Accommodation of a 1S-(-)-benzo[c]phenanthrenyl-N6-dA adduct in the Y-family Dpo4 DNA polymerase active site: structural insights through molecular dynamics simulations

Dynamics and thermal stability of the bypass polymerase, DinB homolog (Dbh)

The DinB homolog polymerase (Dbh) is a member of the Y-family of translesion DNA polymerases that can synthesize using a damaged DNA template. Since Dbh comes from the thermophilic archaeon Sulfolobus acidocaldarius, it is capable of functioning over a wide range of temperatures. Existing X-ray structures were determined at temperatures where the protein is least active. Here we use NMR and circular dichroism to understand how the structure and dynamics of Dbh are affected by temperature (2°C-65°C) and metal ion binding in solution. We measured hydrogen exchange protection factors, temperature coefficients, and chemical shift perturbations with and without magnesium and manganese. We report on regions of the protein that become more dynamic as the temperature is increased toward the functional temperature. Hydrogen exchange protection factors and temperature coefficients reveal that both the thumb and finger domains are very dynamic relative to the palm and little-finger (LF) domains. These trends remain true at high temperature with dynamics increasing as temperatures increase from 35°C to 50°C. Notably, NMR spectra show that the Dbh tertiary structure cold denatures beginning at 25°C and increases in denaturation as the temperature is lowered to 5°C with little change observed by CD. Above 35°C, chemical shift perturbation analysis in the presence and absence of magnesium and manganese reveals three ion binding sites, without DNA bound. In contrast, these bound metals are not apparent in any Dbh crystal structures of the protein without DNA. Two ion binding sites are confirmed to be near the active site, as reported in other Y-family polymerases, and we report a novel ion binding site in the LF domain. Thus, the solution-state structure of the Dbh polymerase is distinct from that of the solid-state structures and shows an unusually high cold denaturation temperature.

Uncovering a unique approach for damaged DNA replication: A computational investigation of a mutagenic tobacco-derived thymine lesion

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone is a potent nicotine carcinogen that leads to many DNA lesions, the most persistent being the O2-[4-oxo-4-(3-pyridyl)butyl]thymine adduct (POB-T). Although the experimental mutagenic profile for the minor groove POB-T lesion has been previously reported, the findings are puzzling in terms of the human polymerases involved. Specifically, while pol κ typically replicates minor groove adducts, in vivo studies indicate pol η replicates POB-T despite being known for processing major groove adducts. Our multiscale modeling approach reveals that the canonical (anti) glycosidic orientation of POB-T can fit in the pol κ active site, but only a unique (syn) POB-T conformation is accommodated by pol η. These distinct binding orientations rationalize the differential in vitro mutagenic spectra based on the preferential stabilization of dGTP and dTTP opposite the lesion for pol κ and η, respectively. Overall, by uncovering the first evidence for the replication of a damaged pyrimidine in the syn glycosidic orientation, the current work provides the insight necessary to clarify a discrepancy in the DNA replication literature, expand the biological role of the critical human pol η, and understand the mutational signature in human cancers associated with tobacco exposure.

Human Y-Family DNA Polymerase κ Is More Tolerant to Changes in Its Active Site Loop than Its Ortholog Escherichia coli DinB

DNA damage is a constant threat and can be bypassed in a process called translesion synthesis, which is typically carried out by Y-family DNA polymerases. Y-family DNA polymerases are conserved in all domains of life and tend to have specificity for certain types of DNA damage. Escherichia coli DinB and its human ortholog pol κ can bypass specific minor groove deoxyguanine adducts efficiently and are inhibited by major groove adducts, as Y-family DNA polymerases make contacts with the minor groove side of the DNA substrate and lack contacts with the major groove at the nascent base pair. DinB is inhibited by major groove adducts more than pol κ, and they each have active site loops of different lengths, with four additional amino acids in the DinB loop. We previously showed that the R35A active site loop mutation in DinB allows for bypass of the major groove adduct N⁶-furfuryl-dA. These observations led us to investigate the different active site loops by creating loop swap chimeras of DinB with a pol κ loop and vice versa by changing the loop residues in a stepwise fashion. We then determined their activity with undamaged DNA or DNA containing N²-furfuryl-dG or N⁶-furfuryl-dA. The DinB proteins with the pol kappa loop have low activity on all templates but have decreased misincorporation compared to either wild-type protein. The kappa proteins with the DinB loop retain activity on all templates and have decreased misincorporation compared to either wild-type protein. We assessed the thermal stability of the proteins and observed an increase in stability in the presence of all DNA templates and additional increases generally only in the presence of the undamaged and N²-furfuryl-dG adduct and dCTP, which correlates with activity. Overall we find that pol κ is more tolerant to changes in the active site loop than DinB.

Formation of S-[2-(N6-Deoxyadenosinyl)ethyl]glutathione in DNA and Replication Past the Adduct by Translesion DNA Polymerases

1,2-Dibromoethane (DBE, ethylene dibromide) is a potent carcinogen due at least in part to its DNA cross-linking effects. DBE cross-links glutathione (GSH) to DNA, notably to sites on 2'-deoxyadenosine and 2'-deoxyguanosine ( Cmarik , J. L. , et al. ( 1991 ) J. Biol. Chem. 267 , 6672 - 6679 ). Adduction at the N6 position of 2'-deoxyadenosine (dA) had not been detected, but this is a site for the linkage of O⁶-alkylguanine DNA alkyltransferase ( Chowdhury , G. , et al. ( 2013 ) Angew. Chem. Int. Ed. 52 , 12879 - 12882 ). We identified and quantified a new adduct, S-[2-(N⁶-deoxyadenosinyl)ethyl]GSH, in calf thymus DNA using LC-MS/MS. Replication studies were performed in duplex oligonucleotides containing this adduct with human DNA polymerases (hPols) η, ι, and κ, as well as with Sulfolobus solfataricus Dpo4, Escherichia coli polymerase I Klenow fragment, and bacteriophage T7 polymerase. hPols η and ι, Dpo4, and Klenow fragment were able to bypass the adduct with only slight impedance; hPol η and ι showed increased misincorporation opposite the adduct compared to that of unmodified 2'-deoxyadenosine. LC-MS/MS analysis of full-length primer extension products by hPol η confirmed the incorporation of dC opposite S-[2-(N⁶-deoxyadenosinyl)ethyl]GSH and also showed the production of a -1 frameshift. These results reveal the significance of N⁶-dA GSH-DBE adducts in blocking replication, as well as producing mutations, by human translesion synthesis DNA polymerases.

Computational Evaluation of Nucleotide Insertion Opposite Expanded and Widened DNA by the Translesion Synthesis Polymerase Dpo4

Expanded (x) and widened (y) deoxyribose nucleic acids (DNA) have an extra benzene ring incorporated either horizontally (xDNA) or vertically (yDNA) between a natural pyrimidine base and the deoxyribose, or between the 5- and 6-membered rings of a natural purine. Far-reaching applications for (x,y)DNA include nucleic acid probes and extending the natural genetic code. Since modified nucleobases must encode information that can be passed to the next generation in order to be a useful extension of the genetic code, the ability of translesion (bypass) polymerases to replicate modified bases is an active area of research. The common model bypass polymerase DNA polymerase IV (Dpo4) has been previously shown to successfully replicate and extend past a single modified nucleobase on a template DNA strand. In the current study, molecular dynamics (MD) simulations are used to evaluate the accommodation of expanded/widened nucleobases in the Dpo4 active site, providing the first structural information on the replication of (x,y)DNA. Our results indicate that the Dpo4 catalytic (palm) domain is not significantly impacted by the (x,y)DNA bases. Instead, the template strand is displaced to accommodate the increased C1’–C1’ base-pair distance. The structural insights unveiled in the present work not only increase our fundamental understanding of Dpo4 replication, but also reveal the process by which Dpo4 replicates (x,y)DNA, and thereby will contribute to the optimization of high fidelity and efficient polymerases for the replication of modified nucleobases.

Discrimination against major groove adducts by Y-family polymerases of the DinB subfamily

Y-family DNA polymerases bypass DNA adducts in a process known as translesion synthesis (TLS). Y-family polymerases make contacts with the minor groove side of the DNA substrate at the nascent base pair. The Y-family polymerases also contact the DNA major groove via the unique little finger domain, but they generally lack contacts with the major groove at the nascent base pair. Escherichia coli DinB efficiently and accurately copies certain minor groove guanosine adducts. In contrast, we previously showed that the presence in the DNA template of the major groove-modified base 1,3-diaza-2-oxophenothiazine (tC) inhibits the activity of E. coli DinB. Even when the DNA primer is extended up to three nucleotides beyond the site of the tC analog, DinB activity is strongly inhibited. These findings prompted us to investigate discrimination against other major groove modifications by DinB and its orthologs. We chose a set of pyrimidines and purines with modifications in the major groove and determined the activity of DinB and several orthologs with these substrates. DinB, human pol kappa, and Sulfolobus solfataricus Dpo4 show differing specificities for the major groove adducts pyrrolo-dC, dP, N(6)-furfuryl-dA, and etheno-dA. In general, DinB was least efficient for bypass of all of these major groove adducts, whereas Dpo4 was most efficient. DinB activity was essentially completely inhibited by the presence of etheno-dA, while pol kappa activity was strongly inhibited. All three of these DNA polymerases were able to bypass N(6)-furfuryl-dA with modest efficiency, with DinB being the least efficient. We also determined that the R35A variant of DinB enhances bypass of N(6)-furfuryl-dA but not etheno-dA. In sum, we find that whereas DinB is specific for bypass of minor groove adducts, it is specifically inhibited by major groove DNA modifications.

Chemistry and structural biology of DNA damage and biological consequences

The formation of adducts by the reaction of chemicals with DNA is a critical step for the initiation of carcinogenesis. The structural analysis of various DNA adducts reveals that conformational and chemical rearrangements and interconversions are a common theme. Conformational changes are modulated both by the nature of adduct and the base sequences neighboring the lesion sites. Equilibria between conformational states may modulate both DNA repair and error-prone replication past these adducts. Likewise, chemical rearrangements of initially formed DNA adducts are also modulated both by the nature of adducts and the base sequences neighboring the lesion sites. In this review, we focus on DNA damage caused by a number of environmental and endogenous agents, and biological consequences.

Quantum mechanics/molecular mechanics investigation of the chemical reaction in Dpo4 reveals water-dependent pathways and requirements for active site reorganization

The nucleotidyl-transfer reaction coupled with the conformational transitions in DNA polymerases is critical for maintaining the fidelity and efficiency of DNA synthesis. We examine here the possible reaction pathways of a Y-family DNA polymerase, Sulfolobus solfataricus DNA polymerase IV (Dpo4), for the correct insertion of dCTP opposite 8-oxoguanine using the quantum mechanics/molecular mechanics (QM/MM) approach, both from a chemistry-competent state and a crystal closed state. The latter examination is important for understanding pre-chemistry barriers to interpret the entire enzyme mechanism, since the crystal closed state is not an ideal state for initiating the chemical reaction. The most favorable reaction path involves initial deprotonation of O3'H via two bridging water molecules to O1A, overcoming an overall potential energy barrier of approximately 20.0 kcal/mol. The proton on O1A-P(alpha) then migrates to the gamma-phosphate oxygen of the incoming nucleotide as O3' attacks P(alpha), and the P(alpha)-O3A bond breaks. The other possible pathway in which the O3'H proton is transferred directly to O1A on P(alpha) has an overall energy barrier of 25.0 kcal/mol. In both reaction paths, the rate-limiting step is the initial deprotonation, and the trigonal-bipyramidal configuration for P(alpha) occurs during the concerted bond formation (O3'-P(alpha)) and breaking (P(alpha)-O3A), indicating the associative nature of the chemical reaction. In contrast, the Dpo4/DNA complex with an imperfect active-site geometry corresponding to the crystal state must overcome a much higher activation energy barrier (29.0 kcal/mol) to achieve a tightly organized site due to hindered O3'H deprotonation stemming from larger distances and distorted conformation of the proton acceptors. This significant difference demonstrates that the pre-chemistry reorganization in Dpo4 costs approximately 4.0 to 9.0 kcal/mol depending on the primer terminus environment. Compared to the higher fidelity DNA polymerase beta from the X-family, Dpo4 has a higher chemical reaction barrier (20.0 vs 15.0 kcal/mol) due to the more solvent-exposed active site.

Regulation of DNA repair fidelity by molecular checkpoints: "gates" in DNA polymerase beta's substrate selection

With an increasing number of structural, kinetic, and modeling studies of diverse DNA polymerases in various contexts, a complex dynamical view of how atomic motions might define molecular "gates" or checkpoints that contribute to polymerase specificity and efficiency is emerging. Such atomic-level information can offer insights into rate-limiting conformational and chemical steps to help piece together mechanistic views of polymerases in action. With recent advances, modeling and dynamics simulations, subject to the well-appreciated limitations, can access transition states and transient intermediates along a reaction pathway, both conformational and chemical, and such information can help bridge the gap between experimentally determined equilibrium structures and mechanistic enzymology data. Focusing on DNA polymerase beta (pol beta), we present an emerging view of the geometric, energetic, and dynamic selection criteria governing insertion rate and fidelity mechanisms of DNA polymerases, as gleaned from various computational studies and based on the large body of existing kinetic and structural data. The landscape of nucleotide insertion for pol beta includes conformational changes, prechemistry, and chemistry "avenues", each with a unique deterministic or stochastic pathway that includes checkpoints for selective control of nucleotide insertion efficiency. For both correct and incorrect incoming nucleotides, pol beta's conformational rearrangements before chemistry include a cascade of slow and subtle side chain rearrangements, followed by active site adjustments to overcome higher chemical barriers, which include critical ion-polymerase geometries; this latter notion of a prechemistry avenue fits well with recent structural and NMR data. The chemical step involves an associative mechanism with several possibilities for the initial proton transfer and for the interaction among the active site residues and bridging water molecules. The conformational and chemical events and associated barriers define checkpoints that control enzymatic efficiency and fidelity. Understanding the nature of such active site rearrangements can facilitate interpretation of existing data and stimulate new experiments that aim to probe enzyme features that contribute to fidelity discrimination across various polymerases via such geometric, dynamic, and energetic selection criteria.

Subtle but variable conformational rearrangements in the replication cycle of Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4) may accommodate lesion bypass

The possible conformational changes of DNA polymerase IV (Dpo4) before and after the nucleotidyl-transfer reaction are investigated at the atomic level by dynamics simulations to gain insight into the mechanism of low-fidelity polymerases and identify slow and possibly critical steps. The absence of significant conformational changes in Dpo4 before chemistry when the incoming nucleotide is removed supports the notion that the "induced-fit" mechanism employed to interpret fidelity in some replicative and repair DNA polymerases does not exist in Dpo4. However, significant correlated movements in the little finger and finger domains, as well as DNA sliding and subtle catalytic-residue rearrangements, occur after the chemical reaction when both active-site metal ions are released. Subsequently, Dpo4's little finger grips the DNA through two arginine residues and pushes it forward. These metal ion correlated movements may define subtle, and possibly characteristic, conformational adjustments that operate in some Y-family polymerase members in lieu of the prominent subdomain motions required for catalytic cycling in other DNA polymerases like polymerase beta. Such subtle changes do not easily provide a tight fit for correct incoming substrates as in higher-fidelity polymerases, but introduce in low-fidelity polymerases different fidelity checks as well as the variable conformational-mobility potential required to bypass different lesions.

DNA adduct bypass polymerization by Sulfolobus solfataricus DNA polymerase Dpo4: analysis and crystal structures of multiple base pair substitution and frameshift products with the adduct 1,N2-ethenoguanine

1,N(2)-Etheno(epsilon)guanine is a mutagenic DNA lesion derived from lipid oxidation products and also from some chemical carcinogens. Gel electrophoretic analysis of the products of primer extension by Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4) indicated preferential incorporation of A opposite 3'-(1,N(2)-epsilon-G)TACT-5', among the four dNTPs tested individually. With the template 3'-(1,N(2)-epsilon-G)CACT-5', both G and A were incorporated. When primer extension was done in the presence of a mixture of all four dNTPs, high pressure liquid chromatography-mass spectrometry analysis of the products indicated that (opposite 3'-(1,N(2)-epsilon-G)CACT-5') the major product was 5'-GTGA-3' and the minor product was 5'-AGTGA-3'. With the template 3'-(1,N(2)-epsilon-G)TACT-5', the following four products were identified by high pressure liquid chromatography-mass spectrometry: 5'-AATGA-3', 5'-ATTGA-3', 5'-ATGA-3', and 5'-TGA-3'. An x-ray crystal structure of Dpo4 was solved (2.1 A) with a primer-template and A placed in the primer to be opposite the 1,N(2)-epsilon-G in the template 3'-(1,N(2)-epsilon-G)TACT 5'. The added A in the primer was paired across the template T with classic Watson-Crick geometry. Similar structures were observed in a ternary Dpo4-DNA-dATP complex and a ternary Dpo4-DNA-ddATP complex, with d(d)ATP opposite the template T. A similar structure was observed with a ddGTP adjacent to the primer and opposite the C next to 1,N(2)-epsilon-G in 3'-(1,N(2)-epsilon-G)CACT-5'. We concluded that Dpo4 uses several mechanisms, including A incorporation opposite 1,N(2)-epsilon-G and also a variation of dNTP-stabilized misalignment, to generate both base pair and frameshift mutations.