Hailey L.
Gahlon
ab,
Alice R.
Walker
c,
G. Andrés
Cisneros
c,
Meindert H.
Lamers‡
d and
David S.
Rueda
*ab
aDepartment of Medicine, Molecular Virology, Imperial College London, Du Cane Road, London W12 0NN, UK. E-mail: david.rueda@imperial.ac.uk
bSingle Molecule Imaging Group, MRC London Institute for Medical Sciences, Imperial College London, Du Cane Road, London, W12 0NN, UK
cDepartment of Chemistry, University of North Texas, 1155 Union Circle, Denton, Texas 76203, USA
dMRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge, CB2 0QH, UK
First published on 22nd October 2018
DNA synthesis, carried out by DNA polymerases, requires balancing speed and accuracy for faithful replication of the genome. High fidelity DNA polymerases contain a 3′–5′ exonuclease domain that can remove misincorporated nucleotides on the 3′ end of the primer strand, a process called proofreading. The E. coli replicative polymerase, DNA polymerase III, has spatially separated (∼55 Å apart) polymerase and exonuclease subunits. Here, we report on the dynamics of E. coli DNA polymerase III proofreading in the presence of its processivity factor, the β2-sliding clamp, at varying base pair termini using single-molecule FRET. We find that the binding kinetics do not depend on the base identity at the termini, indicating a tolerance for DNA mismatches. Further, our single-molecule data and MD simulations show two previously unobserved features: (1) DNA Polymerase III is a highly dynamic protein that adopts multiple conformational states while bound to DNA with matched or mismatched ends, and (2) an exonuclease-deficient DNA polymerase III has reduced conformational flexibility. Overall, our single-molecule experiments provide high time-resolution insight into a mechanism that ensures high fidelity DNA replication to maintain genome integrity.
The DNA polymerase III holoenzyme is the replicative DNA polymerase in E. coli comprising ten proteins with a total mass of ∼10 MDa. The α subunit of the holoenzyme is the replicative DNA polymerase that belongs to the C family of polymerases and performs fast replication (∼103 nt s−1), with high fidelity (∼10−6 error rate) and high processivity (∼105 insertions per binding event).8–11 The ε subunit is the 3′–5′ exonuclease that removes misincorporated nt. In turn, the exonuclease binds the accessory protein θ to form the trimeric complex Pol III core (i.e., α–ε–θ).12,13 The polymerase and exonuclease active sites are separated by ∼55 Å.14 How these two subunits work together to coordinate proofreading while balancing fast and accurate DNA synthesis remains largely unknown. In addition, which amino acid residues are involved in the dynamic transfer of the primer strand between the Pol and Exo domains during proofreading is largely unknown. Previous structural data has indicated that tyrosine 453 in Pol III may stabilize an Exo-conformation through an aromatic stacking interaction with a nucleobase on the primer strand.14
Mechanistic details of polymerase proofreading dynamics remain elusive because of rapid conformational changes that can be hard to detect and quantify with traditional bulk-averaged biochemical approaches. To circumvent this, we have developed a single-molecule Förster resonance energy transfer (smFRET)15,16 assay to monitor the proofreading conformational dynamics of Pol III core in the presence of its processivity factor, the β2-sliding clamp. We tested various primer template termini containing cognate base pairs and mismatches; templates containing G paired opposite a dideoxy C, matched C, A and T (DNA sequences, Table 1). We also tested Pol III core dynamics at the site of a double mismatch (G:AA, DNA sequence, Table 1). Using this smFRET assay, we measured the kinetics and dynamics of DNA Pol III core. The data show that Pol III core-binding rate constants do not change significantly in the presence of matched or mismatched DNA termini. However, the DNA bound Pol III core complex is highly dynamic and samples numerous conformational intermediates along the proofreading pathway. We determined that an exonuclease deficient polymerase mutant has reduced dynamics compared to wild type, linking a key residue, tyrosine 453, in the thumb domain of the polymerase to protein flexibility and proofreading. Lastly, we observed non-equivalent initial binding conformations for matched and mismatched DNA termini, suggesting a functional role for how Pol III binds initially at the 3′ primer terminus. Altogether, our data reveal perturbations in proofreading dynamics at both the DNA and protein level, most notably regarding structural changes at the DNA terminus and with the Exo-deficient Pol III mutant.
DNA | Sequencea |
---|---|
a Template (top) and primer (bottom) strands. Bold T denotes Cy3-labeled amino-dT linker, lower case c denotes dideoxy cytosine terminated primer, underlined bases contain a phosphorothioate bond to prevent exonuclease cleavage. All 39-mer template strands contain a 5′-Biotin TEG for surface immobilization. | |
G:ddC | 5′ CATAATATCC TCAGGAGTCC TTCGTCCTAG TACTACTCA 3′ |
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
|
G:C | 5′ CATAATATCC TCAGGAGTCC TTCGTCCTAG TACTACTCA 3′ |
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
|
G:T | 5′ CATAATATCC TCAGGAGTCC TTCGTCCTAG TACTACTCA 3′ |
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
|
G:A | 5′ CATAATATCC TCAGGAGTCC TTCGTCCTAG TACTACTCA 3′ |
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
|
G:AA | 5′ CATAATATCC TCAGGAGTCC TTCGTCCTAG TACTACTCA 3′ |
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
![]() | ||
Fig. 1 Pol III core binding and dissociation observed by smFRET. (A) Schematic of the smFRET experiment with Cy3 donor emission (blue circle) on DNA and upon protein binding an increase in Cy5 acceptor emission (red circle) via fluorescence energy transfer. (B) Representative intensity trajectory with donor intensity in blue and acceptor intensity in red, anti-correlated signals indicate protein binding events. DNA is labeled with a Cy3 donor and contains a terminal G:ddC terminus (see Table 1 for DNA sequence) and Pol III core is labeled with a Cy5 acceptor on the theta subunit containing an E41C mutation to site-specifically label the Cys residue with maleimide chemistry. Single-molecule experiments were performed in 20 mM Hepes, pH 7.6, 5 mM potassium glutamate, 3 mM magnesium acetate, 2 mM DTT, 2 mM Trolox, 25 nM enzyme and 10 μM dCTP. (C) Corresponding FRET trajectory whereby apparent FRET efficiencies were calculated as FRET = IA/(ID + IA); and IA indicates acceptor intensity and ID indicates donor intensity and examples of (D) double exponential decay graph for kon (pseudo-first order) and (E) single exponential decay graph for koff. |
DNAa | k on,fast (s−1) | k on,slow (s−1) | k off (s−1) | N |
---|---|---|---|---|
a DNA sequences in Table 1. b Mutant is a Y453A modification in the α subunit. | ||||
Wild type | ||||
G:ddC | 0.8 ± 0.1 | 0.030 ± 0.010 | 0.20 ± 0.01 | 104 |
G:C | 0.6 ± 0.1 | 0.020 ± 0.007 | 0.20 ± 0.01 | 94 |
G:T | 0.6 ± 0.1 | 0.010 ± 0.003 | 0.20 ± 0.01 | 103 |
G:A | 0.3 ± 0.1 | 0.020 ± 0.002 | 0.20 ± 0.01 | 102 |
G:AA | N.D. | N.D. | 0.30 ± 0.02 | 107 |
Mutantb | ||||
G:ddC | 0.7 ± 0.1 | 0.05 ± 0.01 | 0.40 ± 0.02 | 99 |
G:C | 0.8 ± 0.1 | 0.04 ± 0.01 | 0.30 ± 0.01 | 141 |
G:T | 0.6 ± 0.1 | 0.05 ± 0.01 | 0.30 ± 0.01 | 104 |
G:A | 1.5 ± 0.3 | 0.04 ± 0.01 | 0.30 ± 0.02 | 143 |
G:AA | N.D. | N.D. | 0.30 ± 0.02 | 104 |
For mismatched DNA, G:T, G:A and G:AA (Table 1), a phosphorothioate modification was placed at the terminal 3′–5′ phosphate linkage on the primer strand, as this modification prevents exonuclease degradation.20,21 Therefore, this allows for monitoring proofreading dynamics on mismatches that are not removed in the time scale of our single-molecule experiments. The off rate constants did not vary significantly for either of the mismatched termini; a koff of 0.2 s−1 was determined for G:T and G:A (Table 2 and Fig. S3F, H, ESI†) and a koff of 0.3 s−1 for G:AA (Table 2 and Fig. S4B, ESI†). The kon,slow did not show appreciable differences amongst the DNA termini, however the kon,fast for G:A DNA yielded a rate constant of 0.3 s−1, which was 2.5 times slower than for the G:ddC DNA of 0.8 s−1 (Table 2).
A set of smFRET trajectories showed dynamic transitions within a long (>60 s) single binding event. These trajectories reveal information about the conformational pathway that Pol III core adopts along the Pol- and Exo-binding landscape. A Hidden Markov Model analysis (HMM) was performed on these smFRET trajectories using the program HaMMy.22 From the HMM fits, transition density plots (TDPs) were generated using the program TDP.22 TDPs are two-dimensional graphs in which the initial and final FRET states are plotted as a heat map. While shorter Pol III core binding events also displayed dynamic transitions, these data were not included in the analysis, as more data points are needed for HMM. The initial FRET value is shown at the x-axis, and the final FRET value, after the transition, is depicted on the y-axis. Here, higher probability FRET states are represented by a yellow-white color and lower probability states by a blue-red color (see legend heat map, Fig. 2 and 3 and Methods for how TDP plots were generated).
![]() | ||
Fig. 2 Conformational dynamics observed for wild type Pol III core as a function of varying DNA termini. Single-molecule experiments were performed in 20 mM Hepes, pH 7.6, 5 mM potassium glutamate, 3 mM magnesium acetate, 2 mM DTT, 2 mM Trolox, 25 nM enzyme and 10 μM dCTP. For the G:ddC DNA, a dideoxyC was inserted at the 3′ primer end to prevent nucleotide insertion. For the G:C DNA, a non-hydrolyzable dCTP analog containing an (α,β)-methylene bridge was utilized to prevent nucleotide incorporation. For the mismatches G:T and G:A, a phosphorothioate modification was incorporated into the primer strand to prevent degradation (see Table 1 for DNA sequences). Long Pol III core binding (>60 s) smFRET trajectories were analyzed using a hidden Markov model with the freely available HaMMy software. FRET trajectories were compiled into transition density plots (TDP) that depict the number of times a transition occurs as a two-dimensional heat map containing the initial and final FRET values on the x and y axis, respectively. (A, n = 33; B, n = 37; C, n = 42; D, n = 51.) |
![]() | ||
Fig. 3 Conformational dynamics observed for mutant Pol III core containing a Y453A mutation in the thumb domain of the α-subunit as a function of varying DNA termini. Single-molecule experiments were performed in 20 mM Hepes, pH 7.6, 5 mM potassium glutamate, 3 mM magnesium acetate, 2 mM DTT, 2 mM Trolox, 25 nM enzyme and 10 μM dCTP. For the G:ddC DNA, a dideoxyC was inserted at the 3′ primer end to prevent nucleotide insertion. For the G:C DNA, a non-hydrolyzable dCTP analog containing an (α,β)-methylene bridge was utilized to prevent nucleotide incorporation. For the mismatches G:T and G:A, a phosphorothioate modification was incorporated into the primer strand to prevent degradation (see Table 1 for DNA sequences). Long Pol III core binding (>60 s) smFRET trajectories were analyzed using a hidden Markov model with the freely available HaMMy software. FRET trajectories were compiled into transition density plots (TDP) that depict the number of times a transition occurs as a two-dimensional heat map containing the initial and final FRET values on the x and y axis, respectively. (A, n = 19; B, n = 10; C, n = 12; D, n = 20). |
The transitions for all TDP plots are symmetric in both directions, demonstrating reversibility in switching between the different FRET states. Overall, the TDP transitions show that wild type Pol III core binds over the entire FRET range between 0.2 and 0.8 and that its binding is highly dynamic. The highest number of transitions observed for all DNA substrates was for G:ddC (Fig. 2A). Further, it has the highest density of transitions between 0.4–0.6 FRET efficiencies. In addition, compared to the other DNA pairs tested, G:ddC populates the 0.8 FRET state with the highest density (Fig. 2A). For G:C, the highest density of transitions is between 0.4–0.6 (Fig. 2B). In case of the mismatches G:T and G:A, the transition densities are highest between 0.2–0.6 (Fig. 2C and D, respectively). Finally, a reduction in densities for the higher FRET states, 0.6–0.8, are reduced for the mismatches in comparison to the cognate pairs G:ddC and G:C; this is especially apparent for the G:A mismatch. While we can observe differences in wild type proofreading dynamics by changing the molecular identity of the terminal base pair, dynamics in proofreading at the protein level are limited using this approach. Therefore, we examined dynamics with an Exo-deficient Pol III mutant.
The binding rate constants were determined for the mutant Pol III and, overall, no significant differences were determined in comparison to wild type (Fig. S5, ESI†). For example, the off rate constants for the mutant ranged from 0.3–0.4 s−1 (Table 2); which is similar to those determined for wild type Pol III. Further, the kon,slow for the mutant did not show appreciable differences amongst the matched and mismatched DNA terminal base pairs. One difference, however, was for the kon,fast for the mutant and G:A (1.5 s−1, Table 1), which was 5 times faster than for the wild type (0.3 s−1, Table 1).
Significant differences in conformational dynamics for the wild type and mutant were determined from the HMM analysis (Fig. 2 and 3, respectively). For the mutant Pol III core, the number of transitions from 0.2–0.8 for the G:ddC DNA is markedly reduced in comparison to the wild type protein (Fig. 3A and 2A, respectively). However, the highest transition densities are still between 0.4–06 for both the mutant and wild type Pol III core for the G:ddC DNA. In the case of G:C, two dominant transition densities are present around 0.4 and 0.6 FRET efficiencies (Fig. 3B). Additionally, a reduction in the transition densities is observed for the lower (0.2) and higher (0.7–0.8) FRET states in the case of the mutant Pol III core as compared with the wild type for the G:C DNA (Fig. 3B and 2B, respectively). For the G:T mismatch, the highest density transition is present at 0.7 and for G:A it occurs at 0.2, 0.4 and 0.6 FRET states (Fig. 3C and D, respectively).
Molecular dynamics (MD) simulations were performed to gain atomic-level insights into the dynamics of the wild-type and mutant systems. These simulations also showed more Pol III motions for the wild type as compared to the mutant (Fig. 5). Eight representative systems were constructed for a Pol III core-clamp model (α, β, ε, θ complex, see Methods section) for wild type systems in the Exo mode, Pol mode with correct dG-dC matched base at the DNA terminus (Fig. S9, ESI†), Pol mode with dG–dA mismatched base at the DNA terminus, and an apo structure with no DNA. A representative image indicating each subdomain is seen in Fig. S9 (ESI†). Each of these was also mutated at 453 from Tyr to Ala for another set of systems. These simulations ran for a comparatively short time, 225 nanoseconds (ns) for each system (1.8 μs total), compared with the FRET experiments due to the constraints of computer hardware and the large size of the system (∼400000 atoms including solvent). Overall, interesting correlations between the simulation data and single-molecule experimental results were obtained.
![]() | ||
Fig. 5 Representative snapshots from the molecular dynamics simulations for (A) Exo mode, (B) Exo mode mutant, (C) Pol-dC, and (D) Pol-dC mutant. Mutation site is highlighted in pink. |
Distances between the site of the Cy5 acceptor (tag location, residue 41 on the θ subunit) and the site on the DNA (alpha C) for the Cy3 donor were calculated over time for each trajectory (Fig. S9, ESI†). The Exo modes show a distance of around 35–40 Å, while the Pol modes show a distance from 50–65 Å. The Pol-dC wild type shows the highest amount of distance and conformational variation over time for the tag distance, spanning a range of ∼50–70 Å. Comparatively, the Pol-dC mutant structure shows a reduction in overall distance with a large drop at 70000 frames (140 ns) (Fig. S10, ESI†). This decrease is caused by the DNA rapidly sliding down, which could in turn have resulted from changes in the hydrogen bonding from the mutation. However, it was observed as an isolated event, and could be an artifact of that single simulation, since this was not observed in the other mutant trajectories. The tag distance variation for wild type Pol-dA is consistent between 52–60 Å and has a substantial decrease in fluctuation compared with Pol-dC, consistent with the smFRET experimental results. The wild type Exo and Exo mutant modes span about 30–40 Å and vary less compared to the Pol-dC and Pol-dC mutant modes, again consistent with the single-molecule FRET data.
To investigate the difference in motion and conformation between wild type and mutant Pol III and with the matched and mismatched termini, we performed principal component analysis (Fig. 6). Principal component analysis is a convenient way to assess relative differences in dynamics and overall motion between systems, and to hone in on specific differences in protein vibrational motion. In this case, the differences in overall structure indicate stable systems and relatively small structural shifts between the systems (Fig. S11–S16, ESI†), but substantial differences in dynamic motion. The first principal component analysis normal mode (PCA1) is shown mapped onto a three-dimensional structure. Further, per residue square fluctuations for PCA1–PCA3 are depicted on the major graph for each panel, and show that the majority of the vibrational motion is contained in the first two modes. There are areas of high fluctuation primarily in the β2-clamp, ε and part of the finger domain in α for Pol-dC (Fig. 6A). For Pol-dA (Fig. 6B), a decrease in square fluctuations appears in the β2-clamp domain, a shifting of location for fluctuations in the finger domain, an increase in ε (Exo domain) and an increase for the primer strand of DNA with the mispaired base. While the locations of the square fluctuations spread out more for Pol-dA as compared to Pol-dC, the amount of fluctuation decreases fairly substantially (Fig. 6B). The Exo mode shows an almost complete loss of fluctuation in the clamp, a decrease in the theta domain and an increase in the alpha and epsilon domains as compared to the Pol modes (Fig. 6C).
The Pol-dC mutant shows a substantial decrease in overall square fluctuations in the first two normal modes, although the locations of high fluctuation remain similar (Fig. 6D). Similarly, the Pol-dA mutant (Fig. S17E, ESI†) shows similar locations of fluctuation, but an overall decrease in square fluctuations for the first two modes. The Apo structure without DNA (Fig. S18G, ESI†) and Apo mutant (Fig. S18H, ESI†) also show large changes in the first few normal modes, which correlates with the smFRET experimental data showing that the Y453A mutation reduces the dynamics of Pol III core.
Previous studies involving E. coli DNA polymerases have revealed new insights into polymerization and proofreading dynamics.23–26 For example, a recent single-molecule flow-based study with Pol III core showed that transfer of the primer strand during proofreading did not disrupt interactions between the ε and β2 subunits.27 Further, this work showed that the ε and β2 interaction remains intact in Pol and Exo mode. Also, a co-localization single-molecule spectroscopy assay that monitored loading of E. coli proteins during replication did not observe changes in lifetimes of the Pol III core in presence of mismatched DNA or an N2-furfuryl-dG lesion.28 Similarly, we did not observe a change in binding kinetic rate constants of the Pol III core in the presence of matched or mismatched DNA termini.
A significant contrast in the conformational dynamics during proofreading is observed for DNA Pol I and DNA Pol III. Work with E. coli DNA Pol I determined the kinetic rate constants for active site switching between the Pol and Exo domains; there, a smFRET assay was developed containing a Cy5-labeled DNA Pol I and Cy3-labeled DNA.29 For Pol I, two FRET states were assigned, one for the Pol-binding mode at 0.82 and another at 0.67 for the Exo-binding orientation. Interestingly, there is a significant difference in the dynamics for DNA Pol I compared to Pol III core. For Pol III core, our transition density plots analysis shows that Pol III binds very dynamically to DNA and more than two FRET states are observed (Fig. 2 and 3), unlike for DNA Pol I. This highlights a contrasting dynamic range for these two polymerases. It is important to note that DNA Pol I and Pol III belong to different polymerase families (A and C-type, respectively) and have their exonuclease domain located in very different positions, which could contribute to the differences in proofreading dynamics. In addition, the smFRET assay with Pol III core is in the presence of the sliding clamp, which could also have an influence on the binding dynamics and lead to an increase in the number of intermediates along the proofreading pathway in comparison to the single protein DNA Pol I.
It was surprising to observe that the matched base pairs (G:C and G:ddC) revealed a higher number of conformational dynamics than for the mismatches (G:T and G:A), shown in the TDP analysis (Fig. 2 and 3) and the MD simulations (Fig. 6). These dynamic differences were also observed in MD simulations where the measured tag distances for the Pol-dA system had a substantial decrease in fluctuation compared with the Pol-dC system (Fig. S10, ESI†). The observed experimental difference in dynamics may be explained by the fact that system is chemically modified to prevent polymerase-mediated elongation (i.e. dideoxy and non-hydrolyzable dCTP). Moreover, these chemical modifications could lead to an enzyme that is probing a variety of conformations to mitigate the disturbance and effectively try to push the forward replication reaction to proceed. In the case of the mismatch, however, it is possible that the protein takes longer while partitioning to the exonuclease domain for proofreading to occur. Indeed, since the MD simulations do not contain the fluorescent organic dyes during the calculations, an alternative explanation for this observation is that these dynamics are attributed to the natural behavior of the protein.
In addition to a difference in dynamics as a function of the terminal base pair, we also observed a varied range of dynamics for the wild type compared to the mutant. This effect is observed by comparing the wild type and mutant TDP plots for each respective DNA (Fig. 2vs.Fig. 3). In addition, we compared the number of dynamic and static events for wild type and mutant Pol III core binding to DNA with a terminal double mismatch (Fig. 4). Here, we found that wild type Pol III has a relatively equal distribution of dynamic and static events at a (49% and 51%, respectively). While, for the mutant, we observed a significant reduction in the number of dynamic events (Fig. 4). In addition, the simulation data monitoring tag distances show that the Pol-dC mutant has a reduction in overall distance with a large drop at 70000 frames (140 ns) (Fig. S10, ESI†). This could be due to a DNA slipping mechanism arising from the mutation from tyrosine to alanine reducing the stability of the DNA in the active site (Fig. S10, ESI†). This difference in Pol vs. Exo mode dynamics has also been reported for DNA PolB in Pyrococcus furiosus.30 In MD simulations, PolB showed a change in conformational dynamics in the Exo mode compared with the Pol mode and it is hypothesized that interactions between PolB and the clamp subunit contributes to the observed change in dynamics compared to the Exo mode.
To further investigate the influence of the mutation, correlation analysis was performed between the Exo wild type and Exo mutant structure (Fig. S19, ESI†). The locations of increased correlation and anticorrelation are similar to that shown in the principal component analysis of the normal modes, with the majority of anticorrelation difference on the DNA and finger domain and the majority of the correlation difference on θ, ε and the finger domain of α. Though the mutation results in a clear change in the overall DNA location and structure, the DNA base remains stable in the exonuclease active site, with the majority of anticorrelation occurring near the mutation location, as one might expect. Further, overall distance changes between Y453 and A453 of ∼4–5.6 Å were observed, suggesting the mutation alters the local positioning of the α helix of the protein (Fig. S19–S21, ESI†). These results are consistent with the experimental finding of relatively similar activity despite the mutation, but with a change in the overall dynamics. Correlation analyses of the Pol mode show similar results, with the strongest differences in correlation between wild type and variant appearing with a mismatched base pair in Pol mode (Fig. S20 and S21, ESI†).
This study reveals a model for the relative conformational dynamics of Pol III core. The present data indicate two main findings. First, wild type Pol III core is more dynamic than the exonuclease deficient Y453A mutant. Second, that matched base pairs evoke higher Pol III dynamics than with mismatched DNA termini (Fig. 7). Regarding the observed reduction in conformational flexibility for the Y453A mutant, this suggests that this tyrosine residue influences the structural and conformational dynamics that are involved in Pol III core proofreading. Structural and simulation data suggests that Y453 stabilizes Exo mode by aromatically stacking with a nucleobase on the primer strand. Our data shows that the loss of this tyrosine lowers the overall protein dynamics of Pol III core and we hypothesize that the loss of this tyrosine moiety removes a stabilization factor necessary for efficient proofreading. Further studies that monitor DNA polymerase dynamics with mutated residues that stabilize and destabilize Exo mode are needed to determine their influence on proofreading and, ultimately, their role in mutagenesis and carcinogenesis in humans.6
f(t) = A![]() | (1) |
f(t) = A1![]() ![]() | (2) |
Footnotes |
† Electronic supplementary information (ESI) available: Additional exponential decay curves, primer extension data, distribution plots and MD simulated results. See DOI: 10.1039/c8cp04112a |
‡ Current address: Department of Cell and Chemical Biology, Leiden University Medical Center, Leiden, The Netherlands. |
This journal is © the Owner Societies 2018 |