Error-prone repair DNA polymerases in prokaryotes and eukaryotes

RAD51B plays an essential role during somatic and meiotic recombination in Physcomitrella

The eukaryotic RecA homologue Rad51 is a key factor in homologous recombination and recombinational repair. Rad51-like proteins have been identified in yeast (Rad55, Rad57 and Dmc1), plants and vertebrates (RAD51B, RAD51C, RAD51D, XRCC2, XRCC3 and DMC1). RAD51 and DMC1 are the strand-exchange proteins forming a nucleofilament for strand invasion, however, the function of the paralogues in the process of homologous recombination is less clear. In yeast the two Rad51 paralogues, Rad55 and Rad57, have been shown to be involved in somatic and meiotic HR and they are essential to the formation of the Rad51/DNA nucleofilament counterbalancing the anti-recombinase activity of the SRS2 helicase. Here, we examined the role of RAD51B in the model bryophyte Physcomitrella patens. Mutant analysis shows that RAD51B is essential for the maintenance of genome integrity, for resistance to DNA damaging agents and for gene targeting. Furthermore, we set up methods to investigate meiosis in Physcomitrella and we demonstrate that the RAD51B protein is essential for meiotic homologous recombination. Finally, we show that all these functions are independent of the SRS2 anti-recombinase protein, which is in striking contrast to what is found in budding yeast where the RAD51 paralogues are fully dependent on the SRS2 anti-recombinase function.

Use of genome sequencing to assess nucleotide structure variation of Staphylococcus aureus strains cultured in spaceflight on Shenzhou-X, under simulated microgravity and on the ground

The extreme environment of space could affect microbial behavior and may increase the risk of infectious disease during spaceflight. However, the molecular genetic changes of methicillin-resistant Staphylococcus aureus (MRSA) in response to the spaceflight environment have not been fully clarified. In the present study, we determined the draft genome sequences for an ancestral S. aureus strain (LCT-SAO) isolated from a clinical sample and three derivative strains, LCT-SAS, LCT-SAM and LCT-SAG, cultured in parallel during the spaceflight Shenzhou-X, under simulated microgravity and on the ground, respectively. To evaluate the impact of short-term spaceflight on the MRSA strains, comparative genomic analysis was implemented. Genome-based mapping of toxin genes and antibiotic resistance genes confirmed that these strains have the conventional pathogenicity and resistance to drugs, as none of the strains showed significant changes in these regions after culturing in the three different environments; this result suggests that spaceflight may not change bacterial virulence or drug resistance. Thirty-nine strain-specific sequence variants (SVs) were identified throughout the genomes, and the three derivatives exhibited almost the same mutation rates. Fifty-nine percent of SVs were located in the intergenic regions of the genomes, indicating that S. aureus may have an extremely robust repair mechanism responsible for recognizing and repairing DNA replication mismatches. It is noteworthy that strain LCT-SAS, cultured in space, presented the most unique SVs (n=9) and shared the fewest SVs with LCT-SAM (n=5) and LCT-SAG (n=4). Furthermore, we identified 10 potential deletion regions and 2 potential insertion regions, with LCT-SAS appearing more fragile than other strains by this measure. These results suggest that the environment of space is inherently complicated, with multiple variables, and cannot be simulated in a simple manner. Our results represent the first analysis of nucleotide structure variation of S. aureus strains in a spaceflight environment and also provide a valuable insight for understanding the mutation strategies of MRSA on earth.

NMR structure of the human Rad18 zinc finger in complex with ubiquitin defines a class of UBZ domains in proteins linked to the DNA damage response

Ubiquitin-mediated interactions are critical for the cellular DNA damage response (DDR). Therefore, many DDR-related proteins contain ubiquitin-binding domains, including ubiquitin-binding zinc fingers (UBZs). The majority of these UBZ domains belong to the C2H2 (type 3 Polη-like) or C2HC (type 4 Rad18-like) family. We have used nuclear magnetic resonance (NMR) spectroscopy to characterize the binding to ubiquitin and determine the structure of the type 4 UBZ domain (UBZ4) from human Rad18, which is a key ubiquitin ligase in the DNA damage tolerance pathway responsible for monoubiquitination of the DNA sliding clamp PCNA. The Rad18-UBZ domain binds ubiquitin with micromolar affinity and adopts a β1-β2-α fold similar to the previously characterized type 3 UBZ domain (UBZ3) from the translesion synthesis DNA polymerase Polη. However, despite nearly identical structures, a disparity in the location of binding-induced NMR chemical shift perturbations shows that the Rad18-UBZ4 and Polη-UBZ3 domains bind ubiquitin in distinctly different modes. The Rad18-UBZ4 domain interacts with ubiquitin with the α-helix and strand β1 as shown by the structure of the Rad18-UBZ domain-ubiquitin complex determined in this work, while the Polη-UBZ3 domain exclusively utilizes the α-helix. Our findings suggest the existence of two classes of UBZ domains in DDR-related proteins with similar structures but unique ubiquitin binding properties and provide context for further study to establish the differential roles of these domains in the complex cellular response to DNA damage.

Impact of abasic site orientation within nucleosomes on human APE1 endonuclease activity

Glycosylases responsible for recognizing DNA lesions and initiating Base Excision Repair (BER) are impeded by the presence of histones, which are essential for compaction of the genetic material in the nucleus. Abasic sites are an abundant mutagenic lesion in the DNA, arising spontaneously and as the product of glycosylase activity, making it a common intermediate in BER. The apurinic/apyrimidinic endonuclease 1 (APE1) recognizes abasic sites and cleaves the DNA backbone adjacent to the lesion, creating the single-strand break essential for the subsequent steps of BER. In this study the endonuclease activity of human APE1 was measured on reconstituted nucleosome core particles (NCPs) with DNA containing enzymatically-created abasic sites (AP) or the abasic site analog tetrahydrofuran (TF) at different rotational positions relative to the histone core surface. The presence of histones on the DNA reduced APE1 activity overall, and the magnitude was greatly influenced by differences in orientation of the lesions along the DNA gyre relative to the histone core. Abasic moieties oriented with their phosphate backbones adjacent to the underlying histones (In) were cleaved less efficiently than those oriented away from the histone core (Out) or between the In and Out orientations (Mid). The impact on APE1 at each orientation was very similar between the AP and TF lesions, highlighting the dependability of the TF abasic analog in APE1 activity measurements in nucleosomes. Measurement of APE1 binding to the NCP substrates reveals a substantial reduction in its interaction with nucleosomes compared to naked DNA, also in a lesion orientation-dependent manner, reinforcing the concept that reduction in APE1 activity on nucleosomes is due to occlusion from its abasic DNA substrate by the histones. These results suggest that APE1 activity in nucleosomes, like BER glycosylases, is primarily regulated by its chance interactions with transiently exposed lesions.

Error-prone polymerase activity causes multinucleotide mutations in humans

About 2% of human genetic polymorphisms have been hypothesized to arise via multinucleotide mutations (MNMs), complex events that generate SNPs at multiple sites in a single generation. MNMs have the potential to accelerate the pace at which single genes evolve and to confound studies of demography and selection that assume all SNPs arise independently. In this paper, we examine clustered mutations that are segregating in a set of 1092 human genomes, demonstrating that the signature of MNM becomes enriched as large numbers of individuals are sampled. We estimate the percentage of linked SNP pairs that were generated by simultaneous mutation as a function of the distance between affected sites and show that MNMs exhibit a high percentage of transversions relative to transitions, findings that are reproducible in data from multiple sequencing platforms and cannot be attributed to sequencing error. Among tandem mutations that occur simultaneously at adjacent sites, we find an especially skewed distribution of ancestral and derived alleles, with GC → AA, GA → TT, and their reverse complements making up 27% of the total. These mutations have been previously shown to dominate the spectrum of the error-prone polymerase Pol ζ, suggesting that low-fidelity DNA replication by Pol ζ is at least partly responsible for the MNMs that are segregating in the human population. We develop statistical estimates of MNM prevalence that can be used to correct phylogenetic and population genetic inferences for the presence of complex mutations.

Enhancement of silencing DNA polymerase β on the radiotherapeutic sensitivity of human esophageal carcinoma cell lines

Human DNA polymerase β (DNA polymeraseβ (polβ)) is a small monomeric protein which is essential for short-patch base excision repair (BER). It plays an important role in regulating the radiation sensitivity of tumor cells in the course of tumor radiation therapy. In this study, qRT-PCR and Western blot assays were used to quantify polβ expression levels in esophageal carcinoma (EC) cells that were transfected with polβ small interfering RNA (siRNA). Cell counting Kit-8 (CCK-8), flow cytometry, and Hoechst/PI stain assays were conducted to evaluate the effects of silencing polβ on the radiotherapeutic sensitivity of EC cells. We found that the expression levels of polβ in EC cells were significantly decreased after transfection with polβ siRNA. Then, we found that polβ silencing increased the sensitivity of EC cells to radiation therapy. In conclusion, our study paves the way for a better understanding of the mechanism of the polβ gene in DNA repair, and we propose that RNA interference technology will have important applications in gene therapy of EC and other cancers in the future.

A genomic island integrated into recA of Vibrio cholerae contains a divergent recA and provides multi-pathway protection from DNA damage

Lateral gene transfer (LGT) has been crucial in the evolution of the cholera pathogen, Vibrio cholerae. The two major virulence factors are present on two different mobile genetic elements, a bacteriophage containing the cholera toxin genes and a genomic island (GI) containing the intestinal adhesin genes. Non-toxigenic V. cholerae in the aquatic environment are a major source of novel DNA that allows the pathogen to morph via LGT. In this study, we report a novel GI from a non-toxigenic V. cholerae strain containing multiple genes involved in DNA repair including the recombination repair gene recA that is 23% divergent from the indigenous recA and genes involved in the translesion synthesis pathway. This is the first report of a GI containing the critical gene recA and the first report of a GI that targets insertion into a specific site within recA. We show that possession of the island in Escherichia coli is protective against DNA damage induced by UV-irradiation and DNA targeting antibiotics. This study highlights the importance of genetic elements such as GIs in the evolution of V. cholerae and emphasizes the importance of environmental strains as a source of novel DNA that can influence the pathogenicity of toxigenic strains.

Physiology of the read-write genome

Discoveries in cytogenetics, molecular biology, and genomics have revealed that genome change is an active cell-mediated physiological process. This is distinctly at variance with the pre-DNA assumption that genetic changes arise accidentally and sporadically. The discovery that DNA changes arise as the result of regulated cell biochemistry means that the genome is best modelled as a read-write (RW) data storage system rather than a read-only memory (ROM). The evidence behind this change in thinking and a consideration of some of its implications are the subjects of this article. Specific points include the following: cells protect themselves from accidental genome change with proofreading and DNA damage repair systems; localized point mutations result from the action of specialized trans-lesion mutator DNA polymerases; cells can join broken chromosomes and generate genome rearrangements by non-homologous end-joining (NHEJ) processes in specialized subnuclear repair centres; cells have a broad variety of natural genetic engineering (NGE) functions for transporting, diversifying and reorganizing DNA sequences in ways that generate many classes of genomic novelties; natural genetic engineering functions are regulated and subject to activation by a range of challenging life history events; cells can target the action of natural genetic engineering functions to particular genome locations by a range of well-established molecular interactions, including protein binding with regulatory factors and linkage to transcription; and genome changes in cancer can usefully be considered as consequences of the loss of homeostatic control over natural genetic engineering functions.

Genetics just got SEXY: Sequences encoding XY

A semi-synthetic organism with an extended genetic alphabet heralds a new era in synthetic biology.

A semi-synthetic organism with an expanded genetic alphabet

Organisms are defined by the information encoded in their genomes, and since the evolution of life, this information has been encoded using a two base pair genetic alphabet (A-T and G-C). In vitro, the alphabet has been expanded to include several unnatural base pairs (UBPs)^1–3. We have developed a class of UBPs formed between nucleotides bearing hydrophobic nucleobases, exemplified by the pair formed between d5SICS and dNaM (d5SICS-dNaM, Fig. 1a), which is efficiently PCR amplified¹ and transcribed^4,5in vitro, and whose unique mechanism of replication has been characterized^6,7. However, expansion of a organism’s genetic alphabet presents new and unprecedented challenges: the unnatural nucleoside triphosphates must enter the cell; endogenous polymerases must be able to faithfully incorporate the unnatural triphosphates into DNA within the complex cellular milieu; and finally, the UBP must be stable in the presence of pathways that maintain the integrity of DNA. Here we show that an exogenously expressed algal nucleotide triphosphate transporter efficiently imports the triphosphates of both d5SICS and dNaM (d5SICSTP and dNaMTP) into E. coli, and that the endogenous replication machinery uses them to accurately replicate a plasmid containing d5SICS-dNaM. Neither the presence of the unnatural triphosphates nor the replication of the UBP introduces a significant growth burden. Lastly, we find that the UBP is not efficiently excised by DNA repair pathways. Thus, the resulting bacterium is the first organism to stably propagate an expanded genetic alphabet.

DNA polymerase V activity is autoregulated by a novel intrinsic DNA-dependent ATPase

Escherichia coli DNA polymerase V (pol V), a heterotrimeric complex composed of UmuD'2C, is marginally active. ATP and RecA play essential roles in the activation of pol V for DNA synthesis including translesion synthesis (TLS). We have established three features of the roles of ATP and RecA. (1) RecA-activated DNA polymerase V (pol V Mut), is a DNA-dependent ATPase; (2) bound ATP is required for DNA synthesis; (3) pol V Mut function is regulated by ATP, with ATP required to bind primer/template (p/t) DNA and ATP hydrolysis triggering dissociation from the DNA. Pol V Mut formed with an ATPase-deficient RecA E38K/K72R mutant hydrolyzes ATP rapidly, establishing the DNA-dependent ATPase as an intrinsic property of pol V Mut distinct from the ATP hydrolytic activity of RecA when bound to single-stranded (ss)DNA as a nucleoprotein filament (RecA*). No similar ATPase activity or autoregulatory mechanism has previously been found for a DNA polymerase.DOI: http://dx.doi.org/10.7554/eLife.02384.001.

Molecular basis of transcriptional fidelity and DNA lesion-induced transcriptional mutagenesis

Maintaining high transcriptional fidelity is essential for life. Some DNA lesions lead to significant changes in transcriptional fidelity. In this review, we will summarize recent progress towards understanding the molecular basis of RNA polymerase II (Pol II) transcriptional fidelity and DNA lesion-induced transcriptional mutagenesis. In particular, we will focus on the three key checkpoint steps of controlling Pol II transcriptional fidelity: insertion (specific nucleotide selection and incorporation), extension (differentiation of RNA transcript extension of a matched over mismatched 3'-RNA terminus), and proofreading (preferential removal of misincorporated nucleotides from the 3'-RNA end). We will also discuss some novel insights into the molecular basis and chemical perspectives of controlling Pol II transcriptional fidelity through structural, computational, and chemical biology approaches.

In vivo evidence that phenylalanine 171 acts as a molecular brake for translesion DNA synthesis across benzo[a]pyrene DNA adducts by human DNA polymerase κ

Humans possess multiple specialized DNA polymerases that continue DNA replication beyond a variety of DNA lesions. DNA polymerase kappa (Pol κ) bypasses benzo[a]pyrene diolepoxide-N(2)-deoxyguanine (BPDE-N(2)-dG) DNA adducts in an almost error-free manner. In the previous work, we changed the amino acids close to the adducts in the active site and examined the bypass efficiency. The substitution of alanine for phenylalanine 171 (F171A) enhanced by 18-fold in vitro, the efficiencies of dCMP incorporation opposite (-)- and (+)-trans-anti-BPDE-N(2)-dG. In the present study, we established human cell lines that express wild-type Pol κ (POLK+/-), F171A (POLK F171A/-) or lack expression of Pol κ (POLK-/-) to examine the in vivo significance. These cell lines were generated with Nalm-6, a human pre-B acute lymphoblastic leukemia cell line, which has high efficiency for gene targeting. Mutations were analyzed with shuttle vectors having (-)- or (+)-trans-anti-BPDE-N(2)-dG in the supF gene. The frequencies of mutations were in the order of POLK-/->POLK+/->POLK F171A/- both in (-)- and (+)-trans-anti-BPDE-N(2)-dG. These results suggest that F171 may function as a molecular brake for bypass across BPDE-N(2)-dG by Pol κ and raise the possibility that the cognate substrates for Pol κ are not BP adducts in DNA but may be lesions in DNA induced by endogenous mutagens.

Estimation of the spontaneous mutation rate per nucleotide site in a Drosophila melanogaster full-sib family

We employed deep genome sequencing of two parents and 12 of their offspring to estimate the mutation rate per site per generation in a full-sib family of Drosophila melanogaster recently sampled from a natural population. Sites that were homozygous for the same allele in the parents and heterozygous in one or more offspring were categorized as candidate mutations and subjected to detailed analysis. In 1.23 × 10(9) callable sites from 12 individuals, we confirmed six single nucleotide mutations. We estimated the false negative rate in the experiment by generating synthetic mutations using the empirical distributions of numbers of nonreference bases at heterozygous sites in the offspring. The proportion of synthetic mutations at callable sites that we failed to detect was <1%, implying that the false negative rate was extremely low. Our estimate of the point mutation rate is 2.8 × 10(-9) (95% confidence interval = 1.0 × 10(-9) - 6.1 × 10(-9)) per site per generation, which is at the low end of the range of previous estimates, and suggests an effective population size for the species of ∼1.4 × 10(6). At one site, point mutations were present in two individuals, indicating that there had been a premeiotic mutation cluster, although surprisingly one individual had a G→A transition and the other a G→T transversion, possibly associated with error-prone mismatch repair. We also detected three short deletion mutations and no insertions, giving a deletion mutation rate of 1.2 × 10(-9) (95% confidence interval = 0.7 × 10(-9) - 11 × 10(-9)).

Translesion DNA synthesis and mutagenesis in prokaryotes

The presence of unrepaired lesions in DNA represents a challenge for replication. Most, but not all, DNA lesions block the replicative DNA polymerases. The conceptually simplest procedure to bypass lesions during DNA replication is translesion synthesis (TLS), whereby the replicative polymerase is transiently replaced by a specialized DNA polymerase that synthesizes a short patch of DNA across the site of damage. This process is inherently error prone and is the main source of point mutations. The diversity of existing DNA lesions and the biochemical properties of Escherichia coli DNA polymerases will be presented. Our main goal is to deliver an integrated view of TLS pathways involving the multiple switches between replicative and specialized DNA polymerases and their interaction with key accessory factors. Finally, a brief glance at how other bacteria deal with TLS and mutagenesis is presented.

Replication, repair, and translesion polymerase bypass of N⁶-oxopropenyl-2'-deoxyadenosine

The oxidative stress products malondialdehyde and base propenal react with DNA bases forming the adduction products 3-(2'-deoxy-β-D-erythro-pentofuranosyl)pyrimido[1,2-a]purin-10(3H)-one (M1dG) and N(6)-(oxypropenyl)-2'-deoxyadenosine (OPdA). M1dG is mutagenic in vivo and miscodes in vitro, but little work has been done on OPdA. To improve our understanding of the effect of OPdA on polymerase activity and mutagenicity, we evaluated the ability of the translesion DNA polymerases hPols η, κ, and ι to bypass OPdA in vitro. hPols η and κ inserted dNTPs opposite the lesion and extended the OPdA-modified primer to the terminus. hPol ι inserted dNTPs opposite OPdA but failed to fully extend the primer. Steady-state kinetic analysis indicated that these polymerases preferentially insert dTTP opposite OPdA, although less efficiently than opposite dA. Minimal incorrect base insertion was observed for all polymerases, and dCTP was the primary mis-insertion event. Examining replicative and repair polymerases revealed little effect of OPdA on the Sulfolobus solfataricus polymerase Dpo1 or the Klenow fragment of Escherichia coli DNA polymerase I. Bacteriophage T7 DNA polymerase displayed a reduced level of OPdA bypass compared to unmodified DNA, and OPdA nearly completely blocked the activity of base excision repair polymerase hPol β. This work demonstrates that bypass of OPdA is generally error-free, modestly decreases the catalytic activity of most polymerases, and blocks hPol β polymerase activity. Although mis-insertion opposite OPdA is relatively weak, the efficiency of bypass may introduce A → G transitions observed in vivo.

Handling the 3-methylcytosine lesion by six human DNA polymerases members of the B-, X- and Y-families

Alkylating agents often generate 3-methylcytosine (3meC) lesions that are efficiently repaired by AlkB homologues. If AlkB homologue proteins are not functional, or the number of 3meC lesions exceeds the cellular repair capacity, the damage will persist in the genome and become substrate of DNA polymerases (Pols). Though alkylating agents are present in our environment and used in the clinics, currently nothing is known about the impact of 3meC on the accuracy and efficiency of human Pols. Here we compared the 3meC bypass properties of six human Pols belonging to the three families: B (Pol δ), X (Pols β and λ) and Y (Pols κ, ι and η). We show that under replicative conditions 3meC impairs B-family, blocks X-family, but not Y-family Pols, in particular Pols η and ι. These Pols successfully synthesize opposite 3meC; Pol ι preferentially misincorporates dTTP and Pol η dATP. The most efficient extenders from 3meC base-paired primers are Pols κ and η. Finally, using xeroderma pigmentosum variant patient cell extracts, we provide evidence that the presence of functional Pol η is mandatory to efficiently overcome 3meC by mediating complete bypass or extension. Our data suggest that Pol η is crucial for efficient 3meC bypass.

Translesion DNA polymerases

Living cells are continually exposed to DNA-damaging agents that threaten their genomic integrity. Although DNA repair processes rapidly target the damaged DNA for repair, some lesions nevertheless persist and block genome duplication by the cell's replicase. To avoid the deleterious consequence of a stalled replication fork, cells use specialized polymerases to traverse the damage. This process, termed "translesion DNA synthesis" (TLS), affords the cell additional time to repair the damage before the replicase returns to complete genome duplication. In many cases, this damage-tolerance mechanism is error-prone, and cell survival is often associated with an increased risk of mutagenesis and carcinogenesis. Despite being tightly regulated by a variety of transcriptional and posttranslational controls, the low-fidelity TLS polymerases also gain access to undamaged DNA where their inaccurate synthesis may actually be beneficial for genetic diversity and evolutionary fitness.

Adaptive translation as a mechanism of stress response and adaptation

The composition of the cellular proteome is commonly thought to strictly adhere to the genetic code. However, accumulating evidence indicates that cells also regulate the synthesis of mutant protein molecules that deviate from the genetic code. Production of mutant proteins generally occurs when cells are stressed or when they undergo environmental adaptation, but production varies in amounts and specificity. The deliberate synthesis of mutant proteins suggests that some of these proteins can be useful in cellular stress response and adaptation. This review describes the occurrence of, the translation mechanisms for, and the functional hypotheses on regulated synthesis of mutant proteins.

The choice of nucleotide inserted opposite abasic sites formed within chromosomal DNA reveals the polymerase activities participating in translesion DNA synthesis

Abasic sites in genomic DNA can be a significant source of mutagenesis in biological systems, including human cancers. Such mutagenesis requires translesion DNA synthesis (TLS) bypass of the abasic site by specialized DNA polymerases. The abasic site bypass specificity of TLS proteins had been studied by multiple means in vivo and in vitro, although the generality of the conclusions reached have been uncertain. Here, we introduce a set of yeast reporter strains for investigating the in vivo specificity of abasic site bypass at numerous random positions within chromosomal DNA. When shifted to 37°C, these strains underwent telomere uncapping and resection that exposed reporter genes within a long 3' ssDNA overhang. Human APOBEC3G cytosine deaminase was expressed to create uracils in ssDNA, which were excised by uracil-DNA N-glycosylase. During repair synthesis, error-prone TLS bypassed the resulting abasic sites. Because of APOBEC3G's strict motif specificity and the restriction of abasic site formation to only one DNA strand, this system provides complete information about the location of abasic sites that led to mutations. We recapitulated previous findings on the roles of REV1 and REV3. Further, we found that sequence context can strongly influence the relative frequency of A or C insertion. We also found that deletion of Pol32, a non-essential common subunit of Pols δ and ζ, resulted in residual low-frequency C insertion dependent on Rev1 catalysis. We summarize our results in a detailed model of the interplay between TLS components leading to error-prone bypass of abasic sites. Our results underscore the utility of this system for studying TLS bypass of many types of lesions within genomic DNA.

N-Aroyl indole thiobarbituric acids as inhibitors of DNA repair and replication stress response polymerases

Using a robust and quantitative assay, we have identified a novel class of DNA polymerase inhibitors that exhibits some specificity against an enzyme involved in resistance to anti-cancer drugs, namely, human DNA polymerase eta (hpol η). In our initial screen, we identified the indole thiobarbituric acid (ITBA) derivative 5-((1-(2-bromobenzoyl)-5-chloro-1H-indol-3-yl)methylene)-2-thioxodihydropyrimidine-4,6(1H,5H)-dione (ITBA-12) as an inhibitor of the Y-family DNA member hpol η, an enzyme that has been associated with increased resistance to cisplatin and doxorubicin treatments. An additional seven DNA polymerases from different subfamilies were tested for inhibition by ITBA-12. Hpol η was the most potently inhibited enzyme (30 ± 3 μM), with hpol β, hpol γ, and hpol κ exhibiting comparable but higher IC50 values of 41 ± 24, 49 ± 6, and 59 ± 11 μM, respectively. The other polymerases tested had IC50 values closer to 80 μM. Steady-state kinetic analysis was used to investigate the mechanism of polymerase inhibition by ITBA-12. Based on changes in the Michaelis constant, it was determined that ITBA-12 acts as an allosteric (or partial) competitive inhibitor of dNTP binding. The parent ITBA scaffold was modified to produce 20 derivatives and establish structure-activity relationships by testing for inhibition of hpol η. Two compounds with N-naphthoyl Ar-substituents, ITBA-16 and ITBA-19, were both found to have improved potency against hpol η with IC50 values of 16 ± 3 μM and 17 ± 3 μM, respectively. Moreover, the specificity of ITBA-16 was improved relative to that of ITBA-12. The presence of a chloro substituent at position 5 on the indole ring appears to be crucial for effective inhibition of hpol η, with the indole N-1-naphthoyl and N-2-naphthoyl analogues being the most potent inhibitors of hpol η. These results provide a framework from which second-generation ITBA derivatives may be developed against specialized polymerases that are involved in mechanisms of radio- and chemo-resistance.

Quick replication fork stop by overproduction of Escherichia coli DinB produces non-proliferative cells with an aberrant chromosome

Escherichia coli dinB encodes the translesion DNA polymerase DinB, which can inhibit progression of replication forks in a dose-dependent manner, independent of exogenous DNA damage. We reported previously that overproduction of DinB from a multicopy dinB plasmid immediately abolished ongoing replication fork progression, and the cells rapidly and drastically lost colony-forming ability, although the mechanisms underlying this lethality by severe replication fork stress remained unclear. Here, we show that the reduced colony-forming ability in the dinB-overexpressing cells is independent of the specific toxin genes that trigger programmed bacterial cell death when replication is blocked by depletion of the dNTP pool. After DinB abolished replication fork progression and colony-forming ability, most of the cells were still viable, as judged by fluorescent dye staining, but contained irregularly shaped nucleoids in which chromosomal DNA was preferentially lost in the replication terminus region relative to the replication origin region. Flow cytometric analysis of the cells revealed chromosomal damage and the eventual appearance of cell populations with less than single-chromosome DNA content, reminiscent of sub-G1 cells with lethal DNA content produced during eukaryotic apoptosis. This reduced DNA content was not observed after replication fork progression was quickly stopped in temperature-sensitive dnaB helicase mutant cells at a non-permissive temperature. Thus, the quick replication stop provoked by excess DinB uniquely generates temporarily viable but non-reproductive cells possessing a fatally depleted chromosomal content, which may represent one of the possible fates of an E. coli cell whose replication is overwhelmingly compromised.

Electrical Current Signatures of DNA Base Modifications in Single Molecules Immobilized in the α-Hemolysin Ion Channel

Nanopore technology holds high potential for next-generation DNA sequencing. This method operates by drawing an individual single-stranded DNA molecule through a nanoscale pore while monitoring the current deflections that occur as the DNA passes through. Individual current levels for the four DNA nucleotides have been established by immobilization of an end biotinylated strand in the pore in which the nucleotide of interest is suspended at the most sensitive region of the ion channel. Due to the inherent reactivity of the DNA bases, many modified nucleotides in the genome exist resulting from oxidative and UV insults, among others. Herein, the current levels for the common DNA damages 8-oxo-7,8-dihydroguanine (OG), spiroiminodihydantoin (Sp), guanidinohydantoin (Gh), uridine (U), abasic sites (AP), thymine dimers (T=T), thymine glycol (Tg) and 5-iodocytosine (5-I-C) were assessed via immobilization experiments. In some cases, the current difference between the damaged and canonical nucleotides was not well resolved; therefore, we took advantage of the chemical reactivity of the new functional groups present to make amine adducts that shifted the current levels outside the range of the native nucleotides. Among adducts studied, only the 2-aminomethyl-18-crown-6 adduct was able to give a large current shift in the immobilization experiment, as well as to be observed in a translocation experiment. The results show potential in providing current level modulators for identification of some types of DNA damage. In principle, any DNA base modification that can be converted chemically or enzymatically to an abasic site could be identified in this way.

Single-molecule DNA repair in live bacteria

Cellular DNA damage is reversed by balanced repair pathways that avoid accumulation of toxic intermediates. Despite their importance, the organization of DNA repair pathways and the function of repair enzymes in vivo have remained unclear because of the inability to directly observe individual reactions in living cells. Here, we used photoactivation, localization, and tracking in live Escherichia coli to directly visualize single fluorescent labeled DNA polymerase I (Pol) and ligase (Lig) molecules searching for DNA gaps and nicks, performing transient reactions, and releasing their products. Our general approach provides enzymatic rates and copy numbers, substrate-search times, diffusion characteristics, and the spatial distribution of reaction sites, at the single-cell level, all in one measurement. Single repair events last 2.1 s (Pol) and 2.5 s (Lig), respectively. Pol and Lig activities increased fivefold over the basal level within minutes of DNA methylation damage; their rates were limited by upstream base excision repair pathway steps. Pol and Lig spent >80% of their time searching for free substrates, thereby minimizing both the number and lifetime of toxic repair intermediates. We integrated these single-molecule observations to generate a quantitative, systems-level description of a model repair pathway in vivo.

Competitive fitness during feast and famine: how SOS DNA polymerases influence physiology and evolution in Escherichia coli

Escherichia coli DNA polymerases (Pol) II, IV, and V serve dual roles by facilitating efficient translesion DNA synthesis while simultaneously introducing genetic variation that can promote adaptive evolution. Here we show that these alternative polymerases are induced as cells transition from exponential to long-term stationary-phase growth in the absence of induction of the SOS regulon by external agents that damage DNA. By monitoring the relative fitness of isogenic mutant strains expressing only one alternative polymerase over time, spanning hours to weeks, we establish distinct growth phase-dependent hierarchies of polymerase mutant strain competitiveness. Pol II confers a significant physiological advantage by facilitating efficient replication and creating genetic diversity during periods of rapid growth. Pol IV and Pol V make the largest contributions to evolutionary fitness during long-term stationary phase. Consistent with their roles providing both a physiological and an adaptive advantage during stationary phase, the expression patterns of all three SOS polymerases change during the transition from log phase to long-term stationary phase. Compared to the alternative polymerases, Pol III transcription dominates during mid-exponential phase; however, its abundance decreases to <20% during long-term stationary phase. Pol IV transcription dominates as cells transition out of exponential phase into stationary phase and a burst of Pol V transcription is observed as cells transition from death phase to long-term stationary phase. These changes in alternative DNA polymerase transcription occur in the absence of SOS induction by exogenous agents and indicate that cell populations require appropriate expression of all three alternative DNA polymerases during exponential, stationary, and long-term stationary phases to attain optimal fitness and undergo adaptive evolution.

Trans-Lesion DNA Polymerases May Be Involved in Yeast Meiosis

Trans-lesion DNA polymerases (TLSPs) enable bypass of DNA lesions during replication and are also induced under stress conditions. Being only weakly dependent on their template during replication, TLSPs introduce mutations into DNA. The low processivity of these enzymes ensures that they fall off their template after a few bases are synthesized and are then replaced by the more accurate replicative polymerase. We find that the three TLSPs of budding yeast Saccharomyces cerevisiae Rev1, PolZeta (Rev3 and Rev7), and Rad30 are induced during meiosis at a time when DNA double-strand breaks (DSBs) are formed and homologous chromosomes recombine. Strains deleted for one or any combination of the three TLSPs undergo normal meiosis. However, in the triple-deletion mutant, there is a reduction in both allelic and ectopic recombination. We suggest that trans-lesion polymerases are involved in the processing of meiotic double-strand breaks that lead to mutations. In support of this notion, we report significant yeast two-hybrid (Y2H) associations in meiosis-arrested cells between the TLSPs and DSB proteins Rev1-Spo11, Rev1-Mei4, and Rev7-Rec114, as well as between Rev1 and Rad30 We suggest that the involvement of TLSPs in processing of meiotic DSBs could be responsible for the considerably higher frequency of mutations reported during meiosis compared with that found in mitotically dividing cells, and therefore may contribute to faster evolutionary divergence than previously assumed.

Adduct formation and repair, and translesion DNA synthesis across the adducts in human cells exposed to 3-nitrobenzanthrone

3-Nitrobenzanthrone (3-nitro-7H-benz[d,e]anthracen-7-one, 3-NBA) is a potent environmental mutagen that is found in diesel exhaust fumes and airborne particulates. It is known to produce several DNA adducts, including three major adducts N-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone (dG-C8-N-ABA), 2-(2'-deoxyadenosin-N(6)-yl)-3-aminobenzanthrone (dA-N(6)-C2-ABA), and 2-(2'-deoxyguanosin-N(2)-yl)-3-aminobenzanthrone (dG-N(2)-C2-ABA) in mammalian cells. In the present study, we measured the quantity of the formation and subsequent reduction of these adducts in human hepatoma HepG2 cells that had been treated with 3-NBA using LC-MS/MS analysis. As a result, dG-C8-N-ABA and dG-N(2)-C2-ABA were identified as major adducts in the HepG2 cells, and dA-N(6)-C2-ABA was found to be a minor adduct. Treatment with 1μg/mL 3-NBA for 24h induced the formation of 2835±1509 dG-C8-N-ABA and 3373±1173 dG-N(2)-C2-ABA per 10(7) dG and 877±330 dA-N(6)-C2-ABA per 10(7) dA in the cells. The cellular DNA repair system removed the dG-C8-N-ABA and dA-N(6)-C2-ABA adducts more efficiently than the dG-N(2)-C2-ABA adducts. After a 24-h repair period, 86.4±11.1% of the dG-N(2)-C2-ABA adducts remained, whereas only 51.7±2.7% of the dG-C8-N-ABA adducts and 37.8±1.7% of the dA-N(6)-C2-ABA adducts were present in the cells. We also evaluated the efficiency of bypasses across these three adducts and their mutagenic potency by introducing site-specific mono-modified plasmids into human cells. This translesion DNA synthesis (TLS) assay showed that dG-C8-N-ABA blocked DNA replication markedly (its replication frequency was 16.9±2.7%), while the replication arrests induced by dG-N(2)-C2-ABA and dA-N(6)-C2-ABA were more moderate (their replication frequencies were 33.3±6.2% and 43.1±7.5%, respectively). Mutagenic TLS was observed more frequently in replication across dG-C8-N-ABA (30.6%) than in replication across dG-N(2)-C2-ABA (12.1%) or dA-N(6)-C2-ABA (12.1%). These findings provide important insights into the molecular mechanism of 3-NBA-mutagenesis.

Biological and therapeutic relevance of nonreplicative DNA polymerases to cancer

Apart from surgical approaches, the treatment of cancer remains largely underpinned by radiotherapy and pharmacological agents that cause damage to cellular DNA, which ultimately causes cancer cell death. DNA polymerases, which are involved in the repair of cellular DNA damage, are therefore potential targets for inhibitors for improving the efficacy of cancer therapy. They can be divided, according to their main function, into two groups, namely replicative and nonreplicative enzymes. At least 15 different DNA polymerases, including their homologs, have been discovered to date, which vary considerably in processivity and fidelity. Many of the nonreplicative (specialized) DNA polymerases replicate DNA in an error-prone fashion, and they have been shown to participate in multiple DNA damage repair and tolerance pathways, which are often aberrant in cancer cells. Alterations in DNA repair pathways involving DNA polymerases have been linked with cancer survival and with treatment response to radiotherapy or to classes of cytotoxic drugs routinely used for cancer treatment, particularly cisplatin, oxaliplatin, etoposide, and bleomycin. Indeed, there are extensive preclinical data to suggest that DNA polymerase inhibition may prove to be a useful approach for increasing the effectiveness of therapies in patients with cancer. Furthermore, specialized DNA polymerases warrant examination of their potential use as clinical biomarkers to select for particular cancer therapies, to individualize treatment for patients.

Evaluating evolutionary models of stress-induced mutagenesis in bacteria

Increased mutation rates under stress allow bacterial populations to adapt rapidly to stressors, including antibiotics. Here we evaluate existing models for the evolution of stress-induced mutagenesis and present a new model arguing that it evolves as a result of a complex interplay between direct selection for increased stress tolerance, second-order selection for increased evolvability and genetic drift. Further progress in our understanding of the evolutionary biology of stress and mutagenesis will require a more detailed understanding both of the patterns of stress encountered by bacteria in nature and of the mutations that are produced under stress.

Leukotriene biosynthesis inhibitor MK886 impedes DNA polymerase activity

Specialized DNA polymerases participate in replication stress responses and in DNA repair pathways that function as barriers against cellular senescence and genomic instability. These events can be co-opted by tumor cells as a mechanism to survive chemotherapeutic and ionizing radiation treatments and as such, represent potential targets for adjuvant therapies. Previously, a high-throughput screen of ∼16,000 compounds identified several first generation proof-of-principle inhibitors of human DNA polymerase kappa (hpol κ). The indole-derived inhibitor of 5-lipoxygenase activating protein (FLAP), MK886, was one of the most potent inhibitors of hpol κ discovered in that screen. However, the specificity and mechanism of inhibition remained largely undefined. In the current study, the specificity of MK886 against human Y-family DNA polymerases and a model B-family DNA polymerase was investigated. MK886 was found to inhibit the activity of all DNA polymerases tested with similar IC(50) values, the exception being a 6- to 8-fold increase in the potency of inhibition against human DNA polymerase iota (hpol ι), a highly error-prone enzyme that uses Hoogsteen base-pairing modes during catalysis. The specificity against hpol ι was partially abrogated by inclusion of the recently annotated 25 a.a. N-terminal extension. On the basis of Michaelis-Menten kinetic analyses and DNA binding assays, the mechanism of inhibition by MK886 appears to be mixed. In silico docking studies were used to produce a series of models for MK886 binding to Y-family members. The docking results indicate that two binding pockets are conserved between Y-family polymerases, while a third pocket near the thumb domain appears to be unique to hpol ι. Overall, these results provide insight into the general mechanism of DNA polymerase inhibition by MK886.

A non-catalytic role of DNA polymerase η in recruiting Rad18 and promoting PCNA monoubiquitination at stalled replication forks

Trans-lesion DNA synthesis (TLS) is a DNA damage-tolerance mechanism that uses low-fidelity DNA polymerases to replicate damaged DNA. The inherited cancer-propensity syndrome xeroderma pigmentosum variant (XPV) results from error-prone TLS of UV-damaged DNA. TLS is initiated when the Rad6/Rad18 complex monoubiquitinates proliferating cell nuclear antigen (PCNA), but the basis for recruitment of Rad18 to PCNA is not completely understood. Here, we show that Rad18 is targeted to PCNA by DNA polymerase eta (Polη), the XPV gene product that is mutated in XPV patients. The C-terminal domain of Polη binds to both Rad18 and PCNA and promotes PCNA monoubiquitination, a function unique to Polη among Y-family TLS polymerases and dissociable from its catalytic activity. Importantly, XPV cells expressing full-length catalytically-inactive Polη exhibit increased recruitment of other error-prone TLS polymerases (Polκ and Polι) after UV irradiation. These results define a novel non-catalytic role for Polη in promoting PCNA monoubiquitination and provide a new potential mechanism for mutagenesis and genome instability in XPV individuals.

Critical Analysis of Strand-Biased Somatic Mutation Signatures in TP53 versus Ig Genes, in Genome-Wide Data and the Etiology of Cancer

Previous analyses of rearranged immunoglobulin (Ig) variable genes (VDJs) concluded that the mechanism of Ig somatic hypermutation (SHM) involves the Ig pre-mRNA acting as a copying template resulting in characteristic strand biased somatic mutation patterns at A:T and G:C base pairs. We have since analysed cancer genome data and found the same mutation strand-biases, in toto or in part, in nonlymphoid cancers. Here we have analysed somatic mutations in a single well-characterised gene TP53. Our goal is to understand the genesis of the strand-biased mutation patterns in TP53—and in genome-wide data—that may arise by “endogenous” mechanisms as opposed to adduct-generated DNA-targeted strand-biased mutations caused by well-characterised “external” carcinogenic influences in cigarette smoke, UV-light, and certain dietary components. The underlying strand-biased mutation signatures in TP53, for many non-lymphoid cancers, bear a striking resemblance to the Ig SHM pattern. A similar pattern can be found in genome-wide somatic mutations in cancer genomes that have also mutated TP53. The analysis implies a role for base-modified RNA template intermediates coupled to reverse transcription in the genesis of many cancers. Thus Ig SHM may be inappropriately activated in many non-lymphoid tissues via hormonal and/or inflammation-related processes leading to cancer.

Why nature really chose phosphate

Phosphoryl transfer plays key roles in signaling, energy transduction, protein synthesis, and maintaining the integrity of the genetic material. On the surface, it would appear to be a simple nucleophile displacement reaction. However, this simplicity is deceptive, as, even in aqueous solution, the low-lying d-orbitals on the phosphorus atom allow for eight distinct mechanistic possibilities, before even introducing the complexities of the enzyme catalyzed reactions. To further complicate matters, while powerful, traditional experimental techniques such as the use of linear free-energy relationships (LFER) or measuring isotope effects cannot make unique distinctions between different potential mechanisms. A quarter of a century has passed since Westheimer wrote his seminal review, 'Why Nature Chose Phosphate' (Science 235 (1987), 1173), and a lot has changed in the field since then. The present review revisits this biologically crucial issue, exploring both relevant enzymatic systems as well as the corresponding chemistry in aqueous solution, and demonstrating that the only way key questions in this field are likely to be resolved is through careful theoretical studies (which of course should be able to reproduce all relevant experimental data). Finally, we demonstrate that the reason that nature really chose phosphate is due to interplay between two counteracting effects: on the one hand, phosphates are negatively charged and the resulting charge-charge repulsion with the attacking nucleophile contributes to the very high barrier for hydrolysis, making phosphate esters among the most inert compounds known. However, biology is not only about reducing the barrier to unfavorable chemical reactions. That is, the same charge-charge repulsion that makes phosphate ester hydrolysis so unfavorable also makes it possible to regulate, by exploiting the electrostatics. This means that phosphate ester hydrolysis can not only be turned on, but also be turned off, by fine tuning the electrostatic environment and the present review demonstrates numerous examples where this is the case. Without this capacity for regulation, it would be impossible to have for instance a signaling or metabolic cascade, where the action of each participant is determined by the fine-tuned activity of the previous piece in the production line. This makes phosphate esters the ideal compounds to facilitate life as we know it.

A single residue unique to DinB-like proteins limits formation of the polymerase IV multiprotein complex in Escherichia coli

The activity of DinB is governed by the formation of a multiprotein complex (MPC) with RecA and UmuD. We identified two highly conserved surface residues in DinB, cysteine 66 (C66) and proline 67 (P67). Mapping on the DinB tertiary structure suggests these are noncatalytic, and multiple-sequence alignments indicate that they are unique among DinB-like proteins. To investigate the role of the C66-containing surface in MPC formation, we constructed the dinB(C66A) derivative. We found that DinB(C66A) copurifies with its interacting partners, RecA and UmuD, to a greater extent than DinB. Notably, copurification of RecA with DinB is somewhat enhanced in the absence of UmuD and is further increased for DinB(C66A). In vitro pulldown assays also indicate that DinB(C66A) binds RecA and UmuD better than DinB. We note that the increased affinity of DinB(C66A) for UmuD is RecA dependent. Thus, the C66-containing binding surface appears to be critical to modulate interaction with UmuD, and particularly with RecA. Expression of dinB(C66A) from the chromosome resulted in detectable differences in dinB-dependent lesion bypass fidelity and homologous recombination. Study of this DinB derivative has revealed a key surface on DinB, which appears to modulate the strength of MPC binding, and has suggested a binding order of RecA and UmuD to DinB. These findings will ultimately permit the manipulation of these enzymes to deter bacterial antibiotic resistance acquisition and to gain insights into cancer development in humans.

DNA metabolism in mycobacterial pathogenesis

Fundamental aspects of the lifestyle of Mycobacterium tuberculosis implicate DNA metabolism in bacillary survival and adaptive evolution. The environments encountered by M. tuberculosis during successive cycles of infection and transmission are genotoxic. Moreover, as an obligate pathogen, M. tuberculosis has the ability to persist for extended periods in a subclinical state, suggesting that active DNA repair is critical to maintain genome integrity and bacterial viability during prolonged infection. In this chapter, we provide an overview of the major DNA metabolic pathways identified in M. tuberculosis, and situate key recent findings within the context of mycobacterial pathogenesis. Unlike many other bacterial pathogens, M. tuberculosis is genetically secluded, and appears to rely solely on chromosomal mutagenesis to drive its microevolution within the human host. In turn, this implies that a balance between high versus relaxed fidelity mechanisms of DNA metabolism ensures the maintenance of genome integrity, while accommodating the evolutionary imperative to adapt to hostile and fluctuating environments. The inferred relationship between mycobacterial DNA repair and genome dynamics is considered in the light of emerging data from whole-genome sequencing studies of clinical M. tuberculosis isolates which have revealed the potential for considerable heterogeneity within and between different bacterial and host populations.

DNA polymerase POLQ and cellular defense against DNA damage

In mammalian cells, POLQ (pol θ) is an unusual specialized DNA polymerase whose in vivo function is under active investigation. POLQ has been implicated by different experiments to play a role in resistance to ionizing radiation and defense against genomic instability, in base excision repair, and in immunological diversification. The protein is formed by an N-terminal helicase-like domain, a C-terminal DNA polymerase domain, and a large central domain that spans between the two. This arrangement is also found in the Drosophila Mus308 protein, which functions in resistance to DNA interstrand crosslinking agents. Homologs of POLQ and Mus308 are found in multicellular eukaryotes, including plants, but a comparison of phenotypes suggests that not all of these genes are functional orthologs. Flies defective in Mus308 are sensitive to DNA interstrand crosslinking agents, while mammalian cells defective in POLQ are primarily sensitive to DNA double-strand breaking agents. Cells from Polq(-/-) mice are hypersensitive to radiation and peripheral blood cells display increased spontaneous and ionizing radiation-induced levels of micronuclei (a hallmark of gross chromosomal aberrations), though mice apparently develop normally. Loss of POLQ in human and mouse cells causes sensitivity to ionizing radiation and other double strand breaking agents and increased DNA damage signaling. Retrospective studies of clinical samples show that higher levels of POLQ gene expression in breast and colorectal cancer are correlated with poorer outcomes for patients. A clear understanding of the mechanism of action and physiologic function of POLQ in the cell is likely to bear clinical relevance.

A proposal: Source of single strand DNA that elicits the SOS response

Chromosome replication is performed by numerous proteins that function together as a "replisome". The replisome machinery duplicates both strands of the parental DNA simultaneously. Upon DNA damage to the cell, replisome action produces single-strand DNA to which RecA binds, enabling its activity in cleaving the LexA repressor and thus inducing the SOS response. How single-strand DNA is produced by a replisome acting on damaged DNA is not clear. For many years it has been assumed the single-strand DNA is generated by the replicative helicase, which continues unwinding DNA even after DNA polymerase stalls at a template lesion. Recent studies indicate another source of the single-strand DNA, resulting from an inherently dynamic replisome that may hop over template lesions on both leading and lagging strands, thereby leaving single-strand gaps in the wake of the replication fork. These single-strand gaps are proposed to be the origin of the single-strand DNA that triggers the SOS response after DNA damage.

Escherichia coli DinB inhibits replication fork progression without significantly inducing the SOS response

The SOS response is readily triggered by replication fork stalling caused by DNA damage or a dysfunctional replicative apparatus in Escherichia coli cells. E. coli dinB encodes DinB DNA polymerase and its expression is upregulated during the SOS response. DinB catalyzes translesion DNA synthesis in place of a replicative DNA polymerase III that is stalled at a DNA lesion. We showed previously that DNA replication was suppressed without exogenous DNA damage in cells overproducing DinB. In this report, we confirm that this was due to a dose-dependent inhibition of ongoing replication forks by DinB. Interestingly, the DinB-overproducing cells did not significantly induce the SOS response even though DNA replication was perturbed. RecA protein is activated by forming a nucleoprotein filament with single-stranded DNA, which leads to the onset of the SOS response. In the DinB-overproducing cells, RecA was not activated to induce the SOS response. However, the SOS response was observed after heat-inducible activation in strain recA441 (encoding a temperature-sensitive RecA) and after replication blockage in strain dnaE486 (encoding a temperature-sensitive catalytic subunit of the replicative DNA polymerase III) at a non-permissive temperature when DinB was overproduced in these cells. Furthermore, since catalytically inactive DinB could avoid the SOS response to a DinB-promoted fork block, it is unlikely that overproduced DinB takes control of primer extension and thus limits single-stranded DNA. These observations suggest that DinB possesses a feature that suppresses DNA replication but does not abolish the cell's capacity to induce the SOS response. We conclude that DinB impedes replication fork progression in a way that does not activate RecA, in contrast to obstructive DNA lesions and dysfunctional replication machinery.

Effects of non-catalytic, distal amino acid residues on activity of E. coli DinB (DNA polymerase IV)

DinB is one of two Y family polymerases in E. coli and is involved in copying damaged DNA. DinB is specialized to bypass deoxyguanosine adducts that occur at the N(2) position, with its cognate lesion being the furfuryl adduct. Active site residues have been identified that make contact with the substrate and carry out deoxynucleotide triphosphate (dNTP) addition to the growing DNA strand. In DNA polymerases, these include negatively charged aspartate and glutamate residues (D8, D103, and E104 in E. coli DNA polymerase IV DinB). These residues position the essential magnesium ions correctly to facilitate nucleophilic attack by the primer hydroxyl group on the α-phosphate group of the incoming dNTP. To study the contribution of DinB residues to lesion bypass, the computational methods THEMATICS and POOL were employed. These methods correctly predict the known active site residues, as well as other residues known to be important for activity. In addition, these methods predict other residues involved in substrate binding as well as more remote residues. DinB variants with mutations at the predicted positions were constructed and assayed for bypass of the N(2) -furfuryl-dG lesion. We find a wide range of effects of predicted residues, including some mutations that abolish damage bypass. Moreover, most of the DinB variants constructed are unable to carry out the extension step of lesion bypass. The use of computational prediction methods represents another tool that will lead to a more complete understanding of translesion DNA synthesis.

REV1 and DNA polymerase zeta in DNA interstrand crosslink repair

DNA interstrand crosslinks (ICLs) are covalent linkages between two strands of DNA, and their presence interferes with essential metabolic processes such as transcription and replication. These lesions are extremely toxic, and their repair is essential for genome stability and cell survival. In this review, we will discuss how the removal of ICLs requires interplay between multiple genome maintenance pathways and can occur in the absence of replication (replication-independent ICL repair) or during S phase (replication-coupled ICL repair), the latter being the predominant pathway used in mammalian cells. It is now well recognized that translesion DNA synthesis (TLS), especially through the activities of REV1 and DNA polymerase zeta (Polζ), is necessary for both ICL repair pathways operating throughout the cell cycle. Recent studies suggest that the convergence of two replication forks upon an ICL initiates a cascade of events including unhooking of the lesion through the actions of structure-specific endonucleases, thereby creating a DNA double-stranded break (DSB). TLS across the unhooked lesion is necessary for restoring the sister chromatid before homologous recombination repair. Biochemical and genetic studies implicate REV1 and Polζ as being essential for performing lesion bypass across the unhooked crosslink, and this step appears to be important for subsequent events to repair the intermediate DSB. The potential role of Fanconi anemia pathway in the regulation of REV1 and Polζ-dependent TLS and the involvement of additional polymerases, including DNA polymerases kappa, nu, and theta, in the repair of ICLs is also discussed in this review.

Structural insights into the assembly of human translesion polymerase complexes

In addition to DNA repair pathways, cells utilize translesion DNA synthesis (TLS) to bypass DNA lesions during replication. During TLS, Y-family DNA polymerase (Polη, Polκ, Polı and Rev1) inserts specific nucleotide opposite preferred DNA lesions, and then Polζ consisting of two subunits, Rev3 and Rev7, carries out primer extension. Here, we report the complex structures of Rev3-Rev7-Rev1(CTD) and Rev3-Rev7-Rev1(CTD)-Polκ(RIR). These two structures demonstrate that Rev1(CTD) contains separate binding sites for Polκ and Rev7. Our BIAcore experiments provide additional support for the notion that the interaction between Rev3 and Rev7 increases the affinity of Rev7 and Rev1. We also verified through FRET experiment that Rev1, Rev3, Rev7 and Polκ form a stable quaternary complex in vivo, thereby suggesting an efficient switching mechanism where the "inserter" polymerase can be immediately replaced by an "extender" polymerase within the same quaternary complex.