RecFOR proteins are essential for Pol V-mediated translesion synthesis and mutagenesis

Sending out an SOS - the bacterial DNA damage response

The term “SOS response” was first coined by Radman in 1974, in an intellectual effort to put together the data suggestive of a concerted gene expression program in cells undergoing DNA damage. A large amount of information about this cellular response has been collected over the following decades. In this review, we will focus on a few of the relevant aspects about the SOS response: its mechanism of control and the stressors which activate it, the diversity of regulated genes in different species, its role in mutagenesis and evolution including the development of antimicrobial resistance, and its relationship with mobile genetic elements.

Single strand gap repair: The presynaptic phase plays a pivotal role in modulating lesion tolerance pathways

During replication, the presence of unrepaired lesions results in the formation of single stranded DNA (ssDNA) gaps that need to be repaired to preserve genome integrity and cell survival. All organisms have evolved two major lesion tolerance pathways to continue replication: Translesion Synthesis (TLS), potentially mutagenic, and Homology Directed Gap Repair (HDGR), that relies on homologous recombination. In Escherichia coli, the RecF pathway repairs such ssDNA gaps by processing them to produce a recombinogenic RecA nucleofilament during the presynaptic phase. In this study, we show that the presynaptic phase is crucial for modulating lesion tolerance pathways since the competition between TLS and HDGR occurs at this stage. Impairing either the extension of the ssDNA gap (mediated by the nuclease RecJ and the helicase RecQ) or the loading of RecA (mediated by RecFOR) leads to a decrease in HDGR and a concomitant increase in TLS. Hence, we conclude that defects in the presynaptic phase delay the formation of the D-loop and increase the time window allowed for TLS. In contrast, we show that a defect in the postsynaptic phase that impairs HDGR does not lead to an increase in TLS. Unexpectedly, we also reveal a strong genetic interaction between recF and recJ genes, that results in a recA deficient-like phenotype in which HDGR is almost completely abolished.

RecA-independent recombination: Dependence on the Escherichia coli RarA protein

Most, but not all, homologous genetic recombination in bacteria is mediated by the RecA recombinase. The mechanistic origin of RecA-independent recombination has remained enigmatic. Here, we demonstrate that the RarA protein makes a major enzymatic contribution to RecA-independent recombination. In particular, RarA makes substantial contributions to intermolecular recombination and to recombination events involving relatively short (<200 bp) homologous sequences, where RecA-mediated recombination is inefficient. The effects are seen here in plasmid-based recombination assays and in vivo cloning processes. Vestigial levels of recombination remain even when both RecA and RarA are absent. Additional pathways for RecA-independent recombination, possibly mediated by helicases, are suppressed by exonucleases ExoI and RecJ. Translesion DNA polymerases may also contribute. Our results provide additional substance to a previous report of a functional overlap between RecA and RarA.

Elucidating Recombination Mediator Function Using Biophysical Tools

Simple Summary

This review recapitulates the initial knowledge acquired with genetics and biochemical experiments on Recombination mediator proteins in different domains of life. We further address how recent in vivo and in vitro biophysical tools were critical to deepen the understanding of RMPs molecular mechanisms in DNA and replication repair, and unveiled unexpected features. For instance, in bacteria, genetic and biochemical studies suggest a close proximity and coordination of action of the RecF, RecR and RecO proteins in order to ensure their RMP function, which is to overcome the single-strand binding protein (SSB) and facilitate the loading of the recombinase RecA onto ssDNA. In contrary to this expectation, using single-molecule fluorescent imaging in living cells, we showed recently that RecO and RecF do not colocalize and moreover harbor different spatiotemporal behavior relative to the replication machinery, suggesting distinct functions. Finally, we address how new biophysics tools could be used to answer outstanding questions about RMP function.

Abstract

The recombination mediator proteins (RMPs) are ubiquitous and play a crucial role in genome stability. RMPs facilitate the loading of recombinases like RecA onto single-stranded (ss) DNA coated by single-strand binding proteins like SSB. Despite sharing a common function, RMPs are the products of a convergent evolution and differ in (1) structure, (2) interaction partners and (3) molecular mechanisms. The RMP function is usually realized by a single protein in bacteriophages and eukaryotes, respectively UvsY or Orf, and RAD52 or BRCA2, while in bacteria three proteins RecF, RecO and RecR act cooperatively to displace SSB and load RecA onto a ssDNA region. Proteins working alongside to the RMPs in homologous recombination and DNA repair notably belongs to the RAD52 epistasis group in eukaryote and the RecF epistasis group in bacteria. Although RMPs have been studied for several decades, molecular mechanisms at the single-cell level are still not fully understood. Here, we summarize the current knowledge acquired on RMPs and review the crucial role of biophysical tools to investigate molecular mechanisms at the single-cell level in the physiological context.

A Comprehensive View of Translesion Synthesis in Escherichia coli

The lesion bypass pathway, translesion synthesis (TLS), exists in essentially all organisms and is considered a pathway for postreplicative gap repair and, at the same time, for lesion tolerance. As with the saying "a trip is not over until you get back home," studying TLS only at the site of the lesion is not enough to understand the whole process of TLS. Recently, a genetic study uncovered that polymerase V (Pol V), a poorly expressed Escherichia coli TLS polymerase, is not only involved in the TLS step per se but also participates in the gap-filling reaction over several hundred nucleotides. The same study revealed that in contrast, Pol IV, another highly expressed TLS polymerase, essentially stays away from the gap-filling reaction. These observations imply fundamentally different ways these polymerases are recruited to DNA in cells. While access of Pol IV appears to be governed by mass action, efficient recruitment of Pol V involves a chaperone-like action of the RecA filament. We present a model of Pol V activation: the 3' tip of the RecA filament initially stabilizes Pol V to allow stable complex formation with a sliding β-clamp, followed by the capture of the terminal RecA monomer by Pol V, thus forming a functional Pol V complex. This activation process likely determines higher accessibility of Pol V than of Pol IV to normal DNA. Finally, we discuss the biological significance of TLS polymerases during gap-filling reactions: error-prone gap-filling synthesis may contribute as a driving force for genetic diversity, adaptive mutation, and evolution.

Pol V-Mediated Translesion Synthesis Elicits Localized Untargeted Mutagenesis during Post-replicative Gap Repair

In vivo, replication forks proceed beyond replication-blocking lesions by way of downstream repriming, generating daughter strand gaps that are subsequently processed by post-replicative repair pathways such as homologous recombination and translesion synthesis (TLS). The way these gaps are filled during TLS is presently unknown. The structure of gap repair synthesis was assessed by sequencing large collections of single DNA molecules that underwent specific TLS events in vivo. The higher error frequency of specialized relative to replicative polymerases allowed us to visualize gap-filling events at high resolution. Unexpectedly, the data reveal that a specialized polymerase, Pol V, synthesizes stretches of DNA both upstream and downstream of a site-specific DNA lesion. Pol V-mediated untargeted mutations are thus spread over several hundred nucleotides, strongly eliciting genetic instability on either side of a given lesion. Consequently, post-replicative gap repair may be a source of untargeted mutations critical for gene diversification in adaptation and evolution.

The recombination mediator proteins RecFOR maintain RecA* levels for maximal DNA polymerase V Mut activity

DNA template damage can potentially block DNA replication. Cells have therefore developed different strategies to repair template lesions. Activation of the bacterial lesion bypass DNA polymerase V (Pol V) requires both the cleavage of the UmuD subunit to UmuD' and the acquisition of a monomer of activated RecA recombinase, forming Pol V Mut. Both of these events are mediated by the generation of RecA* via the formation of a RecA-ssDNA filament during the SOS response. Formation of RecA* is itself modulated by competition with the ssDNA-binding protein (SSB) for binding to ssDNA. Previous observations have demonstrated that RecA filament formation on SSB-coated DNA can be favored in the presence of the recombination mediator proteins RecF, RecO, and RecR. We show here using purified proteins that in the presence of SSB and RecA, a stable RecA-ssDNA filament is not formed, although sufficient RecA* is generated to support some activation of Pol V. The presence of RecFOR increased RecA* generation and allowed Pol V to synthesize longer DNA products and to elongate from an unpaired primer terminus opposite template damage, also without the generation of a stable RecA-ssDNA filament.

The inactivation of rfaP, rarA or sspA gene improves the viability of the Escherichia coli DNA polymerase III holD mutant

The Escherichia coli holD mutant is poorly viable because the stability of holoenzyme polymerase III (Pol III HE) on DNA is compromised. Consequently, the SOS response is induced and the SOS polymerases DinB and Pol II further hinder replication. Mutations that restore the holD mutant viability belong to two classes, those that stabilize Pol III on DNA and those that prevent the deleterious effects of DinB over-production. We identified a dnaX mutation and the inactivation of rfaP and sspA genes as belonging to the first class of holD mutant suppressors. dnaX encodes a Pol III clamp loader subunit that interacts with HolD. rfaP encodes a lipopolysaccharide kinase that acts in outer membrane biogenesis. Its inactivation improves the holD mutant growth in part by affecting potassium import, previously proposed to stabilize Pol III HE on DNA by increasing electrostatic interactions. sspA encodes a global transcriptional regulator and growth of the holD mutant in its absence suggests that SspA controls genes that affect protein-DNA interactions. The inactivation of rarA belongs to the second class of suppressor mutations. rarA inactivation has a weak effect but is additive with other suppressor mutations. Our results suggest that RarA facilitates DinB binding to abandoned forks.

A defect in homologous recombination leads to increased translesion synthesis in E. coli

DNA damage tolerance pathways allow cells to duplicate their genomes despite the presence of replication blocking lesions. Cells possess two major tolerance strategies, namely translesion synthesis (TLS) and homology directed gap repair (HDGR). TLS pathways involve specialized DNA polymerases that are able to synthesize past DNA lesions with an intrinsic risk of causing point mutations. In contrast, HDGR pathways are essentially error-free as they rely on the recovery of missing information from the sister chromatid by RecA-mediated homologous recombination. We have investigated the genetic control of pathway choice between TLS and HDGR in vivo in Escherichia coli In a strain with wild type RecA activity, the extent of TLS across replication blocking lesions is generally low while HDGR is used extensively. Interestingly, recA alleles that are partially impaired in D-loop formation confer a decrease in HDGR and a concomitant increase in TLS. Thus, partial defect of RecA's capacity to invade the homologous sister chromatid increases the lifetime of the ssDNA.RecA filament, i.e. the 'SOS signal'. This increase favors TLS by increasing both the TLS polymerase concentration and the lifetime of the TLS substrate, before it becomes sequestered by homologous recombination. In conclusion, the pathway choice between error-prone TLS and error-free HDGR is controlled by the efficiency of homologous recombination.

Bacterial Proliferation: Keep Dividing and Don't Mind the Gap

DNA Damage Tolerance (DDT) mechanisms help dealing with unrepaired DNA lesions that block replication and challenge genome integrity. Previous in vitro studies showed that the bacterial replicase is able to re-prime downstream of a DNA lesion, leaving behind a single-stranded DNA gap. The question remains of what happens to this gap in vivo. Following the insertion of a single lesion in the chromosome of a living cell, we showed that this gap is mostly filled in by Homology Directed Gap Repair in a RecA dependent manner. When cells fail to repair this gap, or when homologous recombination is impaired, cells are still able to divide, leading to the loss of the damaged chromatid, suggesting that bacteria lack a stringent cell division checkpoint mechanism. Hence, at the expense of losing one chromatid, cell survival and proliferation are ensured.

DNA Damage Tolerance (DDT) mechanisms help dealing with unrepaired DNA lesions that block replication, thus challenging genome integrity. Two DDT mechanisms have previously been described: error prone Translesion Synthesis operated by specialized DNA polymerases and error free bypass that uses the information of the sister chromatid to bypass the lesion. In this work, we set up a novel genetic system that allows to insert a single DNA blocking lesion in the chromosome of a living cell and to visualize the exchange of genetic information between the undamaged and the damaged strand. Using this system, we showed in vivo that the replication fork is able to re-prime downstream of the lesion, leaving a gap. This gap is mostly filled in by the error free pathway through the RecA homologous recombination mechanism. We show that when the gap is left unrepaired, cells are still able to divide by losing the damaged chromatid, which evidences the lack of a stringent cell division checkpoint system.

SOS, the formidable strategy of bacteria against aggressions

The presence of an abnormal amount of single-stranded DNA in the bacterial cell constitutes a genotoxic alarm signal that induces the SOS response, a broad regulatory network found in most bacterial species to address DNA damage. The aim of this review was to point out that beyond being a repair process, SOS induction leads to a very strong but transient response to genotoxic stress, during which bacteria can rearrange and mutate their genome, induce several phenotypic changes through differential regulation of genes, and sometimes acquire characteristics that potentiate bacterial survival and adaptation to changing environments. We review here the causes and consequences of SOS induction, but also how this response can be modulated under various circumstances and how it is connected to the network of other important stress responses. In the first section, we review articles describing the induction of the SOS response at the molecular level. The second section discusses consequences of this induction in terms of DNA repair, changes in the genome and gene expression, and sharing of genomic information, with their effects on the bacteria's life and evolution. The third section is about the fine tuning of this response to fit with the bacteria's 'needs'. Finally, we discuss recent findings linking the SOS response to other stress responses. Under these perspectives, SOS can be perceived as a powerful bacterial strategy against aggressions.

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.

An 'open' structure of the RecOR complex supports ssDNA binding within the core of the complex

Efficient DNA repair is critical for cell survival and the maintenance of genome integrity. The homologous recombination pathway is responsible for the repair of DNA double-strand breaks within cells. Initiation of this pathway in bacteria can be carried out by either the RecBCD or the RecFOR proteins. An important regulatory player within the RecFOR pathway is the RecOR complex that facilitates RecA loading onto DNA. Here we report new data regarding the assembly of Deinococcus radiodurans RecOR and its interaction with DNA, providing novel mechanistic insight into the mode of action of RecOR in homologous recombination. We present a higher resolution crystal structure of RecOR in an 'open' conformation in which the tetrameric RecR ring flanked by two RecO molecules is accessible for DNA binding. We show using small-angle neutron scattering and mutagenesis studies that DNA binding does indeed occur within the RecR ring. Binding of single-stranded DNA occurs without any major conformational changes of the RecOR complex while structural rearrangements are observed on double-stranded DNA binding. Finally, our molecular dynamics simulations, supported by our biochemical data, provide a detailed picture of the DNA binding motif of RecOR and reveal that single-stranded DNA is sandwiched between the two facing oligonucleotide binding domains of RecO within the RecR ring.

Homologous Recombination-Enzymes and Pathways

Homologous recombination is an ubiquitous process that shapes genomes and repairs DNA damage. The reaction is classically divided into three phases: presynaptic, synaptic, and postsynaptic. In Escherichia coli, the presynaptic phase involves either RecBCD or RecFOR proteins, which act on DNA double-stranded ends and DNA single-stranded gaps, respectively; the central synaptic steps are catalyzed by the ubiquitous DNA-binding protein RecA; and the postsynaptic phase involves either RuvABC or RecG proteins, which catalyze branch-migration and, in the case of RuvABC, the cleavage of Holliday junctions. Here, we review the biochemical properties of these molecular machines and analyze how, in light of these properties, the phenotypes of null mutants allow us to define their biological function(s). The consequences of point mutations on the biochemical properties of recombination enzymes and on cell phenotypes help refine the molecular mechanisms of action and the biological roles of recombination proteins. Given the high level of conservation of key proteins like RecA and the conservation of the principles of action of all recombination proteins, the deep knowledge acquired during decades of studies of homologous recombination in bacteria is the foundation of our present understanding of the processes that govern genome stability and evolution in all living organisms.

Simple and efficient purification of Escherichia coli DNA polymerase V: cofactor requirements for optimal activity and processivity in vitro

Most damage induced mutagenesis in Escherichia coli is dependent upon the UmuD'(2)C protein complex, which comprises DNA polymerase V (pol V). Biochemical characterization of pol V has been hindered by the fact that the enzyme is notoriously difficult to purify, largely because overproduced UmuC is insoluble. Here, we report a simple and efficient protocol for the rapid purification of milligram quantities of pol V from just 4 L of bacterial culture. Rather than over producing the UmuC protein, it was expressed at low basal levels, while UmuD'(2)C was expressed in trans from a high copy-number plasmid with an inducible promoter. We have also developed strategies to purify the β-clamp and γ-clamp loader free from contaminating polymerases. Using these highly purified proteins, we determined the cofactor requirements for optimal activity of pol V in vitro and found that pol V shows robust activity on an SSB-coated circular DNA template in the presence of the β/γ-complex and a RecA nucleoprotein filament (RecA*) formed in trans. This strong activity was attributed to the unexpectedly high processivity of pol V Mut (UmuD'(2)C · RecA · ATP), which was efficiently recruited to a primer terminus by SSB.

Presynaptic filament dynamics in homologous recombination and DNA repair

Homologous recombination (HR) is an essential genome stability mechanism used for high-fidelity repair of DNA double-strand breaks and for the recovery of stalled or collapsed DNA replication forks. The crucial homology search and DNA strand exchange steps of HR are catalyzed by presynaptic filaments-helical filaments of a recombinase enzyme bound to single-stranded DNA (ssDNA). Presynaptic filaments are fundamentally dynamic structures, the assembly, catalytic turnover, and disassembly of which must be closely coordinated with other elements of the DNA recombination, repair, and replication machinery in order for genome maintenance functions to be effective. Here, we reviewed the major dynamic elements controlling the assembly, activity, and disassembly of presynaptic filaments; some intrinsic such as recombinase ATP-binding and hydrolytic activities, others extrinsic such as ssDNA-binding proteins, mediator proteins, and DNA motor proteins. We examined dynamic behavior on multiple levels, including atomic- and filament-level structural changes associated with ATP binding and hydrolysis as evidenced in crystal structures, as well as subunit binding and dissociation events driven by intrinsic and extrinsic factors. We examined the biochemical properties of recombination proteins from four model systems (T4 phage, Escherichia coli, Saccharomyces cerevisiae, and Homo sapiens), demonstrating how their properties are tailored for the context-specific requirements in these diverse species. We proposed that the presynaptic filament has evolved to rely on multiple external factors for increased multilevel regulation of HR processes in genomes with greater structural and sequence complexity.

Mechanism of RecO recruitment to DNA by single-stranded DNA binding protein

RecO is a recombination mediator protein (RMP) important for homologous recombination, replication repair and DNA annealing in bacteria. In all pathways, the single-stranded (ss) DNA binding protein, SSB, plays an inhibitory role by protecting ssDNA from annealing and recombinase binding. Conversely, SSB may stimulate each reaction through direct interaction with RecO. We present a crystal structure of Escherichia coli RecO bound to the conserved SSB C-terminus (SSB-Ct). SSB-Ct binds the hydrophobic pocket of RecO in a conformation similar to that observed in the ExoI/SSB-Ct complex. Hydrophobic interactions facilitate binding of SSB-Ct to RecO and RecO/RecR complex in both low and moderate ionic strength solutions. In contrast, RecO interaction with DNA is inhibited by an elevated salt concentration. The SSB mutant lacking SSB-Ct also inhibits RecO-mediated DNA annealing activity in a salt-dependent manner. Neither RecO nor RecOR dissociates SSB from ssDNA. Therefore, in E. coli, SSB recruits RMPs to ssDNA through SSB-Ct, and RMPs are likely to alter the conformation of SSB-bound ssDNA without SSB dissociation to initiate annealing or recombination. Intriguingly, Deinococcus radiodurans RecO does not bind SSB-Ct and weakly interacts with the peptide in the presence of RecR, suggesting the diverse mechanisms of DNA repair pathways mediated by RecO in different organisms.

The Roles of UmuD in Regulating Mutagenesis

All organisms are subject to DNA damage from both endogenous and environmental sources. DNA damage that is not fully repaired can lead to mutations. Mutagenesis is now understood to be an active process, in part facilitated by lower-fidelity DNA polymerases that replicate DNA in an error-prone manner. Y-family DNA polymerases, found throughout all domains of life, are characterized by their lower fidelity on undamaged DNA and their specialized ability to copy damaged DNA. Two E. coli Y-family DNA polymerases are responsible for copying damaged DNA as well as for mutagenesis. These DNA polymerases interact with different forms of UmuD, a dynamic protein that regulates mutagenesis. The UmuD gene products, regulated by the SOS response, exist in two principal forms: UmuD(2), which prevents mutagenesis, and UmuD(2)', which facilitates UV-induced mutagenesis. This paper focuses on the multiple conformations of the UmuD gene products and how their protein interactions regulate mutagenesis.

Architecture of y-family DNA polymerases relevant to translesion DNA synthesis as revealed in structural and molecular modeling studies

DNA adducts, which block replicative DNA polymerases (DNAPs), are often bypassed by lesion-bypass DNAPs, which are mostly in the Y-Family. Y-Family DNAPs can do non-mutagenic or mutagenic dNTP insertion, and understanding this difference is important, because mutations transform normal into tumorigenic cells. Y-Family DNAP architecture that dictates mechanism, as revealed in structural and modeling studies, is considered. Steps from adduct blockage of replicative DNAPs, to bypass by a lesion-bypass DNAP, to resumption of synthesis by a replicative DNAP are described. Catalytic steps and protein conformational changes are considered. One adduct is analyzed in greater detail: the major benzo[a]pyrene adduct (B[a]P-N²-dG), which is bypassed non-mutagenically (dCTP insertion) by Y-family DNAPs in the IV/κ-class and mutagenically (dATP insertion) by V/η-class Y-Family DNAPs. Important architectural differences between IV/κ-class versus V/η-class DNAPs are discussed, including insights gained by analyzing ~400 sequences each for bacterial DNAPs IV and V, along with sequences from eukaryotic DNAPs kappa, eta and iota. The little finger domains of Y-Family DNAPs do not show sequence conservation; however, their structures are remarkably similar due to the presence of a core of hydrophobic amino acids, whose exact identity is less important than the hydrophobic amino acid spacing.

ATP binding, ATP hydrolysis, and protein dimerization are required for RecF to catalyze an early step in the processing and recovery of replication forks disrupted by DNA damage

In Escherichia coli, the recovery of replication following disruption by UV-induced DNA damage requires the RecF protein and occurs through a process that involves stabilization of replication fork DNA, resection of nascent DNA to allow the offending lesion to be repaired, and reestablishment of a productive replisome on the DNA. RecF forms a homodimer and contains an ATP binding cassette ATPase domain that is conserved among eukaryotic SMC (structural maintenance of chromosome) proteins, including cohesin, condensin, and Rad50. Here, we investigated the functions of RecF dimerization, ATP binding, and ATP hydrolysis in the progressive steps involved in recovering DNA synthesis following disruption by DNA damage. RecF point mutations with altered biochemical properties were constructed in the chromosome. We observed that protein dimerization, ATP binding, and ATP hydrolysis were essential for maintaining and processing the arrested replication fork, as well as for restoring DNA synthesis. In contrast, stabilization of the RecF protein dimer partially protected the DNA at the arrested fork from degradation, although overall processing and recovery remained severely impaired.

Interplay of DNA repair, homologous recombination, and DNA polymerases in resistance to the DNA damaging agent 4-nitroquinoline-1-oxide in Escherichia coli

Escherichia coli has three DNA damage-inducible DNA polymerases: DNA polymerase II (Pol II), DNA polymerase IV (Pol IV), and DNA polymerase V (Pol V). While the in vivo function of Pol V is well understood, the precise roles of Pol IV and Pol II in DNA replication and repair are not as clear. Study of these polymerases has largely focused on their participation in the recovery of failed replication forks, translesion DNA synthesis, and origin-independent DNA replication. However, their roles in other repair and recombination pathways in E. coli have not been extensively examined. This study investigated how E. coli's inducible DNA polymerases and various DNA repair and recombination pathways function together to convey resistance to 4-nitroquinoline-1-oxide (NQO), a DNA damaging agent that produces replication blocking DNA base adducts. The data suggest that full resistance to this compound depends upon an intricate interplay among the activities of the inducible DNA polymerases and recombination. The data also suggest new relationships between the different pathways that process recombination intermediates.

A DinB variant reveals diverse physiological consequences of incomplete TLS extension by a Y-family DNA polymerase

The only Y-family DNA polymerase conserved among all domains of life, DinB and its mammalian ortholog pol kappa, catalyzes proficient bypass of damaged DNA in translesion synthesis (TLS). Y-family DNA polymerases, including DinB, have been implicated in diverse biological phenomena ranging from adaptive mutagenesis in bacteria to several human cancers. Complete TLS requires dNTP insertion opposite a replication blocking lesion and subsequent extension with several dNTP additions. Here we report remarkably proficient TLS extension by DinB from Escherichia coli. We also describe a TLS DNA polymerase variant generated by mutation of an evolutionarily conserved tyrosine (Y79). This mutant DinB protein is capable of catalyzing dNTP insertion opposite a replication-blocking lesion, but cannot complete TLS, stalling three nucleotides after an N(2)-dG adduct. Strikingly, expression of this variant transforms a bacteriostatic DNA damaging agent into a bactericidal drug, resulting in profound toxicity even in a dinB(+) background. We find that this phenomenon is not exclusively due to a futile cycle of abortive TLS followed by exonucleolytic reversal. Rather, gene products with roles in cell death and metal homeostasis modulate the toxicity of DinB(Y79L) expression. Together, these results indicate that DinB is specialized to perform remarkably proficient insertion and extension on damaged DNA, and also expose unexpected connections between TLS and cell fate.

Biochemical basis for the essential genetic requirements of RecA and the beta-clamp in Pol V activation

In Escherichia coli, it is genetically well established that the beta-clamp and RecA are essential cofactors that endow DNA polymerase (Pol) V with lesion bypass activity. However, the biochemical basis for these requirements is still largely unknown. Because the process of translesion synthesis (TLS) requires that the specialized DNA polymerase synthesize in a single binding event a TLS patch that is long enough to resist external proofreading, it is critical to monitor Pol V burst synthesis. Here, we dissect the distinct roles that RecA and the beta-clamp perform during the Pol V activation process using physiologically relevant long single-stranded template DNA, similar to those used in genetic assays. Our data show that the beta-clamp endows the complex between Pol V and the template DNA with increased stability. Also, the RecA filament formed in cis on the single-stranded DNA produced downstream from the lesion stretches the template DNA to allow smooth elongation of the nascent strand by Pol V. The concurrent action of both cofactors is required for achieving productive TLS events. The present article presents an integrated view of TLS under physiologically relevant conditions in E. coli that may represent a paradigm for lesion bypass in other organisms.

Coordinating DNA polymerase traffic during high and low fidelity synthesis

With the discovery that organisms possess multiple DNA polymerases (Pols) displaying different fidelities, processivities, and activities came the realization that mechanisms must exist to manage the actions of these diverse enzymes to prevent gratuitous mutations. Although many of the Pols encoded by most organisms are largely accurate, and participate in DNA replication and DNA repair, a sizeable fraction display a reduced fidelity, and act to catalyze potentially error-prone translesion DNA synthesis (TLS) past lesions that persist in the DNA. Striking the proper balance between use of these different enzymes during DNA replication, DNA repair, and TLS is essential for ensuring accurate duplication of the cell's genome. This review highlights mechanisms that organisms utilize to manage the actions of their different Pols. A particular emphasis is placed on discussion of current models for how different Pols switch places with each other at the replication fork during high fidelity replication and potentially error-pone TLS.

RecA433 cells are defective in recF-mediated processing of disrupted replication forks but retain recBCD-mediated functions

RecA is required for recombinational processes and cell survival following UV-induced DNA damage. recA433 is a historically important mutant allele that contains a single amino acid substitution (R243H). This mutation separates the recombination and survival functions of RecA. recA433 mutants remain proficient in recombination as measured by conjugation or transduction, but are hypersensitive to UV-induced DNA damage. The cellular functions carried out by RecA require either recF pathway proteins or recBC pathway proteins to initiate RecA-loading onto the appropriate DNA substrates. In this study, we characterized the ability of recA433 to carry out functions associated with either the recF pathway or recBC pathway. We show that several phenotypic deficiencies exhibited by recA433 mutants are similar to recF mutants but distinct from recBC mutants. In contrast to recBC mutants, recA433 and recF mutants fail to process or resume replication following disruption by UV-induced DNA damage. However, recA433 and recF mutants remain proficient in conjugational recombination and are resistant to formaldehyde-induced protein-DNA crosslinks, functions that are impaired in recBC mutants. The results are consistent with a model in which the recA433 mutation selectively impairs RecA functions associated with the RecF pathway, while retaining the ability to carry out RecBCD pathway-mediated functions. These results are discussed in the context of the recF and recBC pathways and the potential substrates utilized in each case.

Fluence-response dynamics of the UV-induced SOS response in Escherichia coli

Bacteria in nature often suffer sudden stresses, such as ultraviolet (UV) irradiation, nutrient deprivation, and chemotoxins that would cause DNA damage and DNA replication failure, which in turn trigger SOS response. According to the strength and duration of the stress, the SOS system not only repairs DNA damage but also induces mutagenesis, so as to adapt to the changing environment. The key proteins in charge of mutagenesis are UmuD and UmuD'. In this paper, we quantitatively measure the growth rate and cellular levels of proteins UmuD and UmuD' in Escherichia coli after various fluences of UV irradiation. To compare with the experimental observations, an ordinary differential equation model is built to describe the SOS response. Considering the fact that the DNA lesions affect cellular protein production and replication origination, the simulation results fit well with the experimental data. Our results show how the fluence of UV irradiation determines the dynamics of the inducing signal and the mutation frequency of the cell.

SSB as an organizer/mobilizer of genome maintenance complexes

When duplex DNA is altered in almost any way (replicated, recombined, or repaired), single strands of DNA are usually intermediates, and single-stranded DNA binding (SSB) proteins are present. These proteins have often been described as inert, protective DNA coatings. Continuing research is demonstrating a far more complex role of SSB that includes the organization and/or mobilization of all aspects of DNA metabolism. Escherichia coli SSB is now known to interact with at least 14 other proteins that include key components of the elaborate systems involved in every aspect of DNA metabolism. Most, if not all, of these interactions are mediated by the amphipathic C-terminus of SSB. In this review, we summarize the extent of the eubacterial SSB interaction network, describe the energetics of interactions with SSB, and highlight the roles of SSB in the process of recombination. Similar themes to those highlighted in this review are evident in all biological systems.

Maintaining replication fork integrity in UV-irradiated Escherichia coli cells

In dividing cells, the stalling of replication fork complexes by impediments to DNA unwinding or by template imperfections that block synthesis by the polymerase subunits is a serious threat to genomic integrity and cell viability. What happens to stalled forks depends on the nature of the offending obstacle. In UV-irradiated Escherichia coli cells DNA synthesis is delayed for a considerable period, during which forks undergo extensive processing before replication can resume. Thus, restart depends on factors needed to load the replicative helicase, indicating that the replisome may have dissociated. It also requires the RecFOR proteins, which are known to load RecA recombinase on single-stranded DNA, implying that template strands are exposed. To gain a further understanding of how UV irradiation affects replication and how replication resumes after a block, we used fluorescence microscopy and BrdU or radioisotope labelling to examine chromosome replication and cell cycle progression. Our studies confirm that RecFOR promote efficient reactivation of stalled forks and demonstrate that they are also needed for productive replication initiated at the origin, or triggered elsewhere by damage to the DNA. Although delayed, all modes of replication do recover in the absence of these proteins, but nascent DNA strands are degraded more extensively by RecJ exonuclease. However, these strands are also degraded in the presence of RecFOR when restart is blocked by other means, indicating that RecA loading is not sufficient to stabilise and protect the fork. This is consistent with the idea that RecA actively promotes restart. Thus, in contrast to eukaryotic cells, there may be no factor in bacterial cells acting specifically to stabilise stalled forks. Instead, nascent strands may be protected by the simple expedient of promoting restart. We also report that the efficiency of fork reactivation is not affected in polB mutants.

UmuD and RecA directly modulate the mutagenic potential of the Y family DNA polymerase DinB

DinB is the only translesion Y family DNA polymerase conserved among bacteria, archaea, and eukaryotes. DinB and its orthologs possess a specialized lesion bypass function but also display potentially deleterious -1 frameshift mutagenic phenotypes when overproduced. We show that the DNA damage-inducible proteins UmuD(2) and RecA act in concert to modulate this mutagenic activity. Structural modeling suggests that the relatively open active site of DinB is enclosed by interaction with these proteins, thereby preventing the template bulging responsible for -1 frameshift mutagenesis. Intriguingly, residues that define the UmuD(2)-interacting surface on DinB statistically covary throughout evolution, suggesting a driving force for the maintenance of a regulatory protein-protein interaction at this site. Together, these observations indicate that proteins like RecA and UmuD(2) may be responsible for managing the mutagenic potential of DinB orthologs throughout evolution.

Single-stranded DNA-binding protein recruits DNA polymerase V to primer termini on RecA-coated DNA

Translesion DNA synthesis (TLS) by DNA polymerase V (polV) in Escherichia coli involves accessory proteins, including RecA and single-stranded DNA-binding protein (SSB). To elucidate the role of SSB in TLS we used an in vitro exonuclease protection assay and found that SSB increases the accessibility of 3' primer termini located at abasic sites in RecA-coated gapped DNA. The mutant SSB-113 protein, which is defective in protein-protein interactions, but not in DNA binding, was as effective as wild-type SSB in increasing primer termini accessibility, but deficient in supporting polV-catalyzed TLS. Consistently, the heterologous SSB proteins gp32, encoded by phage T4, and ICP8, encoded by herpes simplex virus 1, could replace E. coli SSB in the TLS reaction, albeit with lower efficiency. Immunoprecipitation experiments indicated that polV directly interacts with SSB and that this interaction is disrupted by the SSB-113 mutation. Taken together our results suggest that SSB functions to recruit polV to primer termini on RecA-coated DNA, operating by two mechanisms: 1) increasing the accessibility of 3' primer termini caused by binding of SSB to DNA and 2) a direct SSB-polV interaction mediated by the C terminus of SSB.

Interplay among replicative and specialized DNA polymerases determines failure or success of translesion synthesis pathways

Living cells possess a panel of specialized DNA polymerases that deal with the large diversity of DNA lesions that occur in their genomes. How specialized DNA polymerases gain access to the replication intermediate in the vicinity of the lesion is unknown. Using a model system in which a single replication blocking lesion can be bypassed concurrently by two pathways that leave distinct molecular signatures, we analyzed the complex interplay among replicative and specialized DNA polymerases. The system involves a single N-2-acetylaminofluorene guanine adduct within the NarI frameshift hot spot that can be bypassed concurrently by Pol II or Pol V, yielding a -2 frameshift or an error-free bypass product, respectively. Reconstitution of the two pathways using purified DNA polymerases Pol III, Pol II and Pol V and a set of essential accessory factors was achieved under conditions that recapitulate the known in vivo requirements. With this approach, we have identified the key replication intermediates that are used preferentially by Pol II and Pol V, respectively. Using single-hit conditions, we show that the beta-clamp is critical by increasing the processivity of Pol II during elongation of the slipped -2 frameshift intermediate by one nucleotide which, surprisingly, is enough to support subsequent elongation by Pol III rather than degradation. Finally, the proofreading activity of the replicative polymerase prevents the formation of a Pol II-mediated -1 frameshift product. In conclusion, failure or success of TLS pathways appears to be the net result of a complex interplay among DNA polymerases and accessory factors.

RecA-mediated excision repair: a novel mechanism for repairing DNA lesions at sites of arrested DNA synthesis

In Escherichia coli, bulky DNA lesions are repaired primarily by nucleotide excision repair (NER). Unrepaired lesions encountered by DNA polymerase at the replication fork create a blockage which may be relieved through RecF-dependent recombination. We have designed an assay to monitor the different mechanisms through which a DNA polymerase blocked by a single AAF lesion may be rescued by homologous double-stranded DNA sequences. Monomodified single-stranded plasmids exhibit low survival in non-SOS induced E. coli cells; we show here that the presence of a homologous sequence enhances the survival of the damaged plasmid more than 10-fold in a RecA-dependent way. Remarkably, in an NER proficient strain, 80% of the surviving colonies result from the UvrA-dependent repair of the AAF lesion in a mechanism absolutely requiring RecA and RecF activity, while the remaining 20% of the surviving colonies result from homologous recombination mechanisms. These results uncover a novel mechanism - RecA-mediated excision repair - in which RecA-dependent pairing of the mono-modified single-stranded template with a complementary sequence allows its repair by the UvrABC excinuclease.