Kinetic basis for the differing response to an oxidative lesion by a replicative and a lesion bypass DNA polymerase from Sulfolobus solfataricus

Modulation of Kinase Activities In Vitro by Hepatitis C Virus Protease NS3/NS4A Mediated-Cleavage of Key Immune Modulator Kinases

Hepatitis C Virus NS3/NS4A, a serine protease complex, has been found to interact with many host proteins and cause various adverse effects on cellular function and immune response. For example, the cleavage of important immune factors by NS3/NS4A has been suggested as a mechanism for the hepatitis C virus to evade innate immunity. The spectrum of susceptible substrates for NS3/NS4A cleavage certainly includes important immune modulator kinases such as IKKα, IKKβ, IKKε, and TBK1, as demonstrated in this paper. We show that the kinase activities of these four host kinases were transformed in unexpected ways by NS3/NS4A. Treatment with NS3/NS4A caused a significant reduction in the kinase activities of both IKKα and IKKβ, suggesting that HCV might use its NS3/NS4A protease activity to deactivate the NF-κB-associated innate immune responses. In contrast, the kinase activities of both IKKε and TBK1 were enhanced after NS3/NS4A treatment, and more strikingly, the enhancement was more than 10-fold within 20 min of treatment. Our mass spectroscopic results suggested that the cleavage after Cys89 in the kinase domain of IKKε by NS3/NS4A led to their higher kinase activities, and three potential mechanisms were discussed. The observed kinase activity enhancement might facilitate the activation of both IKKε- and TBK1-dependent cellular antiviral pathways, likely contributing to spontaneous clearance of the virus and observed acute HCV infection. After longer than 20 min cleavage, both IKKε- and TBK1 gradually lost their kinase activities and the relevant antiviral pathways were expected to be inactivated, facilitating the establishment of chronic HCV infection.

Investigating the Conformational Dynamics of a Y-Family DNA Polymerase during Its Folding and Binding to DNA and a Nucleotide

During DNA polymerization, the Y-family DNA polymerases are capable of bypassing various DNA damage, which can stall the replication fork progression. It has been well acknowledged that the structures of the Y-family DNA polymerases have been naturally evolved to undertake this vital task. However, the mechanisms of how these proteins utilize their unique structural and conformational dynamical features to perform the translesion DNA synthesis are less understood. Here, we developed structure-based models to study the precatalytic DNA polymerization process, including DNA and nucleotide binding to DPO4, a paradigmatic Y-family polymerase from Sulfolobus solfataricus. We studied the interplay between the folding and the conformational dynamics of DPO4 and found that DPO4 undergoes first unraveling (unfolding) and then folding for accomplishing the functional "open-to-closed" conformational transition. DNA binding dynamically modulates the conformational equilibrium in DPO4 during the stepwise binding through different types of interactions, leading to different conformational distributions of DPO4 at different DNA binding stages. We observed that nucleotide binding induces modulation of a few contacts surrounding the active site of the DPO4-DNA complex associated with a high free energy barrier. Our simulation results resonate with the experimental evidence that the conformational change at the active site led by nucleotide is the rate-limiting step of nucleotide incorporation. In combination with localized frustration analyses, we underlined the importance of DPO4 conformational dynamics and fluctuations in facilitating DNA and nucleotide binding. Our findings offer mechanistic insights into the processes of DPO4 conformational dynamics associated with the substrate binding and contribute to the understanding of the "structure-dynamics-function" relationship in the Y-family DNA polymerases.

Dynamic Processing of a Common Oxidative DNA Lesion by the First Two Enzymes of the Base Excision Repair Pathway

Base excision repair (BER) is the primary pathway by which eukaryotic cells resolve single base damage. One common example of single base damage is 8-oxo-7,8-dihydro-2'-deoxoguanine (8-oxoG). High incidence and mutagenic potential of 8-oxoG necessitate rapid and efficient DNA repair. How BER enzymes coordinate their activities to resolve 8-oxoG damage while limiting cytotoxic BER intermediates from propagating genomic instability remains unclear. Here we use single-molecule Förster resonance energy transfer (smFRET) and ensemble-level techniques to characterize the activities and interactions of consecutive BER enzymes important for repair of 8-oxoG. In addition to characterizing the damage searching and processing mechanisms of human 8-oxoguanine glycosylase 1 (hOGG1), our data support the existence of a ternary complex between hOGG1, the damaged DNA substrate, and human AP endonuclease 1 (APE1). Our results indicate that hOGG1 is actively displaced from its abasic site containing product by protein-protein interactions with APE1 to ensure timely repair of damaged DNA.

A hand-off of DNA between archaeal polymerases allows high-fidelity replication to resume at a discrete intermediate three bases past 8-oxoguanine

During DNA replication, the presence of 8-oxoguanine (8-oxoG) lesions in the template strand cause the high-fidelity (HiFi) DNA polymerase (Pol) to stall. An early response to 8-oxoG lesions involves ‘on-the-fly’ translesion synthesis (TLS), in which a specialized TLS Pol is recruited and replaces the stalled HiFi Pol for lesion bypass. The length of TLS must be long enough for effective bypass, but it must also be regulated to minimize replication errors by the TLS Pol. The exact position where the TLS Pol ends and the HiFi Pol resumes (i.e. the length of the TLS patch) has not been described. We use steady-state and pre-steady-state kinetic assays to characterize lesion bypass intermediates formed by different archaeal polymerase holoenzyme complexes that include PCNA123 and RFC. After bypass of 8-oxoG by TLS PolY, products accumulate at the template position three base pairs beyond the lesion. PolY is catalytically poor for subsequent extension from this +3 position beyond 8-oxoG, but this inefficiency is overcome by rapid extension of HiFi PolB1. The reciprocation of Pol activities at this intermediate indicates a defined position where TLS Pol extension is limited and where the DNA substrate is handed back to the HiFi Pol after bypass of 8-oxoG.

Investigating the trade-off between folding and function in a multidomain Y-family DNA polymerase

The way in which multidomain proteins fold has been a puzzling question for decades. Until now, the mechanisms and functions of domain interactions involved in multidomain protein folding have been obscure. Here, we develop structure-based models to investigate the folding and DNA-binding processes of the multidomain Y-family DNA polymerase IV (DPO4). We uncover shifts in the folding mechanism among ordered domain-wise folding, backtracking folding, and cooperative folding, modulated by interdomain interactions. These lead to ‘U-shaped’ DPO4 folding kinetics. We characterize the effects of interdomain flexibility on the promotion of DPO4–DNA (un)binding, which probably contributes to the ability of DPO4 to bypass DNA lesions, which is a known biological role of Y-family polymerases. We suggest that the native topology of DPO4 leads to a trade-off between fast, stable folding and tight functional DNA binding. Our approach provides an effective way to quantitatively correlate the roles of protein interactions in conformational dynamics at the multidomain level.

Genetic Control of Oxidative Mutagenesis in Sulfolobus acidocaldarius

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

Characterization of a coupled DNA replication and translesion synthesis polymerase supraholoenzyme from archaea

The ability of the replisome to seamlessly coordinate both high fidelity and translesion DNA synthesis requires a means to regulate recruitment and binding of enzymes from solution. Co-occupancy of multiple DNA polymerases within the replisome has been observed primarily in bacteria and is regulated by posttranslational modifications in eukaryotes, and both cases are coordinated by the processivity clamp. Because of the heterotrimeric nature of the PCNA clamp in some archaea, there is potential to occupy and regulate specific polymerases at defined subunits. In addition to specific PCNA and polymerase interactions (PIP site), we have now identified and characterized a novel protein contact between the Y-family DNA polymerase and the B-family replication polymerase (YB site) bound to PCNA and DNA from Sulfolobus solfataricus. These YB contacts are essential in forming and stabilizing a supraholoenzyme (SHE) complex on DNA, effectively increasing processivity of DNA synthesis. The SHE complex can not only coordinate polymerase exchange within the complex but also provides a mechanism for recruitment of polymerases from solution based on multiequilibrium processes. Our results provide evidence for an archaeal PCNA 'tool-belt' recruitment model of multienzyme function that can facilitate both high fidelity and translesion synthesis within the replisome during DNA replication.

Coordination and Substitution of DNA Polymerases in Response to Genomic Obstacles

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

Backbone assignment of the binary complex of the full length Sulfolobus solfataricus DNA polymerase IV and DNA

Sulfolobus solfataricus DNA polymerase IV (Dpo4), a model Y-family DNA polymerase, bypasses a wide range of DNA lesions in vitro and in vivo. In this paper, we report the backbone chemical shift assignments of the full length Dpo4 in its binary complex with a 14/14-mer DNA substrate. Upon DNA binding, several β-stranded regions in the isolated catalytic core and little finger/linker fragments of Dpo4 become more structured. This work serves as a foundation for our ongoing investigation of conformational dynamics of Dpo4 and future determination of the first solution structures of a DNA polymerase and its binary and ternary complexes.

Error-Free Bypass of 7,8-dihydro-8-oxo-2'-deoxyguanosineby DNA Polymerase of Pseudomonas aeruginosa Phage PaP1

As one of the most common forms of oxidative DNA damage, 7,8-dihydro-8-oxo-2′-deoxyguanosine (8-oxoG) generally leads to G:C to T:A mutagenesis. To study DNA replication encountering 8-oxoG by the sole DNA polymerase (Gp90) of Pseudomonasaeruginosa phage PaP1, we performed steady-state and pre-steady-state kinetic analyses of nucleotide incorporation opposite 8-oxoG by Gp90 D234A that lacks exonuclease activities on ssDNA and dsDNA substrates. Gp90 D234A could bypass 8-oxoG in an error-free manner, preferentially incorporate dCTP opposite 8-oxoG, and yield similar misincorporation frequency to unmodified G. Gp90 D234A could extend beyond C:8-oxoG or A:8-oxoG base pairs with the same efficiency. dCTP incorporation opposite G and dCTP or dATP incorporation opposite 8-oxoG showed fast burst phases. The burst of incorporation efficiency (k_pol/K_d,_dNTP) is decreased as dCTP:G > dCTP:8-oxoG > dATP:8-oxoG. The presence of 8-oxoG in DNA does not affect its binding to Gp90 D234A in a binary complex but it does affect it in a ternary complex with dNTP and Mg²⁺, and dATP misincorporation opposite 8-oxoG further weakens the binding of Gp90 D234A to DNA. This study reveals Gp90 D234A can bypass 8-oxoG in an error-free manner, providing further understanding in DNA replication encountering oxidation lesion for P.aeruginosa phage PaP1.

Advances in Structural and Single-Molecule Methods for Investigating DNA Lesion Bypass and Repair Polymerases

Innovative advances in X-ray crystallography and single-molecule biophysics have yielded unprecedented insight into the mechanisms of DNA lesion bypass and damage repair. Time-dependent X-ray crystallography has been successfully applied to view the bypass of 8-oxo-7,8-dihydro-2'-deoxyguanine (8-oxoG), a major oxidative DNA lesion, and the incorporation of the triphosphate form, 8-oxo-dGTP, catalyzed by human DNA polymerase β. Significant findings of these studies are highlighted here, and their contributions to the current mechanistic understanding of mutagenic translesion DNA synthesis (TLS) and base excision repair are discussed. In addition, single-molecule Förster resonance energy transfer (smFRET) techniques have recently been adapted to investigate nucleotide binding and incorporation opposite undamaged dG and 8-oxoG by Sulfolobus solfataricus DNA polymerase IV (Dpo4), a model Y-family DNA polymerase. The mechanistic response of Dpo4 to a DNA lesion and the complex smFRET technique are described here. In this perspective, we also describe how time-dependent X-ray crystallography and smFRET can be used to achieve the spatial and temporal resolutions necessary to answer some of the mechanistic questions that remain in the fields of TLS and DNA damage repair.

Investigation of Intradomain Motions of a Y-Family DNA Polymerase during Substrate Binding and Catalysis

DNA polymerases catalyze DNA synthesis through a stepwise kinetic mechanism that begins with binding to DNA, followed by selection, binding, and incorporation of a nucleotide into an elongating primer. It is hypothesized that subtle active site adjustments in a polymerase to align reactive moieties limit the rate of correct nucleotide incorporation. DNA damage can impede this process for many DNA polymerases, causing replication fork stalling, genetic mutations, and potentially cell death. However, specialized Y-family DNA polymerases are structurally evolved to efficiently bypass DNA damage in vivo, albeit at the expense of replication fidelity. Dpo4, a model Y-family polymerase from Sulfolobus solfataricus, has been well-studied kinetically, structurally, and computationally, which yielded a mechanistic understanding of how the Y-family DNA polymerases achieve their unique catalytic properties. We previously employed a real-time Förster resonance energy transfer (FRET) technique to characterize the global conformational motions of Dpo4 during DNA binding as well as nucleotide binding and incorporation by monitoring changes in distance between sites on the polymerase and DNA, and even between domains of Dpo4. Here, we extend the utility of our FRET methodology to observe conformational transitions within individual domains of Dpo4 during DNA binding and nucleotide incorporation. The results of this novel, intradomain FRET approach unify findings from many studies to fully clarify the complex DNA binding mechanism of Dpo4. Furthermore, intradomain motions in the Finger domain during nucleotide binding and incorporation, for the first time, report on the rate-limiting step of a single-nucleotide addition catalyzed by Dpo4.

Pre-steady-state kinetic investigation of bypass of a bulky guanine lesion by human Y-family DNA polymerases

3-Nitrobenzanthrone (3-NBA), a byproduct of diesel exhaust, is highly present in the environment and poses a significant health risk. Exposure to 3-NBA results in formation of N-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone (dG^C8-N-ABA), a bulky DNA lesion that is of particular importance due to its mutagenic and carcinogenic potential. If not repaired or bypassed during genomic replication, dG^C8-N-ABA can stall replication forks, leading to senescence and cell death. Here we used pre-steady-state kinetic methods to determine which of the four human Y-family DNA polymerases (hPolη, hPolκ, hPolι, or hRev1) are able to catalyze translesion synthesis of dG^C8-N-ABAin vitro. Our studies demonstrated that hPolη and hPolκ most efficiently bypassed a site-specifically placed dG^C8-N-ABA lesion, making them good candidates for catalyzing translesion synthesis (TLS) of this bulky lesion in vivo. Consistently, our publication (Biochemistry 53, 5323-31) in 2014 has shown that small interfering RNA-mediated knockdown of hPolη and hPolκ in HEK293T cells significantly reduces the efficiency of TLS of dG^C8-N-ABA. In contrast, hPolι and hRev1 were severely stalled by dG^C8-N-ABA and their potential role in vivo was discussed. Subsequently, we determined the kinetic parameters for correct and incorrect nucleotide incorporation catalyzed by hPolη at various positions upstream, opposite, and downstream from dG^C8-N-ABA. Notably, nucleotide incorporation efficiency and fidelity both decreased significantly during dG^C8-N-ABA bypass and the subsequent extension step, leading to polymerase pausing and error-prone DNA synthesis by hPolη. Furthermore, hPolη displayed nucleotide concentration-dependent biphasic kinetics at the two polymerase pause sites, suggesting that multiple enzyme•DNA complexes likely exist during nucleotide incorporation.

Transcriptomes of the Extremely Thermoacidophilic Archaeon Metallosphaera sedula Exposed to Metal "Shock" Reveal Generic and Specific Metal Responses

The extremely thermoacidophilic archaeon Metallosphaera sedula mobilizes metals by novel membrane-associated oxidase clusters and, consequently, requires metal resistance strategies. This issue was examined by "shocking" M. sedula with representative metals (Co(2+), Cu(2+), Ni(2+), UO2 (2+), Zn(2+)) at inhibitory and subinhibitory levels. Collectively, one-quarter of the genome (554 open reading frames [ORFs]) responded to inhibitory levels, and two-thirds (354) of the ORFs were responsive to a single metal. Cu(2+) (259 ORFs, 106 Cu(2+)-specific ORFs) and Zn(2+) (262 ORFs, 131 Zn(2+)-specific ORFs) triggered the largest responses, followed by UO2 (2+) (187 ORFs, 91 UO2 (2+)-specific ORFs), Ni(2+) (93 ORFs, 25 Ni(2+)-specific ORFs), and Co(2+) (61 ORFs, 1 Co(2+)-specific ORF). While one-third of the metal-responsive ORFs are annotated as encoding hypothetical proteins, metal challenge also impacted ORFs responsible for identifiable processes related to the cell cycle, DNA repair, and oxidative stress. Surprisingly, there were only 30 ORFs that responded to at least four metals, and 10 of these responded to all five metals. This core transcriptome indicated induction of Fe-S cluster assembly (Msed_1656-Msed_1657), tungsten/molybdenum transport (Msed_1780-Msed_1781), and decreased central metabolism. Not surprisingly, a metal-translocating P-type ATPase (Msed_0490) associated with a copper resistance system (Cop) was upregulated in response to Cu(2+) (6-fold) but also in response to UO2 (2+) (4-fold) and Zn(2+) (9-fold). Cu(2+) challenge uniquely induced assimilatory sulfur metabolism for cysteine biosynthesis, suggesting a role for this amino acid in Cu(2+) resistance or issues in sulfur metabolism. The results indicate that M. sedula employs a range of physiological and biochemical responses to metal challenge, many of which are specific to a single metal and involve proteins with yet unassigned or definitive functions.

IMPORTANCE

The mechanisms by which extremely thermoacidophilic archaea resist and are negatively impacted by metals encountered in their natural environments are important to understand so that technologies such as bioleaching, which leverage microbially based conversion of insoluble metal sulfides to soluble species, can be improved. Transcriptomic analysis of the cellular response to metal challenge provided both global and specific insights into how these novel microorganisms negotiate metal toxicity in natural and technological settings. As genetics tools are further developed and implemented for extreme thermoacidophiles, information about metal toxicity and resistance can be leveraged to create metabolically engineered strains with improved bioleaching characteristics.

Single-Molecule Investigation of Response to Oxidative DNA Damage by a Y-Family DNA Polymerase

Y-family DNA polymerases are known to bypass DNA lesions in vitro and in vivo and rescue stalled DNA replication machinery. Dpo4, a well-characterized model Y-family DNA polymerase, is known to catalyze translesion synthesis across a variety of DNA lesions including 8-oxo-7,8-dihydro-2'-deoxyguanine (8-oxo-dG). Our previous X-ray crystallographic, stopped-flow Förster resonance energy transfer (FRET), and computational simulation studies have revealed that Dpo4 samples a variety of global conformations as it recognizes and binds DNA. Here we employed single-molecule FRET (smFRET) techniques to investigate the kinetics and conformational dynamics of Dpo4 when it encountered 8-oxo-dG, a major oxidative lesion with high mutagenic potential. Our smFRET data indicated that Dpo4 bound the DNA substrate in multiple conformations, as suggested by three observed FRET states. An incoming correct or incorrect nucleotide affected the distribution and stability of these states with the correct nucleotide completely shifting the equilibrium toward a catalytically competent complex. Furthermore, the presence of the 8-oxo-dG lesion in the DNA stabilized both the binary and ternary complexes of Dpo4. Thus, our smFRET analysis provided a basis for the enhanced efficiency which Dpo4 is known to exhibit when replicating across from 8-oxo-dG.

Mechanistic Basis for the Bypass of a Bulky DNA Adduct Catalyzed by a Y-Family DNA Polymerase

1-Nitropyrene (1-NP), an environmental pollutant, induces DNA damage in vivo and is considered to be carcinogenic. The DNA adducts formed by the 1-NP metabolites stall replicative DNA polymerases but are presumably bypassed by error-prone Y-family DNA polymerases at the expense of replication fidelity and efficiency in vivo. Our running start assays confirmed that a site-specifically placed 8-(deoxyguanosin-N(2)-yl)-1-aminopyrene (dG(1,8)), one of the DNA adducts derived from 1-NP, can be bypassed by Sulfolobus solfataricus DNA polymerase IV (Dpo4), although this representative Y-family enzyme was paused strongly by the lesion. Pre-steady-state kinetic assays were employed to determine the low nucleotide incorporation fidelity and establish a minimal kinetic mechanism for the dG(1,8) bypass by Dpo4. To reveal a structural basis for dCTP incorporation opposite dG(1,8), we solved the crystal structures of the complexes of Dpo4 and DNA containing a templating dG(1,8) lesion in the absence or presence of dCTP. The Dpo4·DNA-dG(1,8) binary structure shows that the aminopyrene moiety of the lesion stacks against the primer/template junction pair, while its dG moiety projected into the cleft between the Finger and Little Finger domains of Dpo4. In the Dpo4·DNA-dG(1,8)·dCTP ternary structure, the aminopyrene moiety of the dG(1,8) lesion, is sandwiched between the nascent and junction base pairs, while its base is present in the major groove. Moreover, dCTP forms a Watson-Crick base pair with dG, two nucleotides upstream from the dG(1,8) site, creating a complex for "-2" frameshift mutation. Mechanistically, these crystal structures provide additional insight into the aforementioned minimal kinetic mechanism.

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

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

Kinetic and structural mechanisms of (5'S)-8,5'-cyclo-2'-deoxyguanosine-induced dna replication stalling

The (5'S)-8,5'-cyclo-2'-deoxyguanosine (S-cdG) lesion is produced from reactions of DNA with hydroxyl radicals generated from ionizing radiation or endogenous oxidative metabolisms. An elevated level of S-cdG has been detected in Xeroderma pigmentosum, Cockayne syndrome, breast cancer patients, and aged mice. S-dG blocks DNA replication and transcription in vitro and in human cells and produces mutant replication and transcription products in vitro and in vivo. Major cellular protection against S-dG includes nucleotide excision repair and translesion DNA synthesis. We used kinetic and crystallographic approaches to elucidate the molecular mechanisms of S-cdG-induced DNA replication stalling using model B-family Sulfolobus solfataricus P2 DNA polymerase B1 (Dpo1) and Y-family S. solfataricus P2 DNA polymerase IV (Dpo4). Dpo1 and Dpo4 inefficiently bypassed S-cdG with dCTP preferably incorporated and dTTP (for Dpo4) or dATP (for Dpo1) misincorporated. Pre-steady-state kinetics and crystallographic data mechanistically explained the low-efficiency bypass. For Dpo1, S-cdG attenuated Kd,dNTP,app and kpol. For Dpo4, the S-cdG-adducted duplex caused a 6-fold decrease in Dpo4:DNA binding affinity and significantly reduced the concentration of the productive Dpo4:DNA:dCTP complex. Consistent with the inefficient bypass, crystal structures of Dpo4:DNA(S-cdG):dCTP (error-free) and Dpo4:DNA(S-cdG):dTTP (error-prone) complexes were catalytically incompetent. In the Dpo4:DNA(S-cdG):dTTP structure, S-cdG induced a loop structure and caused an unusual 5'-template base clustering at the active site, providing the first structural evidence of the previously suggested template loop structure that can be induced by a cyclopurine lesion. Together, our results provided mechanistic insights into S-cdG-induced DNA replication stalling.

Structural and kinetic insights into binding and incorporation of L-nucleotide analogs by a Y-family DNA polymerase

Considering that all natural nucleotides (D-dNTPs) and the building blocks (D-dNMPs) of DNA chains possess D-stereochemistry, DNA polymerases and reverse transcriptases (RTs) likely possess strongD-stereoselectivity by preferably binding and incorporating D-dNTPs over unnatural L-dNTPs during DNA synthesis. Surprisingly, a structural basis for the discrimination against L-dNTPs by DNA polymerases or RTs has not been established although L-deoxycytidine analogs (lamivudine and emtricitabine) and L-thymidine (telbivudine) have been widely used as antiviral drugs for years. Here we report seven high-resolution ternary crystal structures of a prototype Y-family DNA polymerase, DNA, and D-dCTP, D-dCDP, L-dCDP, or the diphosphates and triphosphates of lamivudine and emtricitabine. These structures reveal that relative to D-dCTP, each of these L-nucleotides has its sugar ring rotated by 180° with an unusual O4'-endo sugar puckering and exhibits multiple triphosphate-binding conformations within the active site of the polymerase. Such rare binding modes significantly decrease the incorporation rates and efficiencies of these L-nucleotides catalyzed by the polymerase.

Conformational dynamics of Thermus aquaticus DNA polymerase I during catalysis

Despite the fact that DNA polymerases have been investigated for many years and are commonly used as tools in a number of molecular biology assays, many details of the kinetic mechanism they use to catalyze DNA synthesis remain unclear. Structural and kinetic studies have characterized a rapid, pre-catalytic open-to-close conformational change of the Finger domain during nucleotide binding for many DNA polymerases including Thermus aquaticus DNA polymerase I (Taq Pol), a thermostable enzyme commonly used for DNA amplification in PCR. However, little has been performed to characterize the motions of other structural domains of Taq Pol or any other DNA polymerase during catalysis. Here, we used stopped-flow Förster resonance energy transfer to investigate the conformational dynamics of all five structural domains of the full-length Taq Pol relative to the DNA substrate during nucleotide binding and incorporation. Our study provides evidence for a rapid conformational change step induced by dNTP binding and a subsequent global conformational transition involving all domains of Taq Pol during catalysis. Additionally, our study shows that the rate of the global transition was greatly increased with the truncated form of Taq Pol lacking the N-terminal domain. Finally, we utilized a mutant of Taq Pol containing a de novo disulfide bond to demonstrate that limiting protein conformational flexibility greatly reduced the polymerization activity of Taq Pol.

Mutagenic potential of 8-oxo-7,8-dihydro-2'-deoxyguanosine bypass catalyzed by human Y-family DNA polymerases

One of the most common lesions induced by oxidative DNA damage is 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG). Replicative DNA polymerases poorly traverse this highly mutagenic lesion, suggesting that the replication fork may switch to a polymerase specialized for translesion DNA synthesis (TLS) to catalyze 8-oxodG bypass in vivo. Here, we systematically compared the 8-oxodG bypass efficiencies and fidelities of the TLS-specialized, human Y-family DNA polymerases eta (hPolη), iota (hPolι), kappa (hPolκ), and Rev1 (hRev1) either alone or in combination. Primer extension assays revealed that the times required for hPolη, hRev1, hPolκ, and hPolι to bypass 50% of the 8-oxodG lesions encountered (t₅₀^bypass) were 0.58, 0.86, 108, and 670 s, respectively. Although hRev1 bypassed 8-oxodG efficiently, hRev1 failed to catalyze the extension step of TLS, and a second polymerase was required to extend the lesion bypass products. A high-throughput short oligonucleotide sequencing assay (HT-SOSA) was used to quantify the types and frequencies of incorporation errors produced by the human Y-family DNA polymerases at and near the 8-oxodG site. Although hPolη bypassed 8-oxodG most efficiently, hPolη correctly incorporated dCTP opposite 8-oxodG within only 54.5% of the sequences analyzed. In contrast, hPolι bypassed the lesion least efficiently but correctly incorporated dCTP at a frequency of 65.8% opposite the lesion. The combination of hRev1 and hPolκ was most accurate opposite 8-oxodG (92.3%), whereas hPolκ alone was the least accurate (18.5%). The t₅₀^bypass value and correct dCTP incorporation frequency in the presence of an equal molar concentration of all four Y-family enzymes were 0.60 s and 43.5%, respectively. These values are most similar to those of hPolη alone, suggesting that hPolη outcompetes the other three Y-family polymerases to catalyze 8-oxodG bypass in vitro and possibly in vivo.

Recent insight into the kinetic mechanisms and conformational dynamics of Y-Family DNA polymerases

The kinetic mechanisms by which DNA polymerases catalyze DNA replication and repair have long been areas of active research. Recently discovered Y-family DNA polymerases catalyze the bypass of damaged DNA bases that would otherwise block replicative DNA polymerases and stall replication forks. Unlike DNA polymerases from the five other families, the Y-family DNA polymerases have flexible, solvent-accessible active sites that are able to tolerate various types of damaged template bases and allow for efficient lesion bypass. Their promiscuous active sites, however, also lead to fidelities that are much lower than those observed for other DNA polymerases and give rise to interesting mechanistic properties. Additionally, the Y-family DNA polymerases have several other unique structural features and undergo a set of conformational changes during substrate binding and catalysis different from those observed for replicative DNA polymerases. In recent years, pre-steady-state kinetic methods have been extensively employed to reveal a wealth of information about the catalytic properties of these fascinating noncanonical DNA polymerases. Here, we review many of the recent findings on the kinetic mechanisms of DNA polymerization with undamaged and damaged DNA substrates by the Y-family DNA polymerases, and the conformational dynamics employed by these error-prone enzymes during catalysis.

Identification of an unfolding intermediate for a DNA lesion bypass polymerase

Sulfolobus solfataricus DNA Polymerase IV (Dpo4), a prototype Y-family DNA polymerase, has been well characterized biochemically and biophysically at 37 °C or lower temperatures. However, the physiological temperature of the hyperthermophile S. solfataricus is approximately 80 °C. With such a large discrepancy in temperature, the in vivo relevance of these in vitro studies of Dpo4 has been questioned. Here, we employed circular dichroism spectroscopy and fluorescence-based thermal scanning to investigate the secondary structural changes of Dpo4 over a temperature range from 26 to 119 °C. Dpo4 was shown to display a high melting temperature characteristic of hyperthermophiles. Unexpectedly, the Little Finger domain of Dpo4, which is only found in the Y-family DNA polymerases, was shown to be more thermostable than the polymerase core. More interestingly, Dpo4 exhibited a three-state cooperative unfolding profile with an unfolding intermediate. The linker region between the Little Finger and Thumb domains of Dpo4 was found to be a source of structural instability. Through site-directed mutagenesis, the interactions between the residues in the linker region and the Palm domain were identified to play a critical role in the formation of the unfolding intermediate. Notably, the secondary structure of Dpo4 was not altered when the temperature was increased from 26 to 87.5 °C. Thus, in addition to providing structural insights into the thermal stability and an unfolding intermediate of Dpo4, our work also validated the relevance of the in vitro studies of Dpo4 performed at temperatures significantly lower than 80 °C.