Top PDF DNA Mediated Charge Transport in DNA Repair

DNA Mediated Charge Transport in DNA Repair

DNA Mediated Charge Transport in DNA Repair

Another biologically important target for oxidative stress is the mitochondrion. Mitochondria contain their own DNA and also harbor an abundance of reactive oxygen species as a result of their function in oxidative phosphorylation (21). Mutations in mitochondrial DNA have been found in a variety of tumors and are associated with other diseases, while other DNA perturbations, like large scale rearrangements, are common in mitochondrial DNA (21). Oxidative damage to extracted mitochondrial DNA (22), as well as to mitochondrial DNA within functioning mitochondria (23), promoted by the rhodium photooxidant reveals that DNA lesions can arise from a distance using DNA CT. Again, this damage from a distance was demonstrated by comparing sites of Rh binding versus guanine oxidation. The spatial separation between the Rh binding sites and one electron guanine oxidation sites is striking; oxidation can occur more than 70 bases away from the nearest bound oxidant. Again these data support long range CT through DNA within a cellular organelle, here the mitochondrion (Figure 1.3).
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DNA Mediated Charge Transport Signaling Within the Cell

DNA Mediated Charge Transport Signaling Within the Cell

DNA-mediated signaling within E. coli is required for efficient repair by UvrC A model for how DNA repair proteins with 4Fe-4S clusters utilize DNA-mediated CT chemistry as a means of signaling has been proposed by our lab. 25,27,29,53,54 Within this model, DNA-processing enzymes with 4Fe-4S clusters use DNA-mediated CT in order to scan the genome for damage and localize in the vicinity of damage. A depiction of the model is shown below in Figure 3.6. The model is based on four main postulates: i) DNA CT can occur over long molecular distances (>100 bp), ii) DNA damage attenuates DNA CT, iii) DNA-processing enzymes that share a redox potential can shuttle electrons between one another via DNA-CT, and iv) these enzymes have a higher binding affinity when the 4Fe-4S cluster is in the 3+ oxidation state vs. the 2+ oxidation state. It is hypothesized that if a protein with a 4Fe-4S cluster binds within CT distance of another protein that is oxidized, then it can shuttle an electron to the distally bound protein, so long as there is no intervening damage that would attenuate CT. The distally bound protein would then be reduced, promoting its dissociation from DNA, and signaling for it to search the genome elsewhere for damage. This acts as an effective scan of that segment of the genome. This process would continue until a protein binds in the vicinity of the damage, at which point the distally bound protein would stay bound and proceed to the site of damage to repair it. This mechanism is attractive since it would significantly speed up the search and repair process, 27 which is currently not well understood. Current models for how proteins like glycosylases, which are thought to be at low copy numbers within cells, locate damage within the time required by a cell before it divides do not take into account protein traffic. 55–57 DNA-mediated CT signaling would provide a mechanism for how
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Fundamental mechanisms and biological applications of DNA mediated charge transport

Fundamental mechanisms and biological applications of DNA mediated charge transport

is exquisitely sensitive to perturbations in the intervening base pair stack; DNA binding-proteins, intervening mismatches, and base lesions can serve to attenuate charge migration [20]. This sensitivity of DNA CT to stacking perturbations has led to its application in developing novel electrochemical sensors [21, 22]. DNA CT has also been proposed to play a biological role in the detection of base lesions by DNA repair proteins [23]. Moreover, long-range oxidative DNA damage through CT has been demonstrated to occur within nucleosomes and within the cell nucleus [24, 25]. DNA CT can furthermore be harnassed to promote a variety of redox reactions on DNA triggered from a distance. We have shown that thymine dimers in DNA can be repaired at long range through DNA-mediated CT [26]. Most recently, we have determined that DNA CT can be utilized to promote the formation of disulfide bonds from thiols incorporated into the DNA backbone [27]. It is this chemistry, triggered from a distance, that we considered might also be useful in promoting reactions of proteins bound to DNA. Since p53 contains cysteine residues in close proximity within the DNA binding domain, we wondered if we could selectively oxidize the DNA-bound protein and in so doing, alter DNA binding through long-range CT. This chemistry from a distance, mediated by DNA, would then provide a completely new mechanism to globally regulate p53 binding. Figure 4.1 schematically illustrates this general chemistry.
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Investigations of DNA-Mediated Redox Signaling Between E.coli DNA Repair Pathways

Investigations of DNA-Mediated Redox Signaling Between E.coli DNA Repair Pathways

E. coli endonuclease III (EndoIII) is a DNA glycosylase that excises oxi- dized pyrimidines from DNA, functioning as part of the base excision repair (BER) pathway in order to maintain the integrity of the genome [1]. EndoIII contains a [4Fe4S] 2+ cluster that is relatively insensitive to reduction and oxidation in solution [2]; as a result, it was initially proposed that the cluster served only a structural role within the protein. MutY is another E. coli BER glycosylase, homologous to EndoIII, that also contains a [4Fe4S] 2+ cluster [3]. MutY, found in organisms from bacteria to man, is involved in the repair of oxoG:A mismatches [4]; in humans, inherited defects in MUTYH are associated with a familial form of colon cancer known as MUTYH-associated polyposis (MAP) and many MAP-associated vari- ants are localized near the [4Fe4S] cluster [4]. Furthermore, in the case of MutY, it has been shown that the cluster is not required for folding or stability [3], or di- rect participation in the intrinsic glycosidic bond hydrolysis catalysis [5], making the widespread presence of conserved, noncatalytic [4Fe4S] clusters difficult to explain. Notably, the earliest studies with EndoIII and MutY looked only at free protein in solution, neglecting the effect of DNA binding on redox potential. Experiments carried out on DNA-modified electrodes have demonstrated that, in both EndoIII and MutY, the cluster undergoes a negative shift in potential associated with binding to the DNA polyanion and is activated toward reversible redox activity [6]. In these experiments, DNA monolayers were formed on gold electrodes, and upon addition of EndoIII or MutY, a reversible signal with a midpoint potential ranging from 60 to 95 mV versus NHE was observed. Importantly, the introduction of just a single mismatch or abasic site into DNA led to signal attenuation, showing that electron transfer between the protein and the electrode was through the π -stacked base pairs in a process known as DNA-mediated charge transport (DNA CT) [7]. In this process, charge is funneled from the electrode surface through the π -stack of the DNA bases to reach the redox probe (a protein in this case); the only requirement is that the probe must be electronically coupled to the DNA π -stack. Remarkably, the sensitivity to base stacking observed with EndoIII and MutY was comparable to that obtained using small molecules such as Nile blue or methylene blue that intercalate directly into the base stack.
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Expanding the Repertoire of DNA-Mediated Signaling in DNA Repair

Expanding the Repertoire of DNA-Mediated Signaling in DNA Repair

To determine the efficiency of DNA-mediated charge transfer for each of these individual mutants, each protein (~10 µ M) was incubated on a DNA-modified electrode, and cyclic voltammagrams (CVs) were taken every 3 minutes. The surface was rinsed, and subsequently scanned to verify that any residual protein was removed. This process was repeated for each of the protein mutants. While the magnitudes of the currents varied, each protein retained the same mid-point redox potential of ~80 mV versus NHE. Current intensities were determined for each of the protein mutants and normalized to [4Fe-4S] cluster content (Figure 3.2D). All mutants made have been implicated in human disease, with the exception of the Y104A mutant, which was generated to explore the CT pathway in the protein. This mutant exhibited an electrochemical signal much larger than that of the WT protein (approximately 2-5 times larger). Similarly, the K84H mutant, a mutant implicated in disease, exhibits a signal larger than WT (Figure 3.2D). The G34R mutant, which is ATPase and helicase deficient, exhibits a signal comparable to the WT protein (Figure 3.2D). As predicted in our model for communication between proteins, a larger redox signal would translate into more efficient CT between proteins.
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Activation of Transcription from a Distance: Investigations on the Oxidation of SoxR by DNA Mediated Charge Transport

Activation of Transcription from a Distance: Investigations on the Oxidation of SoxR by DNA Mediated Charge Transport

Examination of guanine oxidation at this 180-mer duplex reveals that damage occurs preferentially at sites which consist of guanine multiplets, and in particular, a 4-guanine site 20 base pairs away from the site of SoxR binding. The overall yield of damage is low, given the short-lived excited state of the Rh photooxidant and its low quantum yield of damage, which is due to rapid back-electron transfer to the long-lived guanine radical (12). Nevertheless, some damage is seen and is not localized near the site of Rh binding, suggesting that the hole equilibrates along the entire length of this DNA. Thus, DNA-mediated CT can occur along a distance long enough to effect SoxR oxidation. In a biological sense, these results highlight the competency of DNA to act as an antenna for oxidative damage in the cell, as the precursor species, oxidative radicals, are able to migrate along the base stack and localize at low potential hot spots that are proximal to the SoxR binding site. Indeed, previous studies have demonstrated that holes formed in DNA are able to be filled oxidation of reduced SoxR (Chapter 3).
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DNA-Mediated Charge Transport for Long-Range Sensing and Protein Detection

DNA-Mediated Charge Transport for Long-Range Sensing and Protein Detection

Conformational gating and charge delocalization also help to explain why such severe attenuation of DNA CT is observed when the structure of the DNA π-stack is distorted. Structural perturbations to the π-stack cause the affected bases to preferentially adopt un- stacked conformations. When it comes to the formation of delocalized domains that facilitate CT, a primarily un-stacked base functions like a rotating disc in a multi-disk combination lock that is stuck on the wrong number. Although the other disks might turn fluidly to the correct combination the disk that is stuck in the wrong orientation will still prevent the lock from opening. Likewise, the bases surrounding a lesion, mismatch, or bound protein may be free to move into CT-active conformations, but the un-stacked base disrupts the formation of a domain over which charge could delocalize, dramatically shutting off DNA CT. Charge delocalization additionally accounts for the fast CT rates that are measured electrochemically: states in which the charge is delocalized over large domains would necessarily be lower in energy than a state in which the charge is localized on an individual base, thus enabling charge injection at the applied potentials used in electrochemistry experiments.
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Regulation of Wild Type and Mutant p53 through DNA mediated Charge Transport

Regulation of Wild Type and Mutant p53 through DNA mediated Charge Transport

Identification of oxidized residues. To identify the cysteines oxidized through DNA CT, we used liquid chromatography coupled to nanoelectrospray, tandem mass spectrometric analysis of the digested peptides that have undergone differential cysteine modification. The order of modifications is shown in Figure 2.6. The mass of the modification reveals the oxidation state of the cysteine residue. The cysteines that were still in a reduced state after irradiation exhibit carbamidomethylation from reaction with iodoacetamide (an approximate mass gain of 57 amu). Half of the sample was digested with trypsin, and the other half was digested with chymotrypsin to provide a more complete survey of the 10 cysteines within p53. Then the samples were desalted, and the cysteines that had been participating in an oxidative modification are reduced and then reacted with N-methyliodoacetamide, with an approximate mass gain of 71 amu. The peptide samples were desalted again and then analyzed by LC-ESI-MS. The reactivity of iodoacetamide and N-methyliodoacetamide was approximately the same; 51 furthermore, there was little preference for peptides with one modification or the other in control MS samples (data not shown).
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DNA-Mediated Hole and Electron Transport

DNA-Mediated Hole and Electron Transport

1.4.5. Trapping rates of electron acceptors versus back electron transfer. Except for several observations of intermediate reaction radicals by transient absorption spectroscopy, 17,68 CT yield reported by the redox reaction over long molecular distances is based on the oxidative damage at a thermal hole trap, guanine or its analogues. 13,26 However, the guanine radical has a relatively long lifetime (millisecond time scale) 36 compared to the gating process of CT: the dynamic motion of DNA bases. Before being trapped as permanent guanine damage, charge can undergo many pathways, which inevitably convolute the concomitant CT process. In particular, fast back electron transfer (BET) within the initially generated radical ion/ion pair can severely diminish the amount of charge that remains. For instance, no detectable permanent guanine damage was observed when Ap or thionin (Th), a DNA intercalator, were used as covalently
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Spectroscopic characterization of DNA-mediated charge transfer

Spectroscopic characterization of DNA-mediated charge transfer

1.1 Schematic illustration of the DNA double helix 3 1.2 Illustration of a Rh(III) modified duplex with 5’-GG-3’ sites up to 200 Å away 4 1.3 Reaction coordinate diagram and couplings for electron transfer 7 1.4 Assemblies in which long-range charge transport was demonstrated 12 1.5 Illustration of duplex DNA self-assembled monolayers on gold 14 1.6 Assemblies highlighting the importance of stacking to charge transfer in DNA 18 1.7 Effect of protein binding on long-range guanine oxidation 20 1.8 Ethidium, guanine, and 7-deazaguanine structures and reactivity 22 1.9 Structures, reactivity, and stacking of modified bases in DNA 24 1.10 Distance dependence of electron transfer through DNA-modified electrodes 26 1.11 Incorporation of a mismatch in DNA-modified gold electrodes 27 1.12 Energetics and coupling involved in various charge transfer mechanisms 31
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DNA-mediated Charge Transport in a Biological Context: Cooperation among Metalloproteins to Find Lesions in the Genome

DNA-mediated Charge Transport in a Biological Context: Cooperation among Metalloproteins to Find Lesions in the Genome

Inspired by our findings that DNA-mediated CT can trigger MutY, we have examined yet another system that includes a metallo transcription factor, SoxR. As mentioned earlier, SoxR is activated in the presence of oxidative stress; however, the specific source for oxidation in the cell is unknown. The protein is reversibly inactivated in vitro by reduction of its [2Fe2S] clusters using dithionite (42). In vivo studies, using redox-cyclers such as paraquat to induce oxidative stress, show that superoxide is not the direct activator of SoxR. Rather, there is depletion of cellular NADPH, which is normally required to keep SoxR in a reduced form. The redox-cyclers then undergo autooxidation and produce superoxide by losing an electron to dioxygen. Plumbagin and phenazine methosulfate, notably, have yielded reversible oxidation of the [2Fe2S] clusters (42). Electrochemistry of SoxR in our lab showed a redox-active signal for the [2Fe2S] cluster of SoxR at +200 mV vs. NHE, indicating the protein undergoes one electron oxidation bound to DNA (44). Due to the shift seen upon binding to DNA, we questioned whether the DNA-bound form of SoxR might be the missing oxidative switch. As DNA is subjected to oxidative damage, and guanine radicals, capable of migrating through the base pair stack, are generated, these guanine radicals can then serve as the oxidant for SoxR. Once oxidized, SoxR can promote transcription under conditions when the cell needs it the most. The oxidative signal then would be the guanine radical. We have learned that the coupling properties of photooxidants used to probe DNA-mediated CT are important factors in observing this process. Therefore, SoxR studies were recently conducted in our lab using a strong DNA intercalator, [Ru(phen)(dppz)(bpyʹ′)] 2+ , where dppz is the moiety
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Magnetic Field Effects and Biophysical Studies on DNA Charge Transport and Repair

Magnetic Field Effects and Biophysical Studies on DNA Charge Transport and Repair

and irradiated with blue light in aqueous solution. HPLC of the DNA before and after incubation show that at CRY1ΔC repairs T□T similar to ecPL (Figure 3.13). When the cryptochrome is added to a monolayer of duplex DNA, each containing a TT dimer in the absence of an applied magnetic field, irradiation with blue light leads to the increase in current for the FAD redox couple (Figure 3.14). Shining light on an identical monolayer in the presence of an applied magnetic field, however, leads to a significant reduction in the yield of charge transferred over the same period of time, consistent with the change observed for photolyase. The diminished signal on the electrode modified with DNA without thymine dimers shows that atCRY1ΔC is preferentially binding to the CPD lesion. Consistent with the experiments on photolyase, the angle of the magnetic field relative to the plane of the electrode significantly influences the yield of repair by cryptochrome (Figure 3.15). A magnetic field perpendicular to the plane of the electrode exhibits the largest effect. Changing the angle of inclination to 45 degrees diminishes the effect, as does applying a field parallel to the plane of the surface.
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DNA Mediated Charge Transport Devices for Protein Detection

DNA Mediated Charge Transport Devices for Protein Detection

Given the anti-cooperative nature of TBP binding observed upon thermodynamic investigations of this protein binding to OCT-DNA monolayers, we also investigated the relative kinetics of TBP binding to these monolayers and to thiolated DNA films. Rotating disk electrode (RDE) experiments were undertaken to determine the binding kinetics of TBP on both high density thiol-DNA and low-density OCT-DNA monolayers. RDEs remove diffusion as a factor when determining kinetics of a system. 44, 48 The loss of an electrochemical DM signal upon TBP binding over time therefore reports on the kinetics of protein binding. Because the number of TBP binding sites is fixed, the solution concentration of protein is in large enough excess to be unaffected by the amount of protein bound to the surface, and the rate of TBP diffusion to the surface is removed as a factor, we can analyze the kinetics of TBP binding to the surfaces with a Langmuir kinetics model. As is evident in Figure 2.13, which shows the decrease in charge determined from the area of the reductive peak plotted as a function of time, the rate of signal decrease for both the high density and ultra low-density monolayers upon TBP binding is almost identical. As is apparent in the figure, the RDEs produce similar overall signal attenuations to stationary electrodes for both types of DMEs. When the data are fit to this Langmuir equation for protein binding kinetics, the k obs for high density
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Investigating DNA-Mediated Charge Transport by Time-Resolved Spectroscopy

Investigating DNA-Mediated Charge Transport by Time-Resolved Spectroscopy

conduit for charge, the probe should interact strongly with the DNA base stack. Such an interaction can be difficult to achieve, considering the geometry of DNA. In general, the only access a diffusing molecule has to the base stack is either at the ends of the DNA strand or within the relatively narrow major and minor grooves which run lengthwise along the sides of the DNA molecule. Probes which are too large, or which are strongly negatively charged and therefore are repelled by the phosphate backbone of DNA, do not easily interface with the DNA π-stack. Second, depending on the function of the probe, it must provide a straightforward means of either initiating or reporting on DNA CT, or both. Often, the photophysical or electrochemical properties of a molecule are utilized for these purposes. Some probes may also report CT events through chemical pathways such as degradation. Third, the probe should not degrade or interact chemically with the DNA strand or with other components of the sample unless this is by design. Not only must the probe be stable enough to persist in solution, but the excited state of the molecule must also be stable if photochemical means are used to initiate or report CT, and the various redox states of the molecule must be able to withstand the charge transfer process. Finally, the ideal probe would be synthetically versatile and easy to build or modify in order to control sensitively the parameters of the experiment. Metallointercalators, transition metal complexes which bind DNA primarily by intercalation, are one class of molecules that fulfill all of these requirements.
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Exploring DNA Mediated Charge Transport with Fast Radical Traps

Exploring DNA Mediated Charge Transport with Fast Radical Traps

DNA charge transport (CT) chemistry has received considerable attention by scientific researchers over the past 15 years since the first provocative publication on long-range CT in a DNA assembly. 1,2 This interest, shared by physicists, chemists and biologists, reflects the potential of DNA CT to provide a sensitive route for signaling, whether in the construction of nanoscale biosensors or as an enzymatic tool to detect damage in the genome. Research into DNA CT chemistry began as a quest to determine whether the DNA double helix, a macromolecular assembly in solution with -stacked base pairs, might share conductive characteristics with -stacked solids. Physicists carried out sophisticated experiments to measure the conductivity of DNA samples, but the means to connect discrete DNA assemblies into the devices to gauge conductivity varied, as did the conditions under which conductivities were determined. Chemists constructed DNA assemblies to measure hole and electron transport in solution using a variety of hole and electron donors. Here, too, DNA CT was seen to depend upon the connections, or coupling, between donors and the DNA base-pair stack. Importantly, these experiments have resolved the debate over whether DNA CT is possible. Moreover these studies have shown that DNA CT, irrespective of the oxidant or reductant used to initiate the chemistry, can occur over long molecular distances but can be exquisitely sensitive to perturbations in the base-pair stack.
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Electrochemical sensors based on DNA mediated charge transport chemistry

Electrochemical sensors based on DNA mediated charge transport chemistry

To take nothing away from the science done in the Barton Labs, one of my favorite things about working for Jackie is that she attracts a wonderful group of people and I have truly enjoyed my experience in lab for that reason. I am sure I could list everyone that I have worked with over the last five years along with something for which I should thank that person, but for the sake of brevity, here is the short list. Shana Kelley and Scott Rajski helped me to settle in to lab and learn the skills I needed to get started. Donato Ceres and Greg Drummond have been excellent co-DNA electrochemists and friends. Julia Salas is a talented undergraduate student who worked with me for two years. Pratip Bhattacharya, Duncan Odom, Chris Treadway, Melanie O'Neill, Matthias Pascaly, Dave Vicic, Eva Rueba, and Jon Hart have all contributed in various ways to increase both my knowledge of science and research as well as my enjoyment of graduate school. I particularly want to thank Kim
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The function of CUX1 in oxidative DNA damage repair is needed to prevent premature senescence of mouse embryo fibroblasts

The function of CUX1 in oxidative DNA damage repair is needed to prevent premature senescence of mouse embryo fibroblasts

Using 8-oxoG cleavage assays with purified proteins, we previously showed that various combinations of Cut repeats and the Cut homeodomain (CR1CR2, CR3HD and CR2CR3HD) are able to stimulate the glycosylase activity of OGG1 [16]. Since each of these recombinant proteins is able to bind to DNA with high affinity, a logical conclusion from these experiments is that high affinity DNA binding contributes to the stimulation of OGG1. Here, we performed structure/function analysis to test this notion and we further characterized the effect of CUX1 on distinct biochemical activities of OGG1. The glycosylase activity of OGG1 was stimulated in the presence of any recombinant CUX1 protein that contains one or more Cut repeats: CR1CR2, CR3HD, CR2CR3HD and CR1 (Figure 5A, compare lane 3 with 4, 5 and 6; Figure 5B, compare lane 2 with 3, 4 and 5). The stimulation of OGG1 appears to be specific to the Cut repeat domain since there was no effect of other regions of
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Increased non-homologous end joining makes DNA-PK a promising target for therapeutic intervention in uveal melanoma

Increased non-homologous end joining makes DNA-PK a promising target for therapeutic intervention in uveal melanoma

A potential competitive relationship between HR and NHEJ is proposed for other cancers [22,23,26] and here, this study shows that it likely exists in UM. This relationship may explain why in clinical trials Melphalan has been shown to have some effectiveness against UM [4], as resistance to Melphalan requires a fully functional HR capacity and indeed FANCD2 expression [39]. UM, thus having a reliance/preference for NHEJ and reduced FANCD2 expression, initially respond to Melphalan treatment. The reasons why they relapse later is less clear and it will be important to determine whether preferential use of NHEJ continues later in the disease or whether residual HR activity is enough to allow resistance to Melphalan. The exact mechanism for preference of UM for NHEJ repair is not clear at this point. Recent evidence has begun to suggest a consensus on how UM repair DNA damage with studies showing as the authors have, that Olaporib is not efficient alone on UM [40] and that PRKDC is upregulated, with NU7026 being an effective regulator of UM proliferation [28]. There are many points to explore in future studies. The use of assays, such as the DR-GFP/EJ5 assay, are required to investigate rigorously the HR status, whilst the anomaly of reduced LIG4 expression may shed light on the interplay between HR and NHEJ in UM. Further investigations will provide a better understanding of the conundrum of selection for preferential DSB repair by UM and may provide new answers for the treatment of this difficult to treat melanoma.
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Repair of Topoisomerase-Mediated DNA Damage in Bacteriophage T4

Repair of Topoisomerase-Mediated DNA Damage in Bacteriophage T4

Figure 5.—Repair of both topoisomerase- mediated damage and I-TevI breaks share similar protein requirements. Cells harboring pAC1000 and either pBS1 (topo site; lanes 1–6) or pBS4 (I-TevI site; lanes 7–12) were infected with K10 (wt) or the indicated K10 mutant (see materials and methods). All samples received m-AMSA at 6 min postinfection, and samples were collected at 24 min postinfection. Purified total DNA was digested with AseI. The blot was hybridized with the flanking (A) and NaeI (B) probes. Repair bands (RC, NC, and LC) are most visible in B. In A, unmodified pAC1000 is indicated by the open arrowhead, while the unmodified and modified pBS bands are indicated by the bottom and top solid arrowheads, respectively. The unmodified and modified large cleavage bands are indicated by the bottom and top asterisks, respectively. The small cleavage bands have migrated off the gel. In B, the bracket indicates the pAC1000 band. The experiments presented in this figure were repeated multiple times with comparable results.
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Studies on protein:protein interactions in mammalian DNA base-excision repair

Studies on protein:protein interactions in mammalian DNA base-excision repair

15 RNA is highly error prone as a genetic template - its structure is exposed to hydrolysis due to the presence of the 2 -OH group of ribose. The transition from the vulnerable RNA molecule to the more stable DNA occurred via reduction of the 2 -OH ribose, making DNA more adequately equipped to hold the genetic information in a more or less intact form. This feature of DNA is mainly due to the higher stability of the phosphodiester bonds in DNA compared to those in RNA. Furthermore, DNA's double­ strand form is composed of a pair of complementary polynucleotide molecules that make DNA more robust and, as noted by Watson and Crick, allow for easier replication; also, the repair machinery can use the intact DNA strand as a template to correct the associated damaged strand. However, as a consequence of the reduction from ribose in nucleic acid to deoxyribose, base-sugar bonds became more susceptible to hydrolysis (Lindahl, 1996). Thus, DNA is a dynamic molecule that is constantly undergoing very slow alterations due to the action of a wide range of physical and chemical agents present both within the cell, and in the external environment. The fact that some of these changes go uncorrected is crucial for an organism to adapt to new conditions and so increase its chances of survival. Organisms that have this ability are not only fit to survive environmental changes but, by allowing variations to be introduced into their genomes, also provide new material for natural selection. There is a continuous interplay between the need to faithfully transmit the genetic information in its intact form and the need to accommodate change. The balance between these two processes depends on the functioning of DNA repair systems.
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