofreading mechanism in B-family replicative polymerases.

Probing the nucleobase selectivity of RNA polymerases with dual-coding substrates

Formycin A (FOR) and pyrazofurin A (PYR) are nucleoside analogs with antiviral and antitumor properties. They are known to interfere with nucleic acid metabolism, but their direct effect on transcription is less understood. We explored how RNA polymerases (RNAPs) from bacteria, mitochondria, and viruses utilize FOR, PYR, and oxidized purine nucleotides. All tested polymerases incorporated FOR in place of adenine and PYR in place of uridine. FOR also exhibited surprising dual-coding behavior, functioning as a cytosine substitute, particularly for viral RNAP. In contrast, 8-oxoadenine and 8-oxoguanine were incorporated in place of uridine in addition to their canonical Watson-Crick codings. Our data suggest that the interconversion of canonical anti and alternative syn conformers underlies dual-coding abilities of FOR and oxidized purines. Structurally distinct RNAPs displayed varying abilities to utilize syn conformers during transcription. By examining base pairings that led to substrate incorporation and the entire spectrum of geometrically compatible pairings, we have gained new insights into the nucleobase selection processes employed by structurally diverse RNAPs. These insights may pave the way for advancements in antiviral therapies.

Collapsed State Mediates the Low Fidelity of the DNA Polymerase β I260 Mutant

DNA polymerase β (Pol β) fills single nucleotide gaps during base excision repair. Deficiencies in Pol β can lead to increased mutagenesis and genomic instability in the cell, resulting in cancer. Our laboratory has previously shown that the I260 M somatic mutation of Pol β, which was first identified in prostate cancer, has reduced nucleotide discrimination in a sequence context-dependent manner. I260 M incorporates the incorrect G opposite A in this context more readily than WT. To identify the molecular mechanism of the reduced fidelity of I260M, we studied incorporation using single turnover kinetics and the nature and rates of conformational changes using steady-state fluorescence and Förster resonance energy transfer (FRET). Our data indicate that the I260 M mutation affects the fingers region of rat Pol β by creating a "collapsed" state in both the open (in the absence of nucleotide) and closed (prior to chemistry) states. I260 M is a temperature-sensitive mutator and binds nucleotides tighter than the WT protein, resulting in reduced fidelity compared to the WT. Additionally, we have generated a kinetic model of WT and I260 M using FRET and single turnover data, which demonstrates that I260 M precatalytic conformation changes differ compared to the WT as it is missing a precatalytic noncovalent step. Taken together, these results suggest that the collapsed state of I260 M may decrease its ability for nucleotide discrimination, illustrating the importance of the "fingers closing" conformational change for polymerase fidelity and accurate DNA synthesis.

Exploring the molecular aspect and updating evolutionary approaches to the DNA polymerase enzymes for biotechnological needs: A comprehensive review

DNA polymerases are essential enzymes that play a key role in living organisms, as they participate in the synthesis and maintenance of the DNA molecule. The intrinsic properties of these enzymes have been widely observed and studied to understand their functions, activities, and behavior, which has allowed their natural power in DNA synthesis to be exploited in modern biotechnology, to the point of making them true pillars of the field. In this context, the laboratory evolution of these enzymes, either by directed evolution or rational design, has led to the generation of a wide range of new DNA polymerases with novel properties, suitable for a variety of biotechnological needs. In this review, we examine DNA polymerases at the molecular level, their biotechnological use, and their evolutionary methods in relation to the novel properties sought, providing a chronological selection of evolved DNA polymerases cited in the literature that we consider to be of great interest. To our knowledge, this work is the first to bring together the molecular, functional and evolutionary aspects of the DNA polymerase enzyme. We believe it will be of great interest to researchers whose aim is to produce new lines of evolved DNA polymerases.

Structural basis for substrate binding and selection by human mitochondrial RNA polymerase

The mechanism by which RNAP selects cognate substrates and discriminates between deoxy and ribonucleotides is of fundamental importance to the fidelity of transcription. Here, we present cryo-EM structures of human mitochondrial transcription elongation complexes that reveal substrate ATP bound in Entry and Insertion Sites. In the Entry Site, the substrate binds along the O helix of the fingers domain of mtRNAP but does not interact with the templating DNA base. Interactions between RNAP and the triphosphate moiety of the NTP in the Entry Site ensure discrimination against nucleosides and their diphosphate and monophosphate derivatives but not against non-cognate rNTPs and dNTPs. Closing of the fingers domain over the catalytic site results in delivery of both the templating DNA base and the substrate into the Insertion Site and recruitment of the catalytic magnesium ions. The cryo-EM data also reveal a conformation adopted by mtRNAP to reject a non-cognate substrate from its active site. Our findings establish a structural basis for substrate binding and suggest a unified mechanism of NTP selection for single-subunit RNAPs.

Coordinated DNA polymerization by Polγ and the region of LonP1 regulated proteolysis

The replicative mitochondrial DNA polymerase, Polγ, and its protein regulation are essential for the integrity of the mitochondrial genome. The intricacies of Polγ regulation and its interactions with regulatory proteins, which are essential for fine-tuning polymerase function, remain poorly understood. Misregulation of the Polγ heterotrimer, consisting of (i) PolG, the polymerase catalytic subunit and (ii) PolG2, the accessory subunit, ultimately results in mitochondrial diseases. Here, we used single particle cryo-electron microscopy to resolve the structure of PolG in its apoprotein state and we captured Polγ at three intermediates within the catalytic cycle: DNA bound, engaged, and an active polymerization state. Chemical crosslinking mass spectrometry, and site-directed mutagenesis uncovered the region of LonP1 engagement of PolG, which promoted proteolysis and regulation of PolG protein levels. PolG2 clinical variants, which disrupted a stable Polγ complex, led to enhanced LonP1-mediated PolG degradation. Overall, this insight into Polγ aids in an understanding of mitochondrial DNA replication and characterizes how machinery of the replication fork may be targeted for proteolytic degradation when improperly functioning.

Non-canonical amino acids uncover the significant impact of Tyr671 on Taq DNA polymerase catalytic activity

Responsible for synthesizing the complementary strand of the DNA template, DNA polymerase is a crucial enzyme in DNA replication, recombination and repair. A highly conserved tyrosine (Tyr), located at the C-terminus of the O-helix in family A DNA polymerases, plays a critical role in enzyme activity and fidelity. Here, we combined the technology of genetic code extension to incorporate non-canonical amino acids and molecular dynamics (MD) simulations to uncover the mechanisms by which Tyr671 impacts substrate binding and conformation transitions in a DNA polymerase from Thermus aquaticus. Five non-canonical amino acids, namely l-3,4-dihydroxyphenylalanine (l-DOPA), p-aminophenylalanine (pAF), p-acetylphenylalanine (pAcF), p-cyanophenylalanine (pCNF) and p-nitrophenylalanine (pNTF), were individually incorporated at position 671. Strikingly, Y671pAF and Y671DOPA were active, but with lower activity compared to Y671F and wild-type. Y671pAF showed a higher fidelity than the Y671F, despite both possessing lower fidelity than the wild-type. Metadynamics and long-timescale MD simulations were carried out to probe the role of mutations in affecting protein structure, including open conformation, open-to-closed conformation transition, closed conformation, and closed-to-open conformation transition. The MD simulations clearly revealed that the size of the 671 amino acid residue and interactions with substrate or nearby residues were critical for Tyr671 to determine enzyme activity and fidelity.

Influence of Nucleotide Context on Non-Specific Amplification of DNA with Bst exo- DNA Polymerase

Isothermal nucleic acids amplification that requires DNA polymerases with strand-displacement activity gained more attention in the last two decades. Among the DNA polymerases with strand-displacement activity, Bst exo- is the most widely used. However, it tends to carry out nonspecific DNA synthesis through multimerization. In this study, the effect of nucleotide sequence on the Bst exo- binding with DNA and on the efficiency of multimerization initiation, are reported. Preference for binding of the "closed" form of Bst exo- to the purine-rich DNA sequences, especially those containing dG at the 3'-end of the growing chain was revealed using molecular docking of the single-stranded trinucleotides (sst) and trinucleotide duplexes (dst). The data obtained in silico were confirmed in the experiments using oligonucleotide templates that differ in the structure of the 3'- and 5'-terminal motifs. It has been shown that templates with the oligopurine 3'-terminal fragment and oligopyrimidine 5'-terminal part contribute to the earlier start of multimerization. The results can be used for design of nucleotide sequences suitable for reliable isothermal amplification. To avoid multimerization, DNA templates and primers containing terminal dA and/or dG nucleotides should be excluded.

Formation and Effects of Upstream DNA-RNA Base Pairing in Telomerase

Telomere elongation by telomerase consists of two types of translocation: duplex translocation during each repeat synthesis and template translocation at the end of repeat synthesis. Our replica exchange molecular dynamics simulations show that in addition to the Watson-Crick interactions in the active site, templating RNA can also form base pairs with the upstream regions of DNA, mostly with the second upstream DNA repeat with respect to the 3'-end. At the end of the repeat synthesis, dG10-P442 and dG11-N446 hydrogen bonds form. Then, active-site base pairs dissociate one by one, and the RNA bases reanneal with the complementary base on the upstream DNA repeat. For each dissociating base pair a new one forms, thus conserving the number of base pairs during template translocation.

Polγ coordinates DNA synthesis and proofreading to ensure mitochondrial genome integrity

Accurate replication of mitochondrial DNA (mtDNA) by DNA polymerase γ (Polγ) is essential for maintaining cellular energy supplies, metabolism, and cell cycle control. To illustrate the structural mechanism for Polγ coordinating polymerase (pol) and exonuclease (exo) activities to ensure rapid and accurate DNA synthesis, we determined four cryo-EM structures of Polγ captured after accurate or erroneous incorporation to a resolution of 2.4-3.0 Å. The structures show that Polγ employs a dual-checkpoint mechanism to sense nucleotide misincorporation and initiate proofreading. The transition from replication to error editing is accompanied by increased dynamics in both DNA and enzyme, in which the polymerase relaxes its processivity and the primer-template DNA unwinds, rotates, and backtracks to shuttle the mismatch-containing primer terminus 32 Å to the exo site for editing. Our structural and functional studies also provide a foundation for analyses of Polγ mutation-induced human diseases and aging.

Reclassification of family A DNA polymerases reveals novel functional subfamilies and distinctive structural features

Family A DNA polymerases (PolAs) form an important and well-studied class of extant polymerases participating in DNA replication and repair. Nonetheless, despite the characterization of multiple subfamilies in independent, dedicated works, their comprehensive classification thus far is missing. We therefore re-examine all presently available PolA sequences, converting their pairwise similarities into positions in Euclidean space, separating them into 19 major clusters. While 11 of them correspond to known subfamilies, eight had not been characterized before. For every group, we compile their general characteristics, examine their phylogenetic relationships and perform conservation analysis in the essential sequence motifs. While most subfamilies are linked to a particular domain of life (including phages), one subfamily appears in Bacteria, Archaea and Eukaryota. We also show that two new bacterial subfamilies contain functional enzymes. We use AlphaFold2 to generate high-confidence prediction models for all clusters lacking an experimentally determined structure. We identify new, conserved features involving structural alterations, ordered insertions and an apparent structural incorporation of a uracil-DNA glycosylase (UDG) domain. Finally, genetic and structural analyses of a subset of T7-like phages indicate a splitting of the 3'-5' exo and pol domains into two separate genes, observed in PolAs for the first time.

Discovery, implications and initial use of semi-synthetic organisms with an expanded genetic alphabet/code

Much recent interest has focused on developing proteins for human use, such as in medicine. However, natural proteins are made up of only a limited number of canonical amino acids with limited functionalities, and this makes the discovery of variants with some functions difficult. The ability to recombinantly express proteins containing non-canonical amino acids (ncAAs) with properties selected to impart the protein with desired properties is expected to dramatically improve the discovery of proteins with different functions. Perhaps the most straightforward approach to such an expansion of the genetic code is through expansion of the genetic alphabet, so that new codon/anticodon pairs can be created to assign to ncAAs. In this review, I briefly summarize more than 20 years of effort leading ultimately to the discovery of synthetic nucleotides that pair to form an unnatural base pair, which when incorporated into DNA, is stably maintained, transcribed and used to translate proteins in Escherichia coli. In addition to discussing wide ranging conceptual implications, I also describe ongoing efforts at the pharmaceutical company Sanofi to employ the resulting 'semi-synthetic organisms' or SSOs, for the production of next-generation protein therapeutics. This article is part of the theme issue 'Reactivity and mechanism in chemical and synthetic biology'.

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.

A two-residue nascent-strand steric gate controls synthesis of 2'-O-methyl- and 2'-O-(2-methoxyethyl)-RNA

Steric exclusion is a key element of enzyme substrate specificity, including in polymerases. Such substrate specificity restricts the enzymatic synthesis of 2'-modified nucleic acids, which are of interest in nucleic-acid-based drug development. Here we describe the discovery of a two-residue, nascent-strand, steric control 'gate' in an archaeal DNA polymerase. We show that engineering of the gate to reduce steric bulk in the context of a previously described RNA polymerase activity unlocks the synthesis of 2'-modified RNA oligomers, specifically the efficient synthesis of both defined and random-sequence 2'-O-methyl-RNA (2'OMe-RNA) and 2'-O-(2-methoxyethyl)-RNA (MOE-RNA) oligomers up to 750 nt. This enabled the discovery of RNA endonuclease catalysts entirely composed of 2'OMe-RNA (2'OMezymes) for the allele-specific cleavage of oncogenic KRAS (G12D) and β-catenin CTNNB1 (S33Y) mRNAs, and the elaboration of mixed 2'OMe-/MOE-RNA aptamers with high affinity for vascular endothelial growth factor. Our results open up these 2'-modified RNAs-used in several approved nucleic acid therapeutics-for enzymatic synthesis and a wider exploration in directed evolution and nanotechnology.

In crystallo observation of three metal ion promoted DNA polymerase misincorporation

Error-free replication of DNA is essential for life. Despite the proofreading capability of several polymerases, intrinsic polymerase fidelity is in general much higher than what base-pairing energies can provide. Although researchers have investigated this long-standing question with kinetics, structural determination, and computational simulations, the structural factors that dictate polymerase fidelity are not fully resolved. Time-resolved crystallography has elucidated correct nucleotide incorporation and established a three-metal-ion-dependent catalytic mechanism for polymerases. Using X-ray time-resolved crystallography, we visualize the complete DNA misincorporation process catalyzed by DNA polymerase η. The resulting molecular snapshots suggest primer 3´-OH alignment mediated by A-site metal ion binding is the key step in substrate discrimination. Moreover, we observe that C-site metal ion binding preceded the nucleotidyl transfer reaction and demonstrate that the C-site metal ion is strictly required for misincorporation. Our results highlight the essential but separate roles of the three metal ions in DNA synthesis.

By observing DNA polymerase misincorporation with time-resolved crystallography, the authors visualize three-metal ion dependent polymerase catalysis and identify A-site metal-mediated primer alignment as a key step in nucleotide discrimination.

Single-molecule Taq DNA polymerase dynamics

Taq DNA polymerase functions at elevated temperatures with fast conformational dynamics-regimes previously inaccessible to mechanistic, single-molecule studies. Here, single-walled carbon nanotube transistors recorded the motions of Taq molecules processing matched or mismatched template-deoxynucleotide triphosphate pairs from 22° to 85°C. By using four enzyme orientations, the whole-enzyme closures of nucleotide incorporations were distinguished from more rapid, 20-μs closures of Taq's fingers domain testing complementarity and orientation. On average, one transient closure was observed for every nucleotide binding event; even complementary substrate pairs averaged five transient closures between each catalytic incorporation at 72°C. The rate and duration of the transient closures and the catalytic events had almost no temperature dependence, leaving all of Taq's temperature sensitivity to its rate-determining open state.

Conformational Dynamics of DNA Polymerases Revealed at the Single-Molecule Level

DNA polymerases are intrinsically dynamic macromolecular machines. The purpose of this review is to describe the single-molecule Förster resonance energy transfer (smFRET) methods that are used to probe the conformational dynamics of DNA polymerases, focusing on E. coli DNA polymerase I. The studies reviewed here reveal the conformational dynamics underpinning the nucleotide selection, proofreading and 5′ nuclease activities of Pol I. Moreover, the mechanisms revealed for Pol I are likely employed across the DNA polymerase family. smFRET methods have also been used to examine other aspects of DNA polymerase activity.

The Rate-limiting Step of DNA Synthesis by DNA Polymerase Occurs in the Fingers-closed Conformation

DNA polymerases maintain genomic integrity by copying DNA with high fidelity, part of which relies on the polymerase fingers opening-closing transition, a series of conformational changes during the DNA synthesis reaction cycle. Fingers opening and closing has been challenging to study, mainly due to the need to synchronise molecular ensembles. We previously studied fingers opening-closing on single polymerase-DNA complexes using single-molecule FRET; however, our work was limited to pre-chemistry reaction steps. Here, we advance our analysis to extensible substrates, and observe DNA polymerase (Pol) conformational changes across the entire DNA polymerisation reaction in real-time, gaining direct access to an elusive post-chemistry step rate-limiting for DNA synthesis. Our results showed that Pol adopts the fingers-closed conformation during polymerisation, and that the post-chemistry rate-limiting step occurs in the fingers-closed conformation. We found that fingers-opening in the Pol-DNA binary complex in the absence of polymerisation is slow (∼5.3 s-1), and comparable to the rate of fingers-opening after polymerisation (3.4 s-1); this indicates that the fingers-opening step itself could be largely responsible for the slow post-chemistry step, with the residual rate potentially accounted for by pyrophosphase release. We also observed that DNA chain-termination of the 3' end of the primer increases substantially the rate of fingers-opening in the Pol-DNA binary complex (5.3 → 29 s-1), demonstrating that the 3'-OH residue is important for the kinetics of fingers conformational changes. Our observations offer mechanistic insight and tools to offer mechanistic insight for all nucleic acid polymerases.

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.

Structure of New Binary and Ternary DNA Polymerase Complexes From Bacteriophage RB69

DNA polymerase plays a critical role in passing the genetic information of any living organism to its offspring. DNA polymerase from enterobacteria phage RB69 (RB69pol) has both polymerization and exonuclease activities and has been extensively studied as a model system for B-family DNA polymerases. Many binary and ternary complex structures of RB69pol are known, and they all contain a single polymerase-primer/template (P/T) DNA complex. Here, we report a crystal structure of the exonuclease-deficient RB69pol with the P/T duplex in a dimeric form at a resolution of 2.2 Å. The structure includes one new closed ternary complex with a single divalent metal ion bound and one new open binary complex in the pre-insertion state with a vacant dNTP-binding pocket. These complexes suggest that initial binding of the correct dNTP in the open state is much weaker than expected and that initial binding of the second divalent metal ion in the closed state is also much weaker than measured. Additional conformational changes are required to convert these complexes to high-affinity states. Thus, the measured affinities for the correct incoming dNTP and divalent metal ions are average values from many conformationally distinctive states. Our structure provides new insights into the order of the complex assembly involving two divalent metal ions. The biological relevance of specific interactions observed between one RB69pol and the P/T duplex bound to the second RB69pol observed within this dimeric complex is discussed.

Structural dynamics and determinants of 2-aminoadenine specificity in DNA polymerase DpoZ of vibriophage ϕVC8

All genetic information in cellular life is stored in DNA copolymers composed of four basic building blocks (ATGC-DNA). In contrast, a group of bacteriophages belonging to families Siphoviridae and Podoviridae has abandoned the usage of one of them, adenine (A), replacing it with 2-aminoadenine (Z). The resulting ZTGC-DNA is more stable than its ATGC-DNA counterpart, owing to the additional hydrogen bond present in the 2-aminoadenine:thymine (Z:T) base pair, while the additional amino group also confers resistance to the host endonucleases. Recently, two classes of replicative proteins found in ZTGC-DNA-containing phages were characterized and one of them, DpoZ from DNA polymerase A (PolA) family, was shown to possess significant Z-vs-A specificity. Here, we present the crystallographic structure of the apo form of DpoZ of vibriophage ϕVC8, composed of the 3′-5′ exonuclease and polymerase domains. We captured the enzyme in two conformations that involve the tip of the thumb subdomain and the exonuclease domain. We highlight insertions and mutations characteristic of ϕVC8 DpoZ and its close homologues. Through mutagenesis and functional assays we suggest that the preference of ϕVC8 DpoZ towards Z relies on a polymerase backtracking process, more efficient when the nascent base pair is A:T than when it is Z:T.

Synthesis and Polymerase Recognition of Threose Nucleic Acid Triphosphates Equipped with Diverse Chemical Functionalities

Expanding the chemical space of evolvable non-natural genetic polymers (XNAs) to include functional groups that enhance protein target binding affinity offers a promising route to therapeutic aptamers with high biological stability. Here we describe the chemical synthesis and polymerase recognition of 10 chemically diverse functional groups introduced at the C-5 position of α-l-threofuranosyl uridine nucleoside triphosphate (tUTP). We show that the set of tUTP substrates is universally recognized by the laboratory-evolved polymerase Kod-RSGA. Insights into the mechanism of TNA synthesis were obtained from a high-resolution X-ray crystal structure of the postcatalytic complex bound to the primer-template duplex. A structural analysis reveals a large cavity in the enzyme active site that can accommodate the side chain of C-5-modified tUTP substrates. Our findings expand the chemical space of evolvable nucleic acid systems by providing a synthetic route to artificial genetic polymers that are uniformly modified with diversity-enhancing functional groups.

Creation, Optimization, and Use of Semi-Synthetic Organisms that Store and Retrieve Increased Genetic Information

With few exceptions, natural proteins are built from only 20 canonical (proteogenic) amino acids which limits the functionality and accordingly the properties they can possess. Genetic code expansion, i.e. the creation of codons and the machinery needed to assign them to non-canonical amino acids (ncAAs), promises to enable the discovery of proteins with novel properties that are otherwise difficult or impossible to obtain. One approach to expanding the genetic code is to expand the genetic alphabet via the development of unnatural nucleotides that pair to form an unnatural base pair (UBP). Semi-synthetic organisms (SSOs), i.e. organisms that stably maintain the UBP, transcribe its component nucleotides into RNA, and use it to translate proteins, would have available to them new codons and the anticodons needed to assign them to ncAAs. This review summarizes the development of a family of UBPs, their use to create SSOs, and the optimization and application of the SSOs to produce candidate therapeutic proteins with improved properties that are now undergoing evaluation in clinical trials.

Selenium atom on phosphate enhances specificity and sensitivity of DNA polymerization and detection

DNA polymerization is of high specificity in vivo. However, its specificity is much lower in vitro, which limits advanced applications of DNA polymerization in ultrasensitive nucleic acid detection. Herein, we report a unique mechanism of single selenium-atom modified dNTP (dNTPαSe) to enhance polymerization specificity. We have found that both dNTPαSe (approximately 660 fold) and Se-DNA (approximately 2.8 fold) have lower binding affinity to DNA polymerase than canonical ones, and the Se-DNA duplex has much lower melting-temperature (Tm) than the corresponding canonical DNA duplex. The reduced affinity and Tm can destabilize the substrate-primer-template-enzyme assembly, thereby largely slowing down the mismatch of DNA polymerization and enhancing the amplification specificity and in turn detection sensitivity. Furthermore, the Se-strategy enables us to develop the selenium enhanced specific isothermal amplification (SEA) for nucleic acid detection with high specificity and sensitivity (up to detection of single-digit copies), allowing convenient detection of clinical HPV and COVID-19 viruses in the low-copy number. Clearly, we have discovered the exciting mechanism for enhancing DNA polymerization accuracy, amplification specificity and detection sensitivity by SEA, up to two orders of magnitude higher.

Allosteric and dynamic control of RNA-dependent RNA polymerase function and fidelity

All RNA viruses encode an RNA-dependent RNA polymerase (RdRp) responsible for genome replication. It is now recognized that enzymes in general, and RdRps specifically, are dynamic macromolecular machines such that their moving parts, including active site loops, play direct functional roles. While X-ray crystallography has provided deep insight into structural elements important for RdRp function, this methodology generally provides only static snapshots, and so is limited in its ability to report on dynamic fluctuations away from the lowest energy conformation. Nuclear magnetic resonance (NMR), molecular dynamics (MD) simulations and other biophysical techniques have brought new insight into RdRp function by their ability to characterize the trajectories, kinetics and thermodynamics of conformational motions. In particular, these methodologies have identified coordinated motions among conserved structural motifs necessary for nucleotide selection and incorporation. Disruption of these motions through amino acid substitutions or inhibitor binding impairs RdRp function. Understanding and re-engineering these motions thus provides exciting new avenues for anti-viral strategies. This chapter outlines the basics of these methodologies, summarizes the dynamic motions observed in different RdRps important for nucleotide selection and incorporation, and illustrates how this information can be leveraged towards rational vaccine strain development and anti-viral drug design.

Multiscale Modeling of Wobble to Watson-Crick-Like Guanine-Uracil Tautomerization Pathways in RNA

Energetically unfavorable Watson–Crick (WC)-like tautomeric forms of nucleobases are known to introduce spontaneous mutations, and contribute to replication, transcription, and translation errors. Recent NMR relaxation dispersion techniques were able to show that wobble (w) G•U mispair exists in equilibrium with the short-lived, low-population WC-like enolic tautomers. Presently, we have investigated the wG•U → WC-like enolic reaction pathway using various theoretical methods: quantum mechanics (QM), molecular dynamics (MD), and combined quantum mechanics/molecular mechanics (QM/MM). The previous studies on QM gas phase calculations were inconsistent with experimental data. We have also explored the environmental effects on the reaction energies by adding explicit water. While the QM-profile clearly becomes endoergic in the presence of water, the QM/MM-profile remains consistently endoergic in the presence and absence of water. Hence, by including microsolvation and QM/MM calculations, the experimental data can be explained. For the G•Uenol→ Genol•U pathway, the latter appears to be energetically more favorable throughout all computational models. This study can be considered as a benchmark of various computational models of wG•U to WC-like tautomerization pathways with and without the environmental effects, and may contribute on further studies of other mispairs as well.

Following replicative DNA synthesis by time-resolved X-ray crystallography

The mechanism of DNA synthesis has been inferred from static structures, but the absence of temporal information raises longstanding questions about the order of events in one of life's most central processes. Here we follow the reaction pathway of a replicative DNA polymerase using time-resolved X-ray crystallography to elucidate the order and transition between intermediates. In contrast to the canonical model, the structural changes observed in the time-lapsed images reveal a catalytic cycle in which translocation precedes catalysis. The translocation step appears to follow a push-pull mechanism where the O-O1 loop of the finger subdomain acts as a pawl to facilitate unidirectional movement along the template with conserved tyrosine residues 714 and 719 functioning as tandem gatekeepers of DNA synthesis. The structures capture the precise order of critical events that may be a general feature of enzymatic catalysis among replicative DNA polymerases.

Specificity Enhancement of Deoxyribonucleic Acid Polymerization for Sensitive Nucleic Acid Detection

Specificity of DNA polymerization plays a critical role in DNA replication and storage of genetic information. Likewise, biotechnological applications, such as nucleic acid detection, DNA amplification, and gene cloning, require high specificity in DNA synthesis catalyzed by DNA polymerases. However, errors in DNA polymerization (such as mis-incorporation and mis-priming) can significantly jeopardize the specificity. Herein, we report our discovery that the specificity of DNA enzymatic synthesis can be substantially enhanced (up to 100-fold higher) by attenuating DNA polymerase kinetics via the phosphorothioate dNTPs. This specificity enhancement allows convenient and sensitive nucleic acid detection, polymerization, PCR, and gene cloning with complex systems (such as human cDNA and genomic DNA). Further, we found that the specificity enhancement offered higher sensitivity (up to 50-fold better) for detecting nucleic acids, such as COVID-19 viral RNAs. Our findings have revealed a simple and convenient strategy for facilitating specificity and sensitivity of nucleic acid detection, amplification, and gene cloning.

Engineering polymerases for applications in synthetic biology

DNA polymerases play a central role in biology by transferring genetic information from one generation to the next during cell division. Harnessing the power of these enzymes in the laboratory has fueled an increase in biomedical applications that involve the synthesis, amplification, and sequencing of DNA. However, the high substrate specificity exhibited by most naturally occurring DNA polymerases often precludes their use in practical applications that require modified substrates. Moving beyond natural genetic polymers requires sophisticated enzyme-engineering technologies that can be used to direct the evolution of engineered polymerases that function with tailor-made activities. Such efforts are expected to uniquely drive emerging applications in synthetic biology by enabling the synthesis, replication, and evolution of synthetic genetic polymers with new physicochemical properties.

Using single-molecule FRET to probe the nucleotide-dependent conformational landscape of polymerase β-DNA complexes

Eukaryotic DNA polymerase β (Pol β) plays an important role in cellular DNA repair, as it fills short gaps in dsDNA that result from removal of damaged bases. Since defects in DNA repair may lead to cancer and genetic instabilities, Pol β has been extensively studied, especially its mechanisms for substrate binding and a fidelity-related conformational change referred to as "fingers closing." Here, we applied single-molecule FRET to measure distance changes associated with DNA binding and prechemistry fingers movement of human Pol β. First, using a doubly labeled DNA construct, we show that Pol β bends the gapped DNA substrate less than indicated by previously reported crystal structures. Second, using acceptor-labeled Pol β and donor-labeled DNA, we visualized dynamic fingers closing in single Pol β-DNA complexes upon addition of complementary nucleotides and derived rates of conformational changes. We further found that, while incorrect nucleotides are quickly rejected, they nonetheless stabilize the polymerase-DNA complex, suggesting that Pol β, when bound to a lesion, has a strong commitment to nucleotide incorporation and thus repair. In summary, the observation and quantification of fingers movement in human Pol β reported here provide new insights into the delicate mechanisms of prechemistry nucleotide selection.

Environmental Effects on Guanine-Thymine Mispair Tautomerization Explored with Quantum Mechanical/Molecular Mechanical Free Energy Simulations

DNA bases can adopt energetically unfavorable tautomeric forms that enable the formation of Watson-Crick-like (WC-like) mispairs, which have been proposed to give rise to spontaneous mutations in DNA and misincorporation errors in DNA replication and translation. Previous NMR and computational studies have indicated that the population of WC-like guanine-thymine (G-T) mispairs depends on the environment, such as the local nucleic acid sequence and solvation. To investigate these environmental effects, herein G-T mispair tautomerization processes are studied computationally in aqueous solution, in A-form and B-form DNA duplexes, and within the active site of a DNA polymerase λ variant. The wobble G-T (wG-T), WC-like G-T*, and WC-like G*-T forms are considered, where * indicates the enol tautomer of the base. The minimum free energy paths for the tautomerization from the wG-T to the WC-like G-T* and from the WC-like G-T* to the WC-like G*-T are computed with mixed quantum mechanical/molecular mechanical (QM/MM) free energy simulations. The reaction free energies and free energy barriers are found to be significantly influenced by the environment. The wG-T→G-T* tautomerization is predicted to be endoergic in aqueous solution and the DNA duplexes but slightly exoergic in the polymerase, with Arg517 and Asn513 providing electrostatic stabilization of G-T*. The G-T*→G*-T tautomerization is also predicted to be slightly more thermodynamically favorable in the polymerase relative to these DNA duplexes. These simulations are consistent with an experimentally driven kinetic misincorporation model suggesting that G-T mispair tautomerization occurs in the ajar polymerase conformation or concertedly with the transition from the ajar to the closed polymerase conformation. Furthermore, the order of the associated two proton transfer reactions is predicted to be different in the polymerase than in aqueous solution and the DNA duplexes. These studies highlight the impact of the environment on the thermodynamics, kinetics, and fundamental mechanisms of G-T mispair tautomerization, which plays a role in a wide range of biochemically important processes.

The Structural Basis for Processing of Unnatural Base Pairs by DNA Polymerases

Unnatural base pairs (UBPs) greatly increase the diversity of DNA and RNA, furthering their broad range of molecular biological and biotechnological approaches. Different candidates have been developed whereby alternative hydrogen-bonding patterns and hydrophobic and packing interactions have turned out to be the most promising base-pairing concepts to date. The key in many applications is the highly efficient and selective acceptance of artificial base pairs by DNA polymerases, which enables amplification of the modified DNA. In this Review, computational as well as experimental studies that were performed to characterize the pairing behavior of UBPs in free duplex DNA or bound to the active site of KlenTaq DNA polymerase are highlighted. The structural studies, on the one hand, elucidate how base pairs lacking hydrogen bonds are accepted by these enzymes and, on the other hand, highlight the influence of one or several consecutive UBPs on the structure of a DNA double helix. Understanding these concepts facilitates optimization of future UBPs for the manifold fields of applications.

Rational Control of Poliovirus RNA-Dependent RNA Polymerase Fidelity by Modulating Motif-D Loop Conformational Dynamics

The conserved structural motif D is an important determinant of the speed and fidelity of viral RNA-dependent RNA polymerases (RdRps). Structural and computational studies have suggested that conformational changes in the motif-D loop that help to reposition the catalytic lysine represent critical steps in nucleotide selection and incorporation. Conformations of the motif-D loop in the poliovirus RdRp are likely controlled in part by noncovalent interactions involving the motif-D residue Glu364. This residue swivels between making interactions with Lys228 and Asn370 to stabilize the open and closed loop conformations, respectively. We show here that we can rationally control the motif-D loop conformation by breaking these interactions. The K228A variant favors a more active closed conformation, leading to increased nucleotide incorporation rates and decreased nucleotide selectivity, and the N370A variant favors a less active open conformation, leading to decreased nucleotide incorporation rates and increased nucleotide selectivity. Similar competing interactions likely control nucleotide incorporation rates and fidelity in other viral RdRps. Rational engineering of these interactions may be important in the generation of live, attenuated vaccine strains, considering the established relationships between RdRp function and viral pathogenesis.

I260Q DNA polymerase β highlights precatalytic conformational rearrangements critical for fidelity

DNA polymerase β (pol β) fills single nucleotide gaps in DNA during base excision repair and non-homologous end-joining. Pol β must select the correct nucleotide from among a pool of four nucleotides with similar structures and properties in order to maintain genomic stability during DNA repair. Here, we use a combination of X-ray crystallography, fluorescence resonance energy transfer and nuclear magnetic resonance to show that pol β‘s ability to access the appropriate conformations both before and upon binding to nucleotide substrates is integral to its fidelity. Importantly, we also demonstrate that the inability of the I260Q mutator variant of pol β to properly navigate this conformational landscape results in error-prone DNA synthesis. Our work reveals that precatalytic conformational rearrangements themselves are an important underlying mechanism of substrate selection by DNA pol β.

Networked Communication between Polymerase and Exonuclease Active Sites in Human Mitochondrial DNA Polymerase

High fidelity human mitochondrial DNA polymerase (Pol γ) contains two active sites, a DNA polymerization site (pol) and a 3'-5' exonuclease site (exo) for proofreading. Although separated by 35 Å, coordination between the pol and exo sites is crucial to high fidelity replication. The biophysical mechanisms for this coordination are not completely understood. To understand the communication between the two active sites, we used a statistical-mechanical model of the protein ensemble to calculate the energetic landscape and local stability. We compared a series of structures of Pol γ, complexed with primer/template DNA, and either a nucleotide substrate or a series of nucleotide analogues, which are differentially incorporated and excised by pol and exo activity. Despite the nucleotide or its analogues being bound in the pol, Pol γ residue stability varied across the protein, particularly in the exo domain. This suggests that substrate presence in the pol can be "sensed" in the exo domain. Consistent with this hypothesis, in silico mutations made in one active site mutually perturbed the energetics of the other. To identify specific regions of the polymerase that contributed to this communication, we constructed an allosteric network connectivity map that further demonstrates specific pol-exo cooperativity. Thus, a cooperative network underlies energetic connectivity. We propose that Pol γ and other dual-function polymerases exploit an energetic coupling network that facilitates domain-domain communication to enhance discrimination between correct and incorrect nucleotides.

Determining selection free energetics from nucleotide pre-insertion to insertion in viral T7 RNA polymerase transcription fidelity control

An elongation cycle of a transcribing RNA polymerase (RNAP) usually consists of multiple kinetics steps, so there exist multiple kinetic checkpoints where non-cognate nucleotides can be selected against. We conducted comprehensive free energy calculations on various nucleotide insertions for viral T7 RNAP employing all-atom molecular dynamics simulations. By comparing insertion free energy profiles between the non-cognate nucleotide species (rGTP and dATP) and a cognate one (rATP), we obtained selection free energetics from the nucleotide pre-insertion to the insertion checkpoints, and further inferred the selection energetics down to the catalytic stage. We find that the insertion of base mismatch rGTP proceeds mainly through an off-path along which both pre-insertion screening and insertion inhibition play significant roles. In comparison, the selection against dATP is found to go through an off-path pre-insertion screening along with an on-path insertion inhibition. Interestingly, we notice that two magnesium ions switch roles of leave and stay during the dATP on-path insertion. Finally, we infer that substantial selection energetic is still required to catalytically inhibit the mismatched rGTP to achieve an elongation error rate ∼10-4 or lower; while no catalytic selection seems to be further needed against dATP to obtain an error rate ∼10-2.

C, Da LT, Yu J. A Viral T7 RNA Polymerase Ratcheting Along DNA With Fidelity Control

RNA polymerase (RNAP) from bacteriophage T7 is a representative single-subunit viral RNAP that can transcribe with high promoter activities without assistances from transcription factors. We accordingly studied this small transcription machine computationally as a model system to understand underlying mechanisms of mechano-chemical coupling and fidelity control in the RNAP transcription elongation. Here we summarize our computational work from several recent publications to demonstrate first how T7 RNAP translocates via Brownian alike motions along DNA right after the catalytic product release. Then we show how the backward translocation motions are prevented at post-translocation upon successful nucleotide incorporation, which is also subject to stepwise nucleotide selection and acts as a pawl for "selective ratcheting". The structural dynamics and energetics features revealed from our atomistic molecular dynamics (MD) simulations and related analyses on the single-subunit T7 RNAP thus provided detailed and quantitative characterizations on the Brownian-ratchet working scenario of a prototypical transcription machine with sophisticated nucleotide selectivity for fidelity control. The presented mechanisms can be more or less general for structurally similar viral or mitochondrial RNAPs and some of DNA polymerases, or even for the RNAP engine of the more complicated transcription machinery in higher organisms.

Single-molecule FRET reveals proofreading complexes in the large fragment of Bacillus stearothermophilus DNA polymerase I

There is increasing interest in the use of DNA polymerases (DNA pols) in next-generation sequencing strategies. These methodologies typically rely on members of the A and B family of DNA polymerases that are classified as high-fidelity DNA polymerases. These enzymes possess the ability to selectively incorporate the correct nucleotide opposite a templating base with an error frequency of only 1 in 10⁶ insertion events. How they achieve this remarkable fidelity has been the subject of numerous investigations, yet the mechanism by which these enzymes achieve this level of accuracy remains elusive. Several smFRET assays were designed to monitor the conformational changes associated with the nucleotide selection mechanism(s) employed by DNA pols. smFRET has also been used to monitor the movement of DNA pols along a DNA substrate as well as to observe the formation of proof-reading complexes. One member among this class of enzymes, the large fragment of Bacillus stearothermophilus DNA polymerase I (Bst pol I LF), contains both 5'→3' polymerase and 3'→5' exonuclease domains, but reportedly lacks exonuclease activity. We have designed a smFRET assay showing that Bst pol I LF forms proofreading complexes. The formation of proofreading complexes at the single molecule level is strongly influenced by the presence of the 3' hydroxyl at the primer-terminus of the DNA substrate. Our assays also identify an additional state, observed in the presence of a mismatched primer-template terminus, that may be involved in the transfer of the primer-terminus from the polymerase to the exonuclease active site.

Expansion of the Genetic Alphabet: A Chemist's Approach to Synthetic Biology

The information available to any organism is encoded in a four nucleotide, two base pair genetic code. Since its earliest days, the field of synthetic biology has endeavored to impart organisms with novel attributes and functions, and perhaps the most fundamental approach to this goal is the creation of a fifth and sixth nucleotide that pair to form a third, unnatural base pair (UBP) and thus allow for the storage and retrieval of increased information. Achieving this goal, by definition, requires synthetic chemistry to create unnatural nucleotides and a medicinal chemistry-like approach to guide their optimization. With this perspective, almost 20 years ago we began designing unnatural nucleotides with the ultimate goal of developing UBPs that function in vivo, and thus serve as the foundation of semi-synthetic organisms (SSOs) capable of storing and retrieving increased information. From the beginning, our efforts focused on the development of nucleotides that bear predominantly hydrophobic nucleobases and thus that pair not based on the complementary hydrogen bonds that are so prominent among the natural base pairs but rather via hydrophobic and packing interactions. It was envisioned that such a pairing mechanism would provide a basal level of selectivity against pairing with natural nucleotides, which we expected would be the greatest challenge; however, this choice mandated starting with analogs that have little or no homology to their natural counterparts and that, perhaps not surprisingly, performed poorly. Progress toward their optimization was driven by the construction of structure–activity relationships, initially from in vitro steady-state kinetic analysis, then later from pre-steady-state and PCR-based assays, and ultimately from performance in vivo, with the results augmented three times with screens that explored combinations of the unnatural nucleotides that were too numerous to fully characterize individually. The structure–activity relationship data identified multiple features required by the UBP, and perhaps most prominent among them was a substituent ortho to the glycosidic linkage that is capable of both hydrophobic packing and hydrogen bonding, and nucleobases that stably stack with flanking natural nucleobases in lieu of the potentially more stabilizing stacking interactions afforded by cross strand intercalation. Most importantly, after the examination of hundreds of unnatural nucleotides and thousands of candidate UBPs, the efforts ultimately resulted in the identification of a family of UBPs that are well recognized by DNA polymerases when incorporated into DNA and that have been used to create SSOs that store and retrieve increased information. In addition to achieving a longstanding goal of synthetic biology, the results have important implications for our understanding of both the molecules and forces that can underlie biological processes, so long considered the purview of molecules benefiting from eons of evolution, and highlight the promise of applying the approaches and methodologies of synthetic and medical chemistry in the pursuit of synthetic biology.

Insights into the effect of minor groove interactions and metal cofactors on mutagenic replication by human DNA polymerase β

DNA polymerases accommodate various base-pair conformations in the event of incorrect insertions. In particular, Watson-Crick-like dG:dTTP base pair has been observed at the insertion site of human DNA polymerase β (pol β). A potential factor contributing to the diverse conformations of base-pair mismatches is minor groove interactions. To gain insights into the effect of minor groove interactions on base-pair conformations, we generated an Asn279Ala polβ mutant that cannot make minor groove contacts with an incoming nucleotide. We conducted structural and kinetic studies of Asn279Ala polβ in complex with incoming dTTP and templating dG or O6-methyl-dG. The crystal structure of the Asn279Ala polβ-G:T complex showed a wobble dG:dTTP base pair, indicating that the previously observed Watson-Crick-like dG:dTTP conformation was induced by the minor groove contact. In contrast, O6-methyl-dG, an analog of the enol tautomer of guanine, formed a Watson-Crick-like base pair with dTTP in the absence of the minor groove contact. These results suggest that the Watson-Crick-like G:T base pair at the insertion site is formed by the rare enol tautomers of G or T, whose population is increased by the minor groove hydrogen bond with Asn279. Kinetic studies showed that Asn279Ala mutation decreased dG:dTTP misincorporation rate six-fold in the presence of Mg²⁺ but increased the rate three-fold in the presence of Mn²⁺, highlighting the effect of minor groove interactions and metal ions on promutagenic replication by polβ.

Investigation of Nascent Base Pair and Polymerase Behavior in the Presence of Mismatches in DNA Polymerase I Using Molecular Dynamics

Optimizing DNA polymerases for a broad range of tasks requires an understanding of the factors influencing polymerase fidelity, but many details of polymerase behavior remain unknown, especially in the presence of mismatched nascent base pairs. Using molecular dynamics, the large fragment of Bacillus stearothermophilus DNA polymerase I is simulated in the presence of all 16 possible standard nucleoside triphosphate-template (dNTP-dN) pairs, including four Watson-Crick pairs and 12 mismatches. The precatalytic steps of nucleotide addition from nucleotide insertion to immediately preceding catalysis are explored using three starting structures representing different stages of nucleotide addition. From these simulations, interactions between dNTPs and the DNA-protein complex formed by the polymerase are elucidated. Patterns of large-scale conformational shifts, classification of nucleotide pairs based on composition, and investigation of the roles of residues interacting with dNTPs are completed on 50+ μs of simulation. The role of molecular dynamics in studies of polymerase behavior is discussed.

Active Site Interactions Impact Phosphoryl Transfer during Replication of Damaged and Undamaged DNA by Escherichia coli DNA Polymerase I

Replicative DNA polymerases are able to discriminate between very similar substrates with high accuracy. One mechanism by which E. coli DNA polymerase I checks for Watson-Crick geometry is through a hydrogen bonding fork between Arg668 and the incoming dNTP and the minor groove of the primer terminus. The importance of the Arg-fork was examined by disrupting it with either a guanine to 3-deazaguanine substitution at the primer terminus or the use of a carbocyclic deoxyribose analog of dUTP. Using thio-substituted dNTPs and differential quench techniques, we determined that when the Arg-fork was disrupted, the rate-limiting step changed from a conformational change to phosphodiester bond formation. This result indicates that Arg668 is involved in the phosphoryl transfer step. We examined the role of the Arg-fork in the replication of four DNA damaged templates, O⁶-methylguanine (O⁶-mG), 8-oxo-7,8-dihydroguanine (oxoG), O²-[4-(3-pyridyl)-4-oxobutyl]thymine (O²-POB-T), and N²-[(7S,8R,9S,10R)-7,8,9,10-tetrahydro-8,9,10-trihydroxybenzo[a]pyren-7-yl]-guanine (N²-BP-G). In general, the guanine to 3-deazaguanine substitution caused a decrease in k_pol that was proportional to k_pol over five orders of magnitude. The linear relationship indicates that the Arg668-fork helps catalyze phosphoryl transfer by the same mechanism with all the substrates. Exceptions to the linear relationship were the incorporations of dTTP opposite G, oxoG, and O⁶mG, which showed large decreases in k_pol, similar to that exhibited by the Watson-Crick base pairs. It was proposed that the incorporation of dTTP opposite G, oxoG, and O⁶mG occurred via Watson-Crick-like structures.

Recent progress in dissecting molecular recognition by DNA polymerases with non-native substrates

DNA polymerases must discriminate the correct Watson-Crick base pair-forming deoxynucleoside triphosphate (dNTP) substrate from three other dNTPs and additional triphosphates found in the cell. The rarity of misincorporations in vivo, then, belies the high tolerance for dNTP analogs observed in vitro. Advances over the last 10 years in single-molecule fluorescence and electronic detection of dNTP analog incorporation enable exploration of the mechanism and limits to base discrimination by DNA polymerases. Such studies reveal transient motions of DNA polymerase during substrate recognition and mutagenesis in the context of erroneous dNTP incorporation that can lead to evolution and genetic disease. Further improvements in time resolution and noise reduction of single-molecule studies will uncover deeper mechanistic understanding of this critical, first step in evolution.

Bunyaviridae RdRps: structure, motifs, and RNA synthesis machinery

Bunyaviridae family is the largest and most diverse family of RNA viruses. It has more than 350 members divided into five genera: Orthobunyavirus, Phlebovirus, Nairovirus, Hantavirus, and Tospovirus. They are present in the five continents, causing recurrent epidemics, epizootics, and considerable agricultural loss. The genome of bunyaviruses is divided into three segments of negative single-stranded RNA according to their relative size: L (Large), M (Medium) and S (Small) segment. Bunyaviridae RNA-dependent RNA polymerase (RdRp) is encoded by the L segment, and is in charge of the replication and transcription of the viral RNA in the cytoplasm of the infected cell. Viral RdRps share a characteristic right hand-like structure with three subdomains: finger, palm, and thumb subdomains that define the formation of the catalytic cavity. In addition to the N-terminal endonuclease domain, eight conserved motifs (A-H) have been identified in the RdRp of Bunyaviridae. In this review, we have summarized the recent insights from the structural and functional studies of RdRp to understand the roles of different motifs shared by RdRps, the mechanism of viral RNA replication, genome segment packaging by the nucleoprotein, cap-snatching, mRNA transcription, and other RNA mechanisms of bunyaviruses.

Triphosphate Reorientation of the Incoming Nucleotide as a Fidelity Checkpoint in Viral RNA-dependent RNA Polymerases

The nucleotide incorporation fidelity of the viral RNA-dependent RNA polymerase (RdRp) is important for maintaining functional genetic information but, at the same time, is also important for generating sufficient genetic diversity to escape the bottlenecks of the host's antiviral response. We have previously shown that the structural dynamics of the motif D loop are closely related to nucleotide discrimination. Previous studies have also suggested that there is a reorientation of the triphosphate of the incoming nucleotide, which is essential before nucleophilic attack from the primer RNA 3'-hydroxyl. Here, we have used ³¹P NMR with poliovirus RdRp to show that the binding environment of the triphosphate is different when correct versus incorrect nucleotide binds. We also show that amino acid substitutions at residues known to interact with the triphosphate can alter the binding orientation/environment of the nucleotide, sometimes lead to protein conformational changes, and lead to substantial changes in RdRp fidelity. The analyses of other fidelity variants also show that changes in the triphosphate binding environment are not always accompanied by changes in the structural dynamics of the motif D loop or other regions known to be important for RdRp fidelity, including motif B. Altogether, our studies suggest that the conformational changes in motifs B and D, and the nucleoside triphosphate reorientation represent separable, "tunable" fidelity checkpoints.

DNA Polymerase ν Rapidly Bypasses O6-Methyl-dG but Not O6-[4-(3-Pyridyl)-4-oxobutyl-dG and O2-Alkyl-dTs

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent tobacco carcinogen that forms mutagenic DNA adducts including O6-methyl-2'-deoxyguanosine (O6-Me-dG), O6-[4-(3-pyridyl)-4-oxobut-1-yl]-dG (O6-POB-dG), O2-methylthymidine (O2-Me-dT), and O2-POB-dT. We evaluated the ability of human DNA polymerase ν to bypass this damage to evaluate the structural constraints on substrates for pol ν and to evaluate if there is kinetic evidence suggesting the in vivo activity of pol ν on tobacco-induced DNA damage. Presteady-state kinetic analysis has indicated that O6-Me-dG is a good substrate for pol ν, while O6-POB-dG and the O2-alkyl-dT adducts are poor substrates for pol ν. The reactivity with O6-Me-dG is high with a preference for dCTP > dGTP > dATP > dTTP. The catalytic activity of pol ν toward O6-Me-dG is high and can potentially be involved in its bypass in vivo. In contrast, pol ν is unlikely to bypass O6-POB-dG or the O2-alkyl-dTs in vivo.

Probing the Conformational Landscape of DNA Polymerases Using Diffusion-Based Single-Molecule FRET

Monitoring conformational changes in DNA polymerases using single-molecule Förster resonance energy transfer (smFRET) has provided new tools for studying fidelity-related mechanisms that promote the rejection of incorrect nucleotides before DNA synthesis. In addition to the previously known open and closed conformations of DNA polymerases, our smFRET assays utilizing doubly labeled variants of Escherichia coli DNA polymerase I were pivotal in identifying and characterizing a partially closed conformation as a primary checkpoint for nucleotide selection. Here, we provide a comprehensive overview of the methods we used for the conformational analysis of wild-type DNA polymerase and some of its low-fidelity derivatives; these methods include strategies for protein labeling and our procedures for solution-based single-molecule fluorescence data acquisition and data analysis. We also discuss alternative single-molecule fluorescence strategies for analyzing the conformations of DNA polymerases in vitro and in vivo.

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.

Molecular events during translocation and proofreading extracted from 200 static structures of DNA polymerase

DNA polymerases in family B are workhorses of DNA replication that carry out the bulk of the job at a high speed with high accuracy. A polymerase in this family relies on a built-in exonuclease for proofreading. It has not been observed at the atomic resolution how the polymerase advances one nucleotide space on the DNA template strand after a correct nucleotide is incorporated, that is, a process known as translocation. It is even more puzzling how translocation is avoided after the primer strand is excised by the exonuclease and returned back to the polymerase active site once an error occurs. The structural events along the bifurcate pathways of translocation and proofreading have been unwittingly captured by hundreds of structures in Protein Data Bank. This study analyzes all available structures of a representative member in family B and reveals the orchestrated event sequence during translocation and proofreading.