The RecOR proteins modulate RecA protein function at 5' ends of single-stranded DNA

Characterization of Staphylococcus aureus RecX protein: Molecular insights into negative regulation of RecA protein and implications in HR processes

Homologous recombination (HR) is essential for genome stability and for maintaining genetic diversity. In eubacteria, RecA protein plays a key role during DNA repair, transcription, and HR. RecA is regulated at multiple levels, but majorly by RecX protein. Moreover, studies have shown RecX is a potent inhibitor of RecA and thus acts as an antirecombinase. Staphylococcus aureus is a major food-borne pathogen that causes skin, bone joint, and bloodstream infections. To date, RecX's role in S. aureus has remained enigmatic. Here, we show that S. aureus RecX (SaRecX) is expressed during exposure to DNA-damaging agents, and purified RecX protein directly interacts physically with RecA protein. The SaRecX is competent to bind with single-stranded DNA preferentially and double-stranded DNA feebly. Significantly, SaRecX impedes the RecA-driven displacement loop and inhibits formation of the strand exchange. Notably, SaRecX also abrogates adenosine triphosphate hydrolysis and abolishes the LexA coprotease activity. These findings highlight the role of the RecX protein as an antirecombinase during HR and play a pivotal role in regulation of RecA during the DNA transactions.

Generation and Repair of Postreplication Gaps in Escherichia coli

When replication forks encounter template lesions, one result is lesion skipping, where the stalled DNA polymerase transiently stalls, disengages, and then reinitiates downstream to leave the lesion behind in a postreplication gap. Despite considerable attention in the 6 decades since postreplication gaps were discovered, the mechanisms by which postreplication gaps are generated and repaired remain highly enigmatic. This review focuses on postreplication gap generation and repair in the bacterium Escherichia coli. New information to address the frequency and mechanism of gap generation and new mechanisms for their resolution are described. There are a few instances where the formation of postreplication gaps appears to be programmed into particular genomic locations, where they are triggered by novel genomic elements.

RecF protein targeting to postreplication (daughter strand) gaps I: DNA binding by RecF and RecFR

In bacteria, the repair of post-replication gaps by homologous recombination requires the action of the recombination mediator proteins RecF, RecO and RecR. Whereas the role of the RecOR proteins to displace the single strand binding protein (SSB) and facilitate RecA loading is clear, how RecF mediates targeting of the system to appropriate sites remains enigmatic. The most prominent hypothesis relies on specific RecF binding to gap ends. To test this idea, we present a detailed examination of RecF and RecFR binding to more than 40 DNA substrates of varying length and structure. Neither RecF nor the RecFR complex exhibited specific DNA binding that can explain the targeting of RecF(R) to post-replication gaps. RecF(R) bound to dsDNA and ssDNA of sufficient length with similar facility. DNA binding was highly ATP-dependent. Most measured Kd values fell into a range of 60-180 nM. The addition of ssDNA extensions on duplex substrates to mimic gap ends or CPD lesions produces only subtle increases or decreases in RecF(R) affinity. Significant RecFR binding cooperativity was evident with many DNA substrates. The results indicate that RecF or RecFR targeting to post-replication gaps must rely on factors not yet identified, perhaps involving interactions with additional proteins.

RecF protein targeting to post-replication (daughter strand) gaps II: RecF interaction with replisomes

The bacterial RecF, RecO, and RecR proteins are an epistasis group involved in loading RecA protein into post-replication gaps. However, the targeting mechanism that brings these proteins to appropriate gaps is unclear. Here, we propose that targeting may involve a direct interaction between RecF and DnaN. In vivo, RecF is commonly found at the replication fork. Over-expression of RecF, but not RecO or a RecF ATPase mutant, is extremely toxic to cells. We provide evidence that the molecular basis of the toxicity lies in replisome destabilization. RecF over-expression leads to loss of genomic replisomes, increased recombination associated with post-replication gaps, increased plasmid loss, and SOS induction. Using three different methods, we document direct interactions of RecF with the DnaN β-clamp and DnaG primase that may underlie the replisome effects. In a single-molecule rolling-circle replication system in vitro, physiological levels of RecF protein trigger post-replication gap formation. We suggest that the RecF interactions, particularly with DnaN, reflect a functional link between post-replication gap creation and gap processing by RecA. RecF's varied interactions may begin to explain how the RecFOR system is targeted to rare lesion-containing post-replication gaps, avoiding the potentially deleterious RecA loading onto thousands of other gaps created during replication.

Mechanism of RecF-RecO-RecR cooperation in bacterial homologous recombination

In bacteria, one type of homologous-recombination-based DNA-repair pathway involves RecFOR proteins that bind at the junction between single-stranded (ss) and double-stranded (ds) DNA. They facilitate the replacement of SSB protein, which initially covers ssDNA, with RecA, which mediates the search for homologous sequences. However, the molecular mechanism of RecFOR cooperation remains largely unknown. We used Thermus thermophilus proteins to study this system. Here, we present a cryo-electron microscopy structure of the RecF-dsDNA complex, and another reconstruction that shows how RecF interacts with two different regions of the tetrameric RecR ring. Lower-resolution reconstructions of the RecR-RecO subcomplex and the RecFOR-DNA assembly explain how RecO is positioned to interact with ssDNA and SSB, which is proposed to lock the complex on a ssDNA-dsDNA junction. Our results integrate the biochemical data available for the RecFOR system and provide a framework for its complete understanding.

ATPase Activity of Bacillus subtilis RecA Affects the Dynamic Formation of RecA Filaments at DNA Double Strand Breaks

RecA plays a central role in DNA repair and is a main actor involved in homologous recombination (HR). In vivo, RecA forms filamentous structures termed "threads," which are essential for HR, but whose nature is still ill defined. We show that RecA from Bacillus subtilis having lower ATP binding activity can still form nucleoprotein filaments in vitro, features lower dsDNA binding activity, but still retains most of wild type RecA activity in vivo. Contrarily, loss of ATPase activity strongly reduced formation of nucleoprotein filaments in vitro, and effectivity to repair double strand breaks (DSBs) in vivo. In the presence of wild type RecA protein, additionally expressed RecA with lowered ATPbinding activity only moderately affected RecA dynamics, while loss of ATPase activity leads to a large reduction of the formation of threads, as well as of their dynamic changes observed in a seconds-scale. Single molecule tracking of RecA revealed incorporation of freely diffusing and nonspecifically DNA-bound molecules into threads upon induction of a single DSB. This change of dynamics was highly perturbed in the absence of ATPase activity, revealing that filamentous forms of RecA as well as their dynamics depend on ATPase activity. Based on the idea that ATPase activity of RecA is most important for DNA strand exchange activity, our data suggest that extension and retraction of threads due is to many local strand invasion events during the search for sequences homologous to the induced DNA break site. IMPORTANCE Single-strand (ss) DNA binding ATPase RecA is the central recombinase in homologous recombination, and therefore essential for DNA repair pathways involving DNA strand exchange reactions. In several bacterial, RecA forms filamentous structures along the long axis of cells after induction of double strand breaks (DSBs) in the chromosome. These striking assemblies likely reflect RecA/ssDNA nucleoprotein filaments, which can extend and remodel within a time frame of few minutes. We show that ATPase activity of RecA is pivotal for these dynamic rearrangements, which include recruitment of freely diffusing molecules into low-mobile molecules within filaments. Our data suggest that ssDNA binding- and unbinding reactions are at the heart of RecA dynamics that power the dynamics of subcellular filamentous assemblies, leading to strand exchange reactions over a distance of several micrometers.

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.

A single-molecule approach to unravel the molecular mechanism of the action of Deinococcus radiodurans RecD2 and its interaction with SSB and RecA in DNA repair

Helicases are ATP-driven molecular machines that directionally remodel nucleic acid polymers in all three domains of life. They are responsible for resolving double-stranded DNA (dsDNA) into single-strands, which is essential for DNA replication, nucleotide excision repair, and homologous recombination. RecD2 from Deinococcus radiodurans (DrRecD2) has important contributions to the organism's unusually high tolerance to gamma radiation and hydrogen peroxide. Although the results from X-ray Crystallography studies have revealed the structural characteristics of the protein, direct experimental evidence regarding the dynamics of the DNA unwinding process by DrRecD2 in the context of other accessory proteins is yet to be found. In this study, we have probed the exact binding event and processivity of DrRecD2 at single-molecule resolution using Protein-induced fluorescence enhancement (smPIFE) and Forster resonance energy transfer (smFRET). We have found that the protein prefers to bind at the 5' terminal end of the single-stranded DNA (ssDNA) by Drift and has helicase activity even in absence of ATP. However, a faster and iterative mode of DNA unwinding was evident in presence of ATP. The rate of translocation of the protein was found to be slower on dsDNA compared to ssDNA. We also showed that DrRecD2 is recruited at the binding site by the single-strand binding protein (SSB) and during the unwinding, it can displace RecA from ssDNA.

The Escherichia coli serS gene promoter region overlaps with the rarA gene

Deletion of the entire gene encoding the RarA protein of Escherichia coli results in a growth defect and additional deficiencies that were initially ascribed to a lack of RarA function. Further work revealed that most of the effects reflected the presence of sequences in the rarA gene that affect expression of the downstream gene, serS. The serS gene encodes the seryl aminoacyl-tRNA synthetase. Decreases in the expression of serS can trigger the stringent response. The sequences that affect serS expression are located in the last 15 nucleotides of the rarA gene.

The rarA gene as part of an expanded RecFOR recombination pathway: Negative epistasis and synthetic lethality with ruvB, recG, and recQ

The RarA protein, homologous to human WRNIP1 and yeast MgsA, is a AAA+ ATPase and one of the most highly conserved DNA repair proteins. With an apparent role in the repair of stalled or collapsed replication forks, the molecular function of this protein family remains obscure. Here, we demonstrate that RarA acts in late stages of recombinational DNA repair of post-replication gaps. A deletion of most of the rarA gene, when paired with a deletion of ruvB or ruvC, produces a growth defect, a strong synergistic increase in sensitivity to DNA damaging agents, cell elongation, and an increase in SOS induction. Except for SOS induction, these effects are all suppressed by inactivating recF, recO, or recJ, indicating that RarA, along with RuvB, acts downstream of RecA. SOS induction increases dramatically in a rarA ruvB recF/O triple mutant, suggesting the generation of large amounts of unrepaired ssDNA. The rarA ruvB defects are not suppressed (and in fact slightly increased) by recB inactivation, suggesting RarA acts primarily downstream of RecA in post-replication gaps rather than in double strand break repair. Inactivating rarA, ruvB and recG together is synthetically lethal, an outcome again suppressed by inactivation of recF, recO, or recJ. A rarA ruvB recQ triple deletion mutant is also inviable. Together, the results suggest the existence of multiple pathways, perhaps overlapping, for the resolution or reversal of recombination intermediates created by RecA protein in post-replication gaps within the broader RecF pathway. One of these paths involves RarA.

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.

Rec(F/O/R) proteins of the nitrogen-fixing cyanobacterium Nostoc PCC7120: In silico and expression analysis

The high radioresistance of Nostoc sp. strain PCC7120 is indicative of a robust DNA repair pathway. In the absence of NHEJ pathway and the canonical RecBCD proteins, the RecF pathway proteins are expected to play an important role in double strand break repair in this organism. The RecF, RecO and RecR proteins which are central to the RecF pathway have not been characterised in the ancient cyanobacteria, several of which are known to be radioresistant. The characterisation of these proteins was initiated through a mix of in silico, expression and complementation analysis. Differential expression of the recF, recO and recR genes was observed both at the transcript and the protein level under normal growth condition, which did not change significantly upon exposure to DNA damage stresses. Expression of RecR as a 23 kDa protein in vivo in Nostoc PCC7120 confirmed the re-annotation of the initiation codon of the gene (alr4977) to a rare initiation codon 'GTT' 267 bases upstream of the annotated initiation codon. Of the three proteins, Nostoc RecO and RecR proteins could complement the corresponding mutations in Escherichia coli, but not RecF. The Nostoc RecO protein exhibited low sequence and structural homology with other bacterial RecO protein, and was predicted to have a longer loop region. Phylogenetic as well as sequence analysis revealed high conservation among bacterial RecR proteins and least for RecO. In silico analysis revealed a comparatively smaller interactome for the Nostoc RecF, RecO and RecR proteins compared to other bacteria, with RecO predicted to interact with both RecF and RecR. The information gathered can form a stepping stone to further characterise these proteins in terms of deciphering their interactome, biochemical and physiological activities. This would help in establishing their importance in RecF pathway of DSB repair in Nostoc PCC7120.

Two components of DNA replication-dependent LexA cleavage

Induction of the SOS response, a cellular system triggered by DNA damage in bacteria, depends on DNA replication for the generation of the SOS signal, ssDNA. RecA binds to ssDNA, forming filaments that stimulate proteolytic cleavage of the LexA transcriptional repressor, allowing expression of > 40 gene products involved in DNA repair and cell cycle regulation. Here, using a DNA replication system reconstituted in vitro in tandem with a LexA cleavage assay, we studied LexA cleavage during DNA replication of both undamaged and base-damaged templates. Only a ssDNA-RecA filament supported LexA cleavage. Surprisingly, replication of an undamaged template supported levels of LexA cleavage like that induced by a template carrying two site-specific cyclobutane pyrimidine dimers. We found that two processes generate ssDNA that could support LexA cleavage. 1) During unperturbed replication, single-stranded regions formed because of stochastic uncoupling of the leading-strand DNA polymerase from the replication fork DNA helicase, and 2) on the damaged template, nascent leading-strand gaps were generated by replisome lesion skipping. The two pathways differed in that RecF stimulated LexA cleavage during replication of the damaged template, but not normal replication. RecF appears to facilitate RecA filament formation on the leading-strand ssDNA gaps generated by replisome lesion skipping.

The extended N-terminus of Mycobacterium smegmatis RecX potentiates its ability to antagonize RecA functions

The members of the RecX family of proteins have a unique capacity to regulate the catalytic activities of RecA/Rad51 proteins in both prokaryotic and eukaryotic organisms. However, our understanding of the functional roles of RecX in pathogenic and non-pathogenic mycobacteria has been limited by insufficient knowledge of the molecular mechanisms of its activity and regulation. Moreover, the significance of a unique 14 amino acid N-terminal extension in Mycobacterium smegmatis RecX (MsRecX) to its function remains unknown. Here, we advance our understanding of the antagonistic roles of mycobacterial RecX proteins and the functional significance of the extended N-terminus of MsRecX. The full-length MsRecX acts as an antagonist of RecA, negatively regulating RecA promoted functions, including DNA strand exchange, LexA cleavage and ATP hydrolysis, but not binding of ATP. The N-terminally truncated MsRecX variants retain the RecA inhibitory activity, albeit with lower efficiencies compared to the full-length protein. Perhaps most importantly, direct visualization of RecA nucleoprotein filaments, which had been incubated with RecX proteins, showed that they promote disassembly of nucleoprotein filaments primarily within the filaments. In addition, interaction of RecX proteins with the RecA nucleoprotein filaments results in the formation of stiff and irregularly shaped nucleoprotein filaments. Thus, these findings add an additional mechanism by which RecX disassembles RecA nucleoprotein filaments. Overall, this study provides strong evidence for the notion that the N-terminal 14 amino acid region of MsRecX plays an important role in the negative regulation of RecA functions and new insights into the molecular mechanism underlying RecX function.

Ancestral niche separation and evolutionary rate differentiation between sister marine flavobacteria lineages

Marine flavobacteria are specialists for polysaccharide degradation. They dominate in habitats enriched with polysaccharides, but are also prevalent in pelagic environments where polysaccharides are less available. These niches are likely occupied by distinct lineages, but evolutionary processes underlying their niche differentiation remain elusive. Here, genomic analyses and physiological assays indicate that the sister flavobacteria lineages Leeuwenhoekiella and Nonlabens likely explore polysaccharide-rich macroalgae and polysaccharide-poor pelagic niches respectively. Phylogenomic analyses inferred that the niche separation likely occurred anciently and coincided with increased sequence evolutionary rate in Nonlabens compared with Leeuwenhoekiella. Further analyses ruled out the known mechanisms likely driving evolutionary rate acceleration, including reduced selection efficiency, decreased generation time and increased mutation rate. In particular, the mutation rates were determined using an unbiased experimental method, which measures the present-day populations and may not reflect ancestral populations. These data collectively lead to a new hypothesis that an ancestral and transient mutation rate increase resulted in evolutionary rate increase in Nonlabens. This hypothesis was supported by inferring that gains and losses of genes involved in SOS response, a mechanism known to drive transiently increased mutation rate, coincided with evolutionary rate acceleration. Our analyses highlight the evolutionary mechanisms underlying niche differentiation of flavobacteria lineages.

RecFOR epistasis group: RecF and RecO have distinct localizations and functions in Escherichia coli

In bacteria, genetic recombination is a major mechanism for DNA repair. The RecF, RecO and RecR proteins are proposed to initiate recombination by loading the RecA recombinase onto DNA. However, the biophysical mechanisms underlying this process remain poorly understood. Here, we used genetics and single-molecule fluorescence microscopy to investigate whether RecF and RecO function together, or separately, in live Escherichia coli cells. We identified conditions in which RecF and RecO functions are genetically separable. Single-molecule imaging revealed key differences in the spatiotemporal behaviours of RecF and RecO. RecF foci frequently colocalize with replisome markers. In response to DNA damage, colocalization increases and RecF dimerizes. The majority of RecF foci are dependent on RecR. Conversely, RecO foci occur infrequently, rarely colocalize with replisomes or RecF and are largely independent of RecR. In response to DNA damage, RecO foci appeared to spatially redistribute, occupying a region close to the cell membrane. These observations indicate that RecF and RecO have distinct functions in the DNA damage response. The observed localization of RecF to the replisome supports the notion that RecF helps to maintain active DNA replication in cells carrying DNA damage.

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.

Crystal structure of RecR, a member of the RecFOR DNA-repair pathway, from Pseudomonas aeruginosa PAO1

DNA damage is usually lethal to all organisms. Homologous recombination plays an important role in the DNA damage-repair process in prokaryotic organisms. Two pathways are responsible for homologous recombination in Pseudomonas aeruginosa: the RecBCD pathway and the RecFOR pathway. RecR is an important regulator in the RecFOR homologous recombination pathway in P. aeruginosa. It forms complexes with RecF and RecO that can facilitate the loading of RecA onto ssDNA in the RecFOR pathway. Here, the crystal structure of RecR from P. aeruginosa PAO1 (PaRecR) is reported. PaRecR crystallizes in space group P6122, with two monomers per asymmetric unit. Analytical ultracentrifugation data show that PaRecR forms a stable dimer, but can exist as a tetramer in solution. The crystal structure shows that dimeric PaRecR forms a ring-like tetramer architecture via crystal symmetry. The presence of a ligand in the Walker B motif of one RecR subunit suggests a putative nucleotide-binding site.

Stable Nuclei of Nucleoprotein Filament and High ssDNA Binding Affinity Contribute to Enhanced RecA E38K Recombinase Activity

RecA plays central roles in the homologous recombination to repair double-stranded DNA break damage in E. coli. A previously identified recA strain surviving high doses of UV radiation includes a dominant RecA E38K mutation. Using single-molecule experiments, we showed that the RecA E38K variant protein assembles nucleoprotein filaments more rapidly than the wild-type RecA. We also used a single-molecule fluorescence resonance energy transfer (smFRET) experiment to compare the nucleation cluster dynamics of wild-type RecA and RecA E38K mutants on various short ssDNA substrates. At shorter ssDNA, nucleation clusters of RecA E38K form dynamically, while only few were seen in wild-type RecA. RecA E38K also forms stable nuclei by specifically lowering the dissociation rate constant, k_d. These observations provide evidence that greater nuclei stability and higher ssDNA binding affinity contribute to the observed enhanced recombination activity of the RecA E38K mutant. Given that assembly of RecA nucleoprotein filaments is the first committed step in recombinational repair processes, enhancement at this step gives rise to a more efficient recombinase.

Advances in structural studies of recombination mediator proteins

Recombination mediator proteins (RMPs) are critical for genome integrity in all organisms. They include phage UvsY, prokaryotic RecF, -O, -R (RecFOR) and eukaryotic Rad52, Breast Cancer susceptibility 2 (BRCA2) and Partner and localizer of BRCA2 (PALB2) proteins. BRCA2 and PALB2 are tumor suppressors implicated in cancer. RMPs regulate binding of RecA-like recombinases to sites of DNA damage to initiate the most efficient non-mutagenic repair of broken chromosome and other deleterious DNA lesions. Mechanistically, RMPs stimulate a single-stranded DNA (ssDNA) hand-off from ssDNA binding proteins (ssbs) such as gp32, SSB and RPA, to recombinases, activating DNA repair only at the time and site of the damage event. This review summarizes structural studies of RMPs and their implications for understanding mechanism and function. Comparative analysis of RMPs is complicated due to their convergent evolution. In contrast to the evolutionary conserved ssbs and recombinases, RMPs are extremely diverse in sequence and structure. Structural studies are particularly important in such cases to reveal common features of the entire family and specific features of regulatory mechanisms for each member. All RMPs are characterized by specific DNA-binding domains and include variable protein interaction motifs. The complexity of such RMPs corresponds to the ever-growing number of DNA metabolism events they participate in under normal and pathological conditions and requires additional comprehensive structure-functional studies.

Structural and Functional Studies of H. seropedicae RecA Protein - Insights into the Polymerization of RecA Protein as Nucleoprotein Filament

The bacterial RecA protein plays a role in the complex system of DNA damage repair. Here, we report the functional and structural characterization of the Herbaspirillum seropedicae RecA protein (HsRecA). HsRecA protein is more efficient at displacing SSB protein from ssDNA than Escherichia coli RecA protein. HsRecA also promotes DNA strand exchange more efficiently. The three dimensional structure of HsRecA-ADP/ATP complex has been solved to 1.7 Å resolution. HsRecA protein contains a small N-terminal domain, a central core ATPase domain and a large C-terminal domain, that are similar to homologous bacterial RecA proteins. Comparative structural analysis showed that the N-terminal polymerization motif of archaeal and eukaryotic RecA family proteins are also present in bacterial RecAs. Reconstruction of electrostatic potential from the hexameric structure of HsRecA-ADP/ATP revealed a high positive charge along the inner side, where ssDNA is bound inside the filament. The properties of this surface may explain the greater capacity of HsRecA protein to bind ssDNA, forming a contiguous nucleoprotein filament, displace SSB and promote DNA exchange relative to EcRecA. Our functional and structural analyses provide insight into the molecular mechanisms of polymerization of bacterial RecA as a helical nucleoprotein filament.

λ Recombination and Recombineering

The bacteriophage λ Red homologous recombination system has been studied over the past 50 years as a model system to define the mechanistic details of how organisms exchange DNA segments that share extended regions of homology. The λ Red system proved useful as a system to study because recombinants could be easily generated by co-infection of genetically marked phages. What emerged from these studies was the recognition that replication of phage DNA was required for substantial Red-promoted recombination in vivo, and the critical role that double-stranded DNA ends play in allowing the Red proteins access to the phage DNA chromosomes. In the past 16 years, however, the λ Red recombination system has gained a new notoriety. When expressed independently of other λ functions, the Red system is able to promote recombination of linear DNA containing limited regions of homology (∼50 bp) with the Escherichia coli chromosome, a process known as recombineering. This review explains how the Red system works during a phage infection, and how it is utilized to make chromosomal modifications of E. coli with such efficiency that it changed the nature and number of genetic manipulations possible, leading to advances in bacterial genomics, metabolic engineering, and eukaryotic genetics.

DNA Metabolism in Balance: Rapid Loss of a RecA-Based Hyperrec Phenotype

The RecA recombinase of Escherichia coli has not evolved to optimally promote DNA pairing and strand exchange, the key processes of recombinational DNA repair. Instead, the recombinase function of RecA protein represents an evolutionary compromise between necessary levels of recombinational DNA repair and the potentially deleterious consequences of RecA functionality. A RecA variant, RecA D112R, promotes conjugational recombination at substantially enhanced levels. However, expression of the D112R RecA protein in E. coli results in a reduction in cell growth rates. This report documents the consequences of the substantial selective pressure associated with the RecA-mediated hyperrec phenotype. With continuous growth, the deleterious effects of RecA D112R, along with the observed enhancements in conjugational recombination, are lost over the course of 70 cell generations. The suppression reflects a decline in RecA D112R expression, associated primarily with a deletion in the gene promoter or chromosomal mutations that decrease plasmid copy number. The deleterious effects of RecA D112R on cell growth can also be negated by over-expression of the RecX protein from Neisseria gonorrhoeae. The effects of the RecX proteins in vivo parallel the effects of the same proteins on RecA D112R filaments in vitro. The results indicate that the toxicity of RecA D112R is due to its persistent binding to duplex genomic DNA, creating barriers for other processes in DNA metabolism. A substantial selective pressure is generated to suppress the resulting barrier to growth.

RecO protein initiates DNA recombination and strand annealing through two alternative DNA binding mechanisms

Recombination mediator proteins (RMPs) are important for genome stability in all organisms. Several RMPs support two alternative reactions: initiation of homologous recombination and DNA annealing. We examined mechanisms of RMPs in both reactions with Mycobacterium smegmatis RecO (MsRecO) and demonstrated that MsRecO interacts with ssDNA by two distinct mechanisms. Zinc stimulates MsRecO binding to ssDNA during annealing, whereas the recombination function is zinc-independent and is regulated by interaction with MsRecR. Thus, different structural motifs or conformations of MsRecO are responsible for interaction with ssDNA during annealing and recombination. Neither annealing nor recombinase loading depends on MsRecO interaction with the conserved C-terminal tail of single-stranded (ss) DNA-binding protein (SSB), which is known to bind Escherichia coli RecO. However, similarly to E. coli proteins, MsRecO and MsRecOR do not dismiss SSB from ssDNA, suggesting that RMPs form a complex with SSB-ssDNA even in the absence of binding to the major protein interaction motif. We propose that alternative conformations of such complexes define the mechanism by which RMPs initiate the repair of stalled replication and support two different functions during recombinational repair of DNA breaks.

Fate of the replisome following arrest by UV-induced DNA damage in Escherichia coli

Accurate replication in the presence of DNA damage is essential to genome stability and viability in all cells. In Escherichia coli, DNA replication forks blocked by UV-induced damage undergo a partial resection and RecF-catalyzed regression before synthesis resumes. These processing events generate distinct structural intermediates on the DNA that can be visualized in vivo using 2D agarose gels. However, the fate and behavior of the stalled replisome remains a central uncharacterized question. Here, we use thermosensitive mutants to show that the replisome's polymerases uncouple and transiently dissociate from the DNA in vivo. Inactivation of α, β, or τ subunits within the replisome is sufficient to signal and induce the RecF-mediated processing events observed following UV damage. By contrast, the helicase-primase complex (DnaB and DnaG) remains critically associated with the fork, leading to a loss of fork integrity, degradation, and aberrant intermediates when disrupted. The results reveal a dynamic replisome, capable of partial disassembly to allow access to the obstruction, while retaining subunits that maintain fork licensing and direct reassembly to the appropriate location after processing has occurred.

DNA repair and genome maintenance in Bacillus subtilis

From microbes to multicellular eukaryotic organisms, all cells contain pathways responsible for genome maintenance. DNA replication allows for the faithful duplication of the genome, whereas DNA repair pathways preserve DNA integrity in response to damage originating from endogenous and exogenous sources. The basic pathways important for DNA replication and repair are often conserved throughout biology. In bacteria, high-fidelity repair is balanced with low-fidelity repair and mutagenesis. Such a balance is important for maintaining viability while providing an opportunity for the advantageous selection of mutations when faced with a changing environment. Over the last decade, studies of DNA repair pathways in bacteria have demonstrated considerable differences between Gram-positive and Gram-negative organisms. Here we review and discuss the DNA repair, genome maintenance, and DNA damage checkpoint pathways of the Gram-positive bacterium Bacillus subtilis. We present their molecular mechanisms and compare the functions and regulation of several pathways with known information on other organisms. We also discuss DNA repair during different growth phases and the developmental program of sporulation. In summary, we present a review of the function, regulation, and molecular mechanisms of DNA repair and mutagenesis in Gram-positive bacteria, with a strong emphasis on B. subtilis.

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.

Mu insertions are repaired by the double-strand break repair pathway of Escherichia coli

Mu is both a transposable element and a temperate bacteriophage. During lytic growth, it amplifies its genome by replicative transposition. During infection, it integrates into the Escherichia coli chromosome through a mechanism not requiring extensive DNA replication. In the latter pathway, the transposition intermediate is repaired by transposase-mediated resecting of the 5′ flaps attached to the ends of the incoming Mu genome, followed by filling the remaining 5 bp gaps at each end of the Mu insertion. It is widely assumed that the gaps are repaired by a gap-filling host polymerase. Using the E. coli Keio Collection to screen for mutants defective in recovery of stable Mu insertions, we show in this study that the gaps are repaired by the machinery responsible for the repair of double-strand breaks in E. coli—the replication restart proteins PriA-DnaT and homologous recombination proteins RecABC. We discuss alternate models for recombinational repair of the Mu gaps.

Transposon activity shapes genome structure and evolution. The movement of these elements generates target site duplications as a result of staggered cuts in the target made initially by the transposase. For replicative transposons, the single-stranded gaps generated after the initial strand transfer event are filled by target-primed replication. However, the majority of known transposable elements transpose by a non-replicative mechanism. Despite a wealth of information available for the mechanism of transposase action, little is known about how the cell repairs gaps left in the wake of transposition of these majority elements. Phage Mu is unique in using both replicative and non-replicative modes of transposition. Our study finds that during its non-replicative pathway, the gaps created by Mu insertion are repaired by the primary machinery for double-strand break repair in E. coli, not by gap-filling polymerases as previously thought. This first report of specific host processes involved in repair of transposon insertions in bacteria is likely to have a broad significance, given also that double-strand break repair pathways have been implicated in repair of the retroviral and Line retroelement insertions.

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.

A mechanism for single-stranded DNA-binding protein (SSB) displacement from single-stranded DNA upon SSB-RecO interaction

Displacement of single-stranded DNA (ssDNA)-binding protein (SSB) from ssDNA is necessary for filament formation of RecA on ssDNA to initiate homologous recombination. The interaction between RecO and SSB is considered to be important for SSB displacement; however, the interaction has not been characterized at the atomic level. In this study, to clarify the mechanism underlying SSB displacement from ssDNA upon RecO binding, we examined the interaction between Thermus thermophilus RecO and cognate SSB by NMR analysis. We found that SSB interacts with the C-terminal positively charged region of RecO. Based on this result, we constructed some RecO mutants. The R127A mutant had considerably decreased binding affinity for SSB and could not anneal SSB-coated ssDNAs. Further, the mutant in the RecOR complex prevented the recovery of ssDNA-dependent ATPase activity of RecA from inhibition by SSB. These results indicated that the region surrounding Arg-127 is the binding site of SSB. We also performed NMR analysis using the C-terminal peptide of SSB and found that the acidic region of SSB is involved in the interaction with RecO, as seen in other protein-SSB interactions. Taken together with the findings of previous studies, we propose a model for SSB displacement from ssDNA where the acidic C-terminal region of SSB weakens the ssDNA binding affinity of SSB when the dynamics of the C-terminal region are suppressed by interactions with other proteins, including RecO.

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.

A major role of the RecFOR pathway in DNA double-strand-break repair through ESDSA in Deinococcus radiodurans

In Deinococcus radiodurans, the extreme resistance to DNA-shattering treatments such as ionizing radiation or desiccation is correlated with its ability to reconstruct a functional genome from hundreds of chromosomal fragments. The rapid reconstitution of an intact genome is thought to occur through an extended synthesis-dependent strand annealing process (ESDSA) followed by DNA recombination. Here, we investigated the role of key components of the RecF pathway in ESDSA in this organism naturally devoid of RecB and RecC proteins. We demonstrate that inactivation of RecJ exonuclease results in cell lethality, indicating that this protein plays a key role in genome maintenance. Cells devoid of RecF, RecO, or RecR proteins also display greatly impaired growth and an important lethal sectoring as bacteria devoid of RecA protein. Other aspects of the phenotype of recFOR knock-out mutants paralleled that of a DeltarecA mutant: DeltarecFOR mutants are extremely radiosensitive and show a slow assembly of radiation-induced chromosomal fragments, not accompanied by DNA synthesis, and reduced DNA degradation. Cells devoid of RecQ, the major helicase implicated in repair through the RecF pathway in E. coli, are resistant to gamma-irradiation and have a wild-type DNA repair capacity as also shown for cells devoid of the RecD helicase; in contrast, DeltauvrD mutants show a markedly decreased radioresistance, an increased latent period in the kinetics of DNA double-strand-break repair, and a slow rate of fragment assembly correlated with a slow rate of DNA synthesis. Combining RecQ or RecD deficiency with UvrD deficiency did not significantly accentuate the phenotype of DeltauvrD mutants. In conclusion, RecFOR proteins are essential for DNA double-strand-break repair through ESDSA whereas RecJ protein is essential for cell viability and UvrD helicase might be involved in the processing of double stranded DNA ends and/or in the DNA synthesis step of ESDSA.

Fungal BRCA2 ortholog Brh2 brings 5' end strand invasion back on stage

In this issue of Molecular Cell, Mazloum and Holloman (2009b) propose that 5' end strand invasion promoted by collaboration between Rad51 and Brh2 could be used for bypassing lesions during DNA replication, potentially advancing understanding of BRCA2 tumor suppressor function.

SSB protein diffusion on single-stranded DNA stimulates RecA filament formation

Single stranded (ss)DNA generated in the cell during DNA metabolism is stabilized and protected by binding of single stranded DNA binding (SSB) proteins. E. coli SSB, a representative homotetrameric SSB, binds to ssDNA by wrapping the DNA using its four subunits. However, such a tightly wrapped, high affinity protein-DNA complex still needs to be removed or repositioned quickly for unhindered action of other proteins. Here, we show, using single molecule two and three-color FRET, that tetrameric SSB can spontaneously migrate along ssDNA. Diffusional migration of SSB helps in the local displacement of SSB by an elongating RecA filament. SSB diffusion also melts short DNA hairpins transiently and stimulates RecA filament elongation on DNA with secondary structure. This first observation of diffusional movement of a protein on ssDNA introduces a new paradigm for how an SSB protein can be redistributed, while remaining tightly bound to ssDNA during recombination and repair processes.

The clamp loader assembles the beta clamp onto either a 3' or 5' primer terminus: the underlying basis favoring 3' loading

Clamp loaders assemble sliding clamps onto 3' primed sites for DNA polymerases. Clamp loaders are thought to be specific for a 3' primed site, and unable to bind a 5' site. We demonstrate here that the Escherichia coli gamma complex clamp loader can load the beta clamp onto a 5' primed site, although with at least 20-fold reduced efficiency relative to loading at a 3' primed site. Preferential clamp loading at a 3' site does not appear to be due to DNA binding, as the clamp loader forms an avid complex with beta at a 5' site. Preferential loading at a 3' versus a 5' site occurs at the ATP hydrolysis step, needed to close the ring around DNA. We also address DNA structural features that are recognized for preferential loading at a 3' site. Although the single-stranded template strand extends in opposite directions from 3' and 5' primed sites, thus making it a favorite candidate for distinguishing between 3' and 5' sites, the single-strand polarity at a primed template junction does not determine 3' site selection for clamp loading. Instead, we find that clamp loader recognition of a 3' site lies in the duplex portion of the primed site, not the single-strand portion. We present evidence that the beta clamp facilitates its own loading specificity for a 3' primed site. Implications to eukaryotic clamp loader complexes are proposed.

DNA-binding by Haemophilus influenzae and Escherichia coli YbaB, members of a widely-distributed bacterial protein family

Background

Genes orthologous to the ybaB loci of Escherichia coli and Haemophilus influenzae are widely distributed among eubacteria. Several years ago, the three-dimensional structures of the YbaB orthologs of both E. coli and H. influenzae were determined, revealing a novel "tweezer"-like structure. However, a function for YbaB had remained elusive, with an early study of the H. influenzae ortholog failing to detect DNA-binding activity. Our group recently determined that the Borrelia burgdorferi YbaB ortholog, EbfC, is a DNA-binding protein. To reconcile those results, we assessed the abilities of both the H. influenzae and E. coli YbaB proteins to bind DNA to which B. burgdorferi EbfC can bind.

Results

Both the H. influenzae and the E. coli YbaB proteins bound to tested DNAs. DNA-binding was not well competed with poly-dI-dC, indicating some sequence preferences for those two proteins. Analyses of binding characteristics determined that both YbaB orthologs bind as homodimers. Different DNA sequence preferences were observed between H. influenzae YbaB, E. coli YbaB and B. burgdorferi EbfC, consistent with amino acid differences in the putative DNA-binding domains of these proteins.

Conclusion

Three distinct members of the YbaB/EbfC bacterial protein family have now been demonstrated to bind DNA. Members of this protein family are encoded by a broad range of bacteria, including many pathogenic species, and results of our studies suggest that all such proteins have DNA-binding activities. The functions of YbaB/EbfC family members in each bacterial species are as-yet unknown, but given the ubiquity of these DNA-binding proteins among Eubacteria, further investigations are warranted.

Contribution of RecFOR machinery of homologous recombination to cell survival after loss of a restriction-modification gene complex

Loss of a type II restriction-modification (RM) gene complex, such as EcoRI, from a bacterial cell leads to death of its descendent cells through attack by residual restriction enzymes on undermethylated target sites of newly synthesized chromosomes. Through such post-segregational host killing, these gene complexes impose their maintenance on their host cells. This finding led to the rediscovery of type II RM systems as selfish mobile elements. The host prokaryote cells were found to cope with such attacks through a variety of means. The RecBCD pathway of homologous recombination in Escherichia coli repairs the lethal lesions on the chromosome, whilst it destroys restricted non-self DNA. recBCD homologues, however, appear very limited in distribution among bacterial genomes, whereas homologues of the RecFOR proteins, responsible for another pathway, are widespread in eubacteria, just like the RM systems. In the present work, therefore, we examined the possible contribution of the RecFOR pathway to cell survival after loss of an RM gene complex. A recF mutation reduced survival in an otherwise rec-positive background and, more severely, in a recBC sbcBC background. We also found that its effect is prominent in the presence of specific non-null mutant forms of the RecBCD enzyme: the resistance to killing seen with recC1002, recC1004, recC2145 and recB2154 is severely reduced to the level of a null recBC allele when combined with a recF, recO or recR mutant allele. Such resistance was also dependent on RecJ and RecQ functions. UV resistance of these non-null recBCD mutants is also reduced by recF, recJ or recQ mutation. These results demonstrate that the RecFOR pathway of recombination can contribute greatly to resistance to RM-mediated host killing, depending on the genetic background.

Differential requirements of two recA mutants for constitutive SOS expression in Escherichia coli K-12

Background

Repairing DNA damage begins with its detection and is often followed by elicitation of a cellular response. In E. coli, RecA polymerizes on ssDNA produced after DNA damage and induces the SOS Response. The RecA-DNA filament is an allosteric effector of LexA auto-proteolysis. LexA is the repressor of the SOS Response. Not all RecA-DNA filaments, however, lead to an SOS Response. Certain recA mutants express the SOS Response (recA^C) in the absence of external DNA damage in log phase cells.

Methodology/Principal Findings

Genetic analysis of two recA^C mutants was used to determine the mechanism of constitutive SOS (SOS^C) expression in a population of log phase cells using fluorescence of single cells carrying an SOS reporter system (sulAp-gfp). SOS^C expression in recA4142 mutants was dependent on its initial level of transcription, recBCD, recFOR, recX, dinI, xthA and the type of medium in which the cells were grown. SOS^C expression in recA730 mutants was affected by none of the mutations or conditions tested above.

Conclusions/Significance

It is concluded that not all recA^C alleles cause SOS^C expression by the same mechanism. It is hypothesized that RecA4142 is loaded on to a double-strand end of DNA and that the RecA filament is stabilized by the presence of DinI and destabilized by RecX. RecFOR regulate the activity of RecX to destabilize the RecA filament. RecA730 causes SOS^C expression by binding to ssDNA in a mechanism yet to be determined.

RecR-mediated modulation of RecF dimer specificity for single- and double-stranded DNA

RecF pathway proteins play an important role in the restart of stalled replication and DNA repair in prokaryotes. Following DNA damage, RecF, RecR, and RecO initiate homologous recombination (HR) by loading of the RecA recombinase on single-stranded (ss) DNA, protected by ssDNA-binding protein. The specific role of RecF in this process is not well understood. Previous studies have proposed that RecF directs the RecOR complex to boundaries of damaged DNA regions by recognizing single-stranded/double-stranded (ss/ds) DNA junctions. RecF belongs to ABC-type ATPases, which function through an ATP-dependent dimerization. Here, we demonstrate that the RecF of Deinococcus radiodurans interacts with DNA as an ATP-dependent dimer, and that the DNA binding and ATPase activity of RecF depend on both the structure of DNA substrate, and the presence of RecR. We found that RecR interacts as a tetramer with the RecF dimer. RecR increases the RecF affinity to dsDNA without stimulating ATP hydrolysis but destabilizes RecF binding to ssDNA and dimerization, likely due to increasing the ATPase rate. The DNA-dependent binding of RecR to the RecF-DNA complex occurs through specific protein-protein interactions without significant contributions from RecR-DNA interactions. Finally, RecF neither alone nor in complex with RecR preferentially binds to the ss/dsDNA junction. Our data suggest that the specificity of the RecFOR complex toward the boundaries of DNA damaged regions may result from a network of protein-protein and DNA-protein interactions, rather than a simple recognition of the ss/dsDNA junction by RecF.

RecFOR and RecOR as distinct RecA loading pathways

The molecular role of the RecF protein in loading RecA protein onto single-stranded DNA (ssDNA)-binding protein-coated ssDNA has been obscured by the facility with which the RecO and RecR proteins alone perform this function. We now show that RecFOR and RecOR define distinct RecA loading functions that operate optimally in different contexts. RecFOR, but not RecOR, is most effective when RecF(R) is bound near an ssDNA/double-stranded (dsDNA) junction. However, RecF(R) has no enhanced binding affinity for such a junction. RecO and RecR proteins are both required under all conditions in which the RecFOR pathway operates. The RecOR pathway is uniquely distinguished by a required interaction between RecO protein and the ssDNA binding protein C terminus. The RecOR pathway is more efficient for RecA loading onto ssDNA when no proximal dsDNA is available. A merger of new and published results leads to a new model for RecFOR function.

Molecular Design and Functional Organization of the RecA Protein

The bacterial RecA protein participates in a remarkably diverse set of functions, all of which are involved in the maintenance of genomic integrity. RecA is a central component in both the catalysis of recombinational DNA repair and the regulation of the cellular SOS response. Despite the mechanistic differences of its functions, all require formation of an active RecA/ATP/DNA complex. RecA is a classic allosterically regulated enzyme, and ATP binding results in a dramatic increase in DNA binding affinity and a cooperative assembly of RecA subunits to form an ordered, helical nucleoprotein filament. The molecular events that underlie this ATP-induced structural transition are becoming increasingly clear. This review focuses on descriptions of our current understanding of the molecular design and allosteric regulation of RecA. We present a comprehensive list of all published recA mutants and use the results of various genetic and biochemical studies, together with available structural information, to develop ideas regarding the design of RecA functional domains and their catalytic organization.

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.

RecR forms a ring-like tetramer that encircles dsDNA by forming a complex with RecF

In the RecFOR pathway, the RecF and RecR proteins form a complex that binds to DNA and exerts multiple functions, including directing the loading of RecA onto single-stranded (ss) DNA regions near double-stranded (ds) DNA–ssDNA junctions and preventing it from forming a filament beyond the ssDNA region. However, neither the structure of the RecFR complex nor its DNA-binding mechanism was previously identified. Here, size-exclusion chromatography and small-angle X-ray scattering data indicate that Thermus thermophilus (tt) RecR binds to ttRecF to form a globular structure consisting of four ttRecR and two ttRecF monomers. In addition, a low resolution model shows a cavity in the central part of the complex, suggesting that ttRecR forms a ring-like tetramer inside the ttRecFR complex. Mutant ttRecR proteins lacking the N- or C-terminal interfaces that are required for tetramer formation are unable to form a complex with ttRecF. Furthermore, a ttRecFR complex containing the DNA-binding deficient ttRecR K23E/R27E double mutant, which contains mutations lying inside the ring, exhibits significantly reduced dsDNA binding. Thus, we propose that the ring-like ttRecR tetramer has a key role in tethering the ttRecFR complex onto dsDNA and that the ring structure may function as a clamp protein.

Structural basis for inhibition of homologous recombination by the RecX protein

The RecA/RAD51 nucleoprotein filament is central to the reaction of homologous recombination (HR). Filament activity must be tightly regulated in vivo as unrestrained HR can cause genomic instability. Our mechanistic understanding of HR is restricted by lack of structural information about the regulatory proteins that control filament activity. Here, we describe a structural and functional analysis of the HR inhibitor protein RecX and its mode of interaction with the RecA filament. RecX is a modular protein assembled of repeated three-helix motifs. The relative arrangement of the repeats generates an elongated and curved shape that is well suited for binding within the helical groove of the RecA filament. Structure-based mutagenesis confirms that conserved basic residues on the concave side of RecX are important for repression of RecA activity. Analysis of RecA filament dynamics in the presence of RecX shows that RecX actively promotes filament disassembly. Collectively, our data support a model in which RecX binding to the helical groove of the filament causes local dissociation of RecA protomers, leading to filament destabilisation and HR inhibition.

Bacillus subtilis RecO nucleates RecA onto SsbA-coated single-stranded DNA

Subsaturating amounts of Bacillus subtilis SsbA, independently of the order of addition, partially inhibit the single-stranded DNA-dependent dATPase activity of RecA. This negative effect is fully overcome when a substoichiometric amount of RecO is added. SsbA added prior to RecA does not stimulate the dATP-dependent DNA strand exchange activity; however, added after RecA it enhances the extent of strand exchange. The addition of RecO stimulates RecA-mediated joint molecule formation, although it limits the accumulation of final recombination products. Thus we suggest that RecO has a dual activity: RecO acts as a RecA mediator enabling RecA to utilize SsbA-coated single-stranded DNA as a polymerization substrate and controls RecA-mediated DNA strand exchange by limiting its extent. We herein discuss the possible mechanisms of RecO involvement in the regulation of double strand break repair and genetic transformation.

SSB antagonizes RecX-RecA interaction

The RecX protein of Escherichia coli inhibits the extension of RecA protein filaments on DNA, presumably by binding to and blocking the growing filament end. The direct binding of RecX protein to single-stranded DNA is weak, and previous reports suggested that direct binding to DNA did not explain the effects of RecX. We now demonstrate that elevated concentrations of SSB greatly moderate the effects of RecX protein. High concentrations of the yeast RPA protein have the same effect, suggesting that the effect is not species-specific or even specific to bacterial SSB proteins. A direct SSB-RecX interaction is thus unlikely. We suggest that SSB is blocking access to single-stranded DNA. The evident competition between RecX and SSB implies that the mechanism of RecX action may involve RecX binding to both RecA protein and to DNA. We speculate that the interaction of RecX protein and RecA may enable an enhanced DNA binding by RecX protein. The effects of SSB are increased if the SSB C terminus is removed.

The SOS Regulatory Network

All organisms possess a diverse set of genetic programs that are used to alter cellular physiology in response to environmental cues. The gram-negative bacterium, Escherichia coli, mounts what is known as the "SOS response" following DNA damage, replication fork arrest, and a myriad of other environmental stresses. For over 50 years, E. coli has served as the paradigm for our understanding of the transcriptional, and physiological changes that occur following DNA damage (400). In this chapter, we summarize the current view of the SOS response and discuss how this genetic circuit is regulated. In addition to examining the E. coli SOS response, we also include a discussion of the SOS regulatory networks in other bacteria to provide a broader perspective on how prokaryotes respond to DNA damage.

D-loop formation by Brh2 protein of Ustilago maydis

Brh2, the ortholog of the BRCA2 tumor suppressor in Ustilago maydis, works hand in hand with Rad51 to promote repair of DNA by homologous recombination. Previous studies established that Brh2 can stimulate DNA strand exchange by enabling Rad51 nucleoprotein filament formation on replication protein A-coated ssDNA. But, more recently, it was noted that Brh2 has an inherent DNA annealing activity, raising the notion that it might have roles in recombination in addition to or beyond the mediator function. Here, we found that Brh2 can autonomously promote the formation of D-loops in reactions with plasmid DNA and homologous single-stranded oligonucleotides. The reaction differs from that catalyzed by Rad51 in having no requirement for cofactors or preloading phase on ssDNA. D-loop formation was most effective when Brh2 was mixed with plasmid DNA before addition of single-stranded oligomer. D-loop formation catalyzed by Rad51 was also enhanced when Brh2 was premixed with plasmid DNA. Brh2 rendered defective in Rad51 interaction by mutation in the BRC element was still capable of promoting D-loop formation. However, the mutant protein was unable to enhance the Rad51-catalyzed reaction. The results suggest a model in which Brh2 binding to plasmid DNA attracts and helps capture Rad51-coated ssDNA.