Escherichia coli RecO protein anneals ssDNA complexed with its cognate ssDNA-binding protein: A common step in genetic recombination

DNA Self-Assembly Optimization by Betaine and Its Analogs

The self-assembly yield of DNA nanostructures can be exponentially lower with increasing structural complexity. Few optimizing strategies are available in the DNA nanotechnology field for the assembly yield improvement. Here, betaine and its analogs are applied as supplementary ingredients in DNA self-assembly. Such a simple implementation results in effective yield improvement. Through a comprehensive investigation, a reliable yield improvement of two- to threefold is achieved for a number of DNA nanostructures with considerable complexity.

Prokaryotic DNA Crossroads: Holliday Junction Formation and Resolution

Cells are continually exposed to a multitude of internal and external stressors, which give rise to various types of DNA damage. To protect the integrity of their genetic material, cells are equipped with a repertoire of repair proteins that engage in various repair mechanisms, facilitated by intricate networks of protein-protein and protein-DNA interactions. Among these networks is the homologous recombination (HR) system, a molecular repair mechanism conserved in all three domains of life. On one hand, HR ensures high-fidelity, template-dependent DNA repair, while on the other hand, it results in the generation of combinatorial genetic variations through allelic exchange. Despite substantial progress in understanding this pathway in bacteria, yeast, and humans, several critical questions remain unanswered, including the molecular processes leading to the exchange of DNA segments, the coordination of protein binding, conformational switching during branch migration, and the resolution of Holliday Junctions (HJs). This Review delves into our current understanding of the HR pathway in bacteria, shedding light on the roles played by various proteins or their complexes at different stages of HR. In the first part of this Review, we provide a brief overview of the end resection processes and the strand-exchange reaction, offering a concise depiction of the mechanisms that culminate in the formation of HJs. In the latter half, we expound upon the alternative methods of branch migration and HJ resolution more comprehensively and holistically, considering the historical research timelines. Finally, when we consolidate our knowledge about HR within the broader context of genome replication and the emergence of resistant species, it becomes evident that the HR pathway is indispensable for the survival of bacteria in diverse ecological niches.

Molecular insights into the prototypical single-stranded DNA-binding protein from E. coli

The SSB protein of Escherichia coli functions to bind single-stranded DNA wherever it occurs during DNA metabolism. Depending upon conditions, SSB occurs in several different binding modes. In the course of its function, SSB diffuses on ssDNA and transfers rapidly between different segments of ssDNA. SSB interacts with many other proteins involved in DNA metabolism, with 22 such SSB-interacting proteins, or SIPs, defined to date. These interactions chiefly involve the disordered and conserved C-terminal residues of SSB. When not bound to ssDNA, SSB can aggregate to form a phase-separated biomolecular condensate. Current understanding of the properties of SSB and the functional significance of its many intermolecular interactions are summarized in this review.

Allosteric effects of E. coli SSB and RecR proteins on RecO protein binding to DNA

Escherichia coli single stranded (ss) DNA binding protein (SSB) plays essential roles in DNA maintenance. It binds ssDNA with high affinity through its N-terminal DNA binding core and recruits at least 17 different SSB interacting proteins (SIPs) that are involved in DNA replication, recombination, and repair via its nine amino acid acidic tip (SSB-Ct). E. coli RecO, a SIP, is an essential recombination mediator protein in the RecF pathway of DNA repair that binds ssDNA and forms a complex with E. coli RecR protein. Here, we report ssDNA binding studies of RecO and the effects of a 15 amino acid peptide containing the SSB-Ct monitored by light scattering, confocal microscope imaging, and analytical ultracentrifugation (AUC). We find that one RecO monomer can bind the oligodeoxythymidylate, (dT)15, while two RecO monomers can bind (dT)35 in the presence of the SSB-Ct peptide. When RecO is in molar excess over ssDNA, large RecO-ssDNA aggregates occur that form with higher propensity on ssDNA of increasing length. Binding of RecO to the SSB-Ct peptide inhibits RecO-ssDNA aggregation. RecOR complexes can bind ssDNA via RecO, but aggregation is suppressed even in the absence of the SSB-Ct peptide, demonstrating an allosteric effect of RecR on RecO binding to ssDNA. Under conditions where RecO binds ssDNA but does not form aggregates, SSB-Ct binding enhances the affinity of RecO for ssDNA. For RecOR complexes bound to ssDNA, we also observe a shift in RecOR complex equilibrium towards a RecR4O complex upon binding SSB-Ct. These results suggest a mechanism by which SSB recruits RecOR to facilitate loading of RecA onto ssDNA gaps.

Crystal Structure of the Recombination Mediator Protein RecO from Campylobacter jejuni and Its Interaction with DNA and a Zinc Ion

Homologous recombination is involved in repairing DNA damage, contributing to maintaining the integrity and stability of viral and cellular genomes. In bacteria, the recombination mediator proteins RecO and RecR are required to load the RecA recombinase on ssDNA for homologous recombination. To structurally and functionally characterize RecO, we determined the crystal structure of RecO from Campylobacter jejuni (cjRecO) at a 1.8 Å resolution and biochemically assessed its capacity to interact with DNA and a metal ion. cjRecO folds into a curved rod-like structure that consists of an N-terminal domain (NTD), C-terminal domain (CTD), and Zn²⁺-binding domain (ZnD). The ZnD at the end of the rod-like structure coordinates three cysteine residues and one histidine residue to accommodate a Zn²⁺ ion. Based on an extensive comparative analysis of RecO structures and sequences, we propose that the Zn²⁺-binding consensus sequence of RecO is CxxC…C/HxxC/H/D. The interaction with Zn²⁺ is indispensable for the protein stability of cjRecO but does not seem to be required for the recombination mediator function. cjRecO also interacts with ssDNA as part of its biological function, potentially using the positively charged patch in the NTD and CTD. However, cjRecO displays a low ssDNA-binding affinity, suggesting that cjRecO requires RecR to efficiently recognize ssDNA for homologous recombination.

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.

Allosteric effects of SSB C-terminal tail on assembly of E. coli RecOR proteins

Escherichia coli RecO is a recombination mediator protein that functions in the RecF pathway of homologous recombination, in concert with RecR, and interacts with E. coli single stranded (ss) DNA binding (SSB) protein via the last 9 amino acids of the C-terminal tails (SSB-Ct). Structures of the E. coli RecR and RecOR complexes are unavailable; however, crystal structures from other organisms show differences in RecR oligomeric state and RecO stoichiometry. We report analytical ultracentrifugation studies of E. coli RecR assembly and its interaction with RecO for a range of solution conditions using both sedimentation velocity and equilibrium approaches. We find that RecR exists in a pH-dependent dimer-tetramer equilibrium that explains the different assembly states reported in previous studies. RecO binds with positive cooperativity to a RecR tetramer, forming both RecR₄O and RecR₄O₂ complexes. We find no evidence of a stable RecO complex with RecR dimers. However, binding of RecO to SSB-Ct peptides elicits an allosteric effect, eliminating the positive cooperativity and shifting the equilibrium to favor a RecR₄O complex. These studies suggest a mechanism for how SSB binding to RecO influences the distribution of RecOR complexes to facilitate loading of RecA onto SSB coated ssDNA to initiate homologous recombination.

Single stranded DNA annealing is a conserved activity of telomere resolvases

Bacterial species of the genera Agrobacterium and Borrelia possess chromosomes terminated by hairpin telomeres. Replication produces dimeric replication intermediates fused via replicated telomere junctions. A specialized class of enzymes, referred to as telomere resolvases, promotes the resolution of the replicated intermediate into linear monomers terminated by hairpin telomeres. Telomere resolution is catalyzed via DNA cleavage and rejoining events mechanistically similar to those promoted by topoisomerase-IB and tyrosine recombinase enzymes. Examination of the borrelial telomere resolvase, ResT, revealed unanticipated multifunctionality; aside from its expected telomere resolution activity ResT possessed a singled-stranded DNA (ssDNA) annealing activity that extended to both naked ssDNA and ssDNA complexed with its cognate single-stranded DNA binding protein (SSB). At present, the role this DNA annealing activity plays in vivo remains unknown. We have demonstrated here that single-stranded DNA annealing is also a conserved property of the agrobacterial telomere resolvase, TelA. This activity in TelA similarly extends to both naked ssDNA and ssDNA bound by its cognate SSB. TelA's annealing activity was shown to stem from the N-terminal domain; removal of this domain abolished annealing without affecting telomere resolution. Further, independent expression of the N-terminal domain of TelA produced a functional annealing protein. We suggest that the apparent conservation of annealing activity in two telomere resolvases, from distantly related bacterial species, implies a role for this activity in hairpin telomere metabolism. Our demonstration of the separation of the telomere resolution and annealing activities of TelA provides a platform for future experiments aimed at identifying the role DNA annealing performs in vivo.

Are the intrinsically disordered linkers involved in SSB binding to accessory proteins?

Escherichia coli single strand (ss) DNA binding (SSB) protein protects ssDNA intermediates and recruits at least 17 SSB interacting proteins (SIPs) during genome maintenance. The SSB C-termini contain a 9 residue acidic tip and a 56 residue intrinsically disordered linker (IDL). The acidic tip interacts with SIPs; however a recent proposal suggests that the IDL may also interact with SIPs. Here we examine the binding to four SIPs (RecO, PriC, PriA and χ subunit of DNA polymerase III) of three peptides containing the acidic tip and varying amounts of the IDL. Independent of IDL length, we find no differences in peptide binding to each individual SIP indicating that binding is due solely to the acidic tip. However, the tip shows specificity, with affinity decreasing in the order: RecO > PriA ∼ χ > PriC. Yet, RecO binding to the SSB tetramer and an SSB–ssDNA complex show significant thermodynamic differences compared to the peptides alone, suggesting that RecO interacts with another region of SSB, although not the IDL. SSB containing varying IDL deletions show different binding behavior, with the larger linker deletions inhibiting RecO binding, likely due to increased competition between the acidic tip interacting with DNA binding sites within SSB.

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.

Novel RNA and DNA strand exchange activity of the PALB2 DNA binding domain and its critical role for DNA repair in cells

BReast Cancer Associated proteins 1 and 2 (BRCA1, -2) and Partner and Localizer of BRCA2 (PALB2) protein are tumour suppressors linked to a spectrum of malignancies, including breast cancer and Fanconi anemia. PALB2 coordinates functions of BRCA1 and BRCA2 during homology-directed repair (HDR) and interacts with several chromatin proteins. In addition to protein scaffold function, PALB2 binds DNA. The functional role of this interaction is poorly understood. We identified a major DNA-binding site of PALB2, mutations in which reduce RAD51 foci formation and the overall HDR efficiency in cells by 50%. PALB2 N-terminal DNA-binding domain (N-DBD) stimulates the function of RAD51 recombinase. Surprisingly, it possesses the strand exchange activity without RAD51. Moreover, N-DBD stimulates the inverse strand exchange and can use DNA and RNA substrates. Our data reveal a versatile DNA interaction property of PALB2 and demonstrate a critical role of PALB2 DNA binding for chromosome repair in cells.

Desiccation and radiation stress tolerance in cyanobacteria

Cyanobacteria are among the oldest living organisms on this planet, existing since more than 3 billion years. They are ideal organisms for investigating biological processes such as photosynthesis, respiration, circadian rhythm, photoregulation of gene expression, developmental gene rearrangements, and specialized cell differentiation. They are nearly ubiquitous in distribution, have colonized a wide range of ecosystems including soil, air, dry rock, and aquatic systems, and even occupy extreme niches that are inaccessible to other organisms. Such wide ecological distribution reflects their capacity to acclimate to extreme environments. They show great adaptive abilities and have survived various adverse physiological growth conditions like desiccation, high temperatures, extreme pH, cold, osmosis, salt, light, nitrogen, and high salinity. Their ancient origin and surviving through numerous stresses during evolution indicates their remarkable capabilities to survive and prevail under different environmental and man-made stresses. It has been hypothesized that similar and overlap stress response mechanisms help them to survive different stresses. It has been stated that responses against stresses like radiation has been accidental-exhibited because of similar response against desiccation stress, which has prevailed more during evolution. These overlaps and similarities in stress responses have been instrumental in making these organisms a large class of biological entities today. Present review discuss about stress tolerance in cyanobacteria against two extreme stresses - desiccation and gamma radiation. It also discuss the commonality and underlying molecular mechanisms in these two stress responses.

Template-switching during replication fork repair in bacteria

Replication forks frequently are challenged by lesions on the DNA template, replication-impeding DNA secondary structures, tightly bound proteins or nucleotide pool imbalance. Studies in bacteria have suggested that under these circumstances the fork may leave behind single-strand DNA gaps that are subsequently filled by homologous recombination, translesion DNA synthesis or template-switching repair synthesis. This review focuses on the template-switching pathways and how the mechanisms of these processes have been deduced from biochemical and genetic studies. I discuss how template-switching can contribute significantly to genetic instability, including mutational hotspots and frequent genetic rearrangements, and how template-switching may be elicited by replication fork damage.

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.

In vivo and in vitro characterization of DdrC, a DNA damage response protein in Deinococcus radiodurans bacterium

The bacterium Deinococcus radiodurans possesses a set of Deinococcus-specific genes highly induced after DNA damage. Among them, ddrC (dr0003) was recently re-annotated, found to be in the inverse orientation and called A2G07_00380. Here, we report the first in vivo and in vitro characterization of the corrected DdrC protein to better understand its function in irradiated cells. In vivo, the ΔddrC null mutant is sensitive to high doses of UV radiation and the ddrC deletion significantly increases UV-sensitivity of ΔuvrA or ΔuvsE mutant strains. We show that the expression of the DdrC protein is induced after γ-irradiation and is under the control of the regulators, DdrO and IrrE. DdrC is rapidly recruited into the nucleoid of the irradiated cells. In vitro, we show that DdrC is able to bind single- and double-stranded DNA with a preference for the single-stranded DNA but without sequence or shape specificity and protects DNA from various nuclease attacks. DdrC also condenses DNA and promotes circularization of linear DNA. Finally, we show that the purified protein exhibits a DNA strand annealing activity. Altogether, our results suggest that DdrC is a new DNA binding protein with pleiotropic activities. It might maintain the damaged DNA fragments end to end, thus limiting their dispersion and extensive degradation after exposure to ionizing radiation. DdrC might also be an accessory protein that participates in a single strand annealing pathway whose importance in DNA repair becomes apparent when DNA is heavily damaged.

SSB and the RecG DNA helicase: an intimate association to rescue a stalled replication fork

In E. coli, the regression of stalled DNA replication forks is catalyzed by the DNA helicase RecG. One means of gaining access to the fork is by binding to the single strand binding protein or SSB. This interaction occurs via the wedge domain of RecG and the intrinsically disordered linker (IDL) of SSB, in a manner similar to that of SH3 domains binding to PXXP motif-containing ligands in eukaryotic cells. During loading, SSB remodels the wedge domain so that the helicase domains bind to the parental, duplex DNA, permitting the helicase to translocate using thermal energy. This translocation may be used to clear the fork of obstacles, prior to the initiation of fork regression.

Noncanonical views of homology-directed DNA repair

DNA repair is essential to maintain genomic integrity and initiate genetic diversity. While gene conversion and classical nonhomologous end-joining are the most physiologically predominant forms of DNA repair mechanisms, emerging lines of evidence suggest the usage of several noncanonical homology-directed repair (HDR) pathways in both prokaryotes and eukaryotes in different contexts. Here we review how these alternative HDR pathways are executed, specifically focusing on the determinants that dictate competition between them and their relevance to cancers that display complex genomic rearrangements or maintain their telomeres by homology-directed DNA synthesis.

The IDL of E. coli SSB links ssDNA and protein binding by mediating protein-protein interactions

The E. coli single strand DNA binding protein (SSB) is essential to viability where it functions in two seemingly disparate roles: it binds to single stranded DNA (ssDNA) and to target proteins that comprise the SSB interactome. The link between these roles resides in a previously under-appreciated region of the protein known as the intrinsically disordered linker (IDL). We present a model wherein the IDL is responsible for mediating protein-protein interactions critical to each role. When interactions occur between SSB tetramers, cooperative binding to ssDNA results. When binding occurs between SSB and an interactome partner, storage or loading of that protein onto the DNA takes place. The properties of the IDL that facilitate these interactions include the presence of repeats, a putative polyproline type II helix and, PXXP motifs that may facilitate direct binding to the OB-fold in a manner similar to that observed for SH3 domain binding of PXXP ligands in eukaryotic systems.

The tale of SSB

The E. coli single stranded DNA binding protein (SSB) is essential to all aspects of DNA metabolism. Here, it has two seemingly disparate but equally important roles: it binds rapidly and cooperatively to single stranded DNA (ssDNA) and it binds to partner proteins that constitute the SSB interactome. These two roles are not disparate but are instead, intimately linked. A model is presented wherein the intrinsically disordered linker (IDL) is directly responsible for mediating protein-protein interactions. It does this by binding, via PXXP motifs, to the OB-fold (aka SH3 domain) of a nearby protein. When the nearby protein is another SSB tetramer, this leads to a highly efficient ssDNA binding reaction that rapidly and cooperatively covers and protects the exposed nucleic acid from degradation. Alternatively, when the nearby protein is a member of the SSB interactome, loading of the enzyme onto the DNA takes places.

Holliday junction trap shows how cells use recombination and a junction-guardian role of RecQ helicase

DNA repair by homologous recombination (HR) underpins cell survival and fuels genome instability, cancer, and evolution. However, the main kinds and sources of DNA damage repaired by HR in somatic cells and the roles of important HR proteins remain elusive. We present engineered proteins that trap, map, and quantify Holliday junctions (HJs), a central DNA intermediate in HR, based on catalytically deficient mutant RuvC protein of Escherichia coli. We use RuvCDefGFP (RDG) to map genomic footprints of HR at defined DNA breaks in E. coli and demonstrate genome-scale directionality of double-strand break (DSB) repair along the chromosome. Unexpectedly, most spontaneous HR-HJ foci are instigated, not by DSBs, but rather by single-stranded DNA damage generated by replication. We show that RecQ, the E. coli ortholog of five human cancer proteins, nonredundantly promotes HR-HJ formation in single cells and, in a novel junction-guardian role, also prevents apparent non-HR-HJs promoted by RecA overproduction. We propose that one or more human RecQ orthologs may act similarly in human cancers overexpressing the RecA ortholog RAD51 and find that cancer genome expression data implicate the orthologs BLM and RECQL4 in conjunction with EME1 and GEN1 as probable HJ reducers in such cancers. Our results support RecA-overproducing E. coli as a model of the many human tumors with up-regulated RAD51 and provide the first glimpses of important, previously elusive reaction intermediates in DNA replication and repair in single living cells.

The E. coli DNA Replication Fork

DNA replication in Escherichia coli initiates at oriC, the origin of replication and proceeds bidirectionally, resulting in two replication forks that travel in opposite directions from the origin. Here, we focus on events at the replication fork. The replication machinery (or replisome), first assembled on both forks at oriC, contains the DnaB helicase for strand separation, and the DNA polymerase III holoenzyme (Pol III HE) for DNA synthesis. DnaB interacts transiently with the DnaG primase for RNA priming on both strands. The Pol III HE is made up of three subassemblies: (i) the αɛθ core polymerase complex that is present in two (or three) copies to simultaneously copy both DNA strands, (ii) the β2 sliding clamp that interacts with the core polymerase to ensure its processivity, and (iii) the seven-subunit clamp loader complex that loads β2 onto primer-template junctions and interacts with the α polymerase subunit of the core and the DnaB helicase to organize the two (or three) core polymerases. Here, we review the structures of the enzymatic components of replisomes, and the protein-protein and protein-DNA interactions that ensure they remain intact while undergoing substantial dynamic changes as they function to copy both the leading and lagging strands simultaneously during coordinated replication.

The Borrelia burgdorferi telomere resolvase, ResT, anneals ssDNA complexed with its cognate ssDNA-binding protein

Spirochetes of the genus Borrelia possess unusual genomes that consist in a linear chromosome and multiple linear and circular plasmids. The linear replicons are terminated by covalently closed hairpin ends, referred to as hairpin telomeres. The hairpin telomeres represent a simple solution to the end-replication problem. Deoxyribonucleic acid replication initiates internally and proceeds bidirectionally toward the hairpin telomeres. The telomere resolvase, ResT, forms the hairpin telomeres from replicated telomere intermediates in a reaction with similarities to those promoted by type IB topoisomerases and tyrosine recombinases. ResT has also been shown to possess DNA single-strand annealing activity. We report here that ResT promotes single-strand annealing of both free DNA strands and ssDNA complexed with single-stranded DNA binding protein (SSB). The annealing of complementary strands bound by SSB requires a ResT-SSB interaction that is mediated by the conserved amphipathic C-terminal tail of SSB. These properties of ResT are similar to those demonstrated for the recombination mediator protein, RecO, of the RecF pathway. Borrelia burgdorferi is unusual in lacking identifiable homologs of the RecFOR proteins. We propose that ResT may provide missing RecFOR functions.

Structural basis for DNA 5´-end resection by RecJ

The resection of DNA strand with a 5´ end at double-strand breaks is an essential step in recombinational DNA repair. RecJ, a member of DHH family proteins, is the only 5´ nuclease involved in the RecF recombination pathway. Here, we report the crystal structures of Deinococcus radiodurans RecJ in complex with deoxythymidine monophosphate (dTMP), ssDNA, the C-terminal region of single-stranded DNA-binding protein (SSB-Ct) and a mechanistic insight into the RecF pathway. A terminal 5´-phosphate-binding pocket above the active site determines the 5´-3´ polarity of the deoxy-exonuclease of RecJ; a helical gateway at the entrance to the active site admits ssDNA only; and the continuous stacking interactions between protein and nine nucleotides ensure the processive end resection. The active site of RecJ in the N-terminal domain contains two divalent cations that coordinate the nucleophilic water. The ssDNA makes a 180° turn at the scissile phosphate. The C-terminal domain of RecJ binds the SSB-Ct, which explains how RecJ and SSB work together to efficiently process broken DNA ends for homologous recombination.

An Overview of the Molecular Mechanisms of Recombinational DNA Repair

Recombinational DNA repair is a universal aspect of DNA metabolism and is essential for genomic integrity. It is a template-directed process that uses a second chromosomal copy (sister, daughter, or homolog) to ensure proper repair of broken chromosomes. The key steps of recombination are conserved from phage through human, and an overview of those steps is provided in this review. The first step is resection by helicases and nucleases to produce single-stranded DNA (ssDNA) that defines the homologous locus. The ssDNA is a scaffold for assembly of the RecA/RAD51 filament, which promotes the homology search. On finding homology, the nucleoprotein filament catalyzes exchange of DNA strands to form a joint molecule. Recombination is controlled by regulating the fate of both RecA/RAD51 filaments and DNA pairing intermediates. Finally, intermediates that mature into Holliday structures are disjoined by either nucleolytic resolution or topological dissolution.

Imaging and energetics of single SSB-ssDNA molecules reveal intramolecular condensation and insight into RecOR function

Escherichia coli single-stranded DNA (ssDNA) binding protein (SSB) is the defining bacterial member of ssDNA binding proteins essential for DNA maintenance. SSB binds ssDNA with a variable footprint of ∼30-70 nucleotides, reflecting partial or full wrapping of ssDNA around a tetramer of SSB. We directly imaged single molecules of SSB-coated ssDNA using total internal reflection fluorescence (TIRF) microscopy and observed intramolecular condensation of nucleoprotein complexes exceeding expectations based on simple wrapping transitions. We further examined this unexpected property by single-molecule force spectroscopy using magnetic tweezers. In conditions favoring complete wrapping, SSB engages in long-range reversible intramolecular interactions resulting in condensation of the SSB-ssDNA complex. RecO and RecOR, which interact with SSB, further condensed the complex. Our data support the idea that RecOR--and possibly other SSB-interacting proteins-function(s) in part to alter long-range, macroscopic interactions between or throughout nucleoprotein complexes by microscopically altering wrapping and bridging distant sites.

RecF and RecR Play Critical Roles in the Homologous Recombination and Single-Strand Annealing Pathways of Mycobacteria

Mycobacteria encode three DNA double-strand break repair pathways: (i) RecA-dependent homologous recombination (HR), (ii) Ku-dependent nonhomologous end joining (NHEJ), and (iii) RecBCD-dependent single-strand annealing (SSA). Mycobacterial HR has two presynaptic pathway options that rely on the helicase-nuclease AdnAB and the strand annealing protein RecO, respectively. Ablation of adnAB or recO individually causes partial impairment of HR, but loss of adnAB and recO in combination abolishes HR. RecO, which can accelerate annealing of single-stranded DNA in vitro, also participates in the SSA pathway. The functions of RecF and RecR, which, in other model bacteria, function in concert with RecO as mediators of RecA loading, have not been examined in mycobacteria. Here, we present a genetic analysis of recF and recR in mycobacterial recombination. We find that RecF, like RecO, participates in the AdnAB-independent arm of the HR pathway and in SSA. In contrast, RecR is required for all HR in mycobacteria and for SSA. The essentiality of RecR as an agent of HR is yet another distinctive feature of mycobacterial DNA repair.IMPORTANCE This study clarifies the molecular requirements for homologous recombination in mycobacteria. Specifically, we demonstrate that RecF and RecR play important roles in both the RecA-dependent homologous recombination and RecA-independent single-strand annealing pathways. Coupled with our previous findings (R. Gupta, M. Ryzhikov, O. Koroleva, M. Unciuleac, S. Shuman, S. Korolev, and M. S. Glickman, Nucleic Acids Res 41:2284-2295, 2013, http://dx.doi.org/10.1093/nar/gks1298), these results revise our view of mycobacterial recombination and place the RecFOR system in a central position in homology-dependent DNA repair.

DNA-pairing and annealing processes in homologous recombination and homology-directed repair

The formation of heteroduplex DNA is a central step in the exchange of DNA sequences via homologous recombination, and in the accurate repair of broken chromosomes via homology-directed repair pathways. In cells, heteroduplex DNA largely arises through the activities of recombination proteins that promote DNA-pairing and annealing reactions. Classes of proteins involved in pairing and annealing include RecA-family DNA-pairing proteins, single-stranded DNA (ssDNA)-binding proteins, recombination mediator proteins, annealing proteins, and nucleases. This review explores the properties of these pairing and annealing proteins, and highlights their roles in complex recombination processes including the double Holliday junction (DhJ) formation, synthesis-dependent strand annealing, and single-strand annealing pathways--DNA transactions that are critical both for genome stability in individual organisms and for the evolution of species.

RecQ helicase and RecJ nuclease provide complementary functions to resect DNA for homologous recombination

Recombinational DNA repair by the RecF pathway of Escherichia coli requires the coordinated activities of RecA, RecFOR, RecQ, RecJ, and single-strand DNA binding (SSB) proteins. These proteins facilitate formation of homologously paired joint molecules between linear double-stranded (dsDNA) and supercoiled DNA. Repair starts with resection of the broken dsDNA by RecQ, a 3'→5' helicase, RecJ, a 5'→3' exonuclease, and SSB protein. The ends of a dsDNA break can be blunt-ended, or they may possess either 5'- or 3'-single-stranded DNA (ssDNA) overhangs of undefined length. Here we show that RecJ nuclease alone can initiate nucleolytic resection of DNA with 5'-ssDNA overhangs, and that RecQ helicase can initiate resection of DNA with blunt-ends or 3'-ssDNA overhangs by DNA unwinding. We establish that in addition to its well-known ssDNA exonuclease activity, RecJ can display dsDNA exonuclease activity, degrading 100-200 nucleotides of the strand terminating with a 5'-ssDNA overhang. The dsDNA product, with a 3'-ssDNA overhang, is an optimal substrate for RecQ, which unwinds this intermediate to reveal the complementary DNA strand with a 5'-end that is degraded iteratively by RecJ. On the other hand, RecJ cannot resect duplex DNA that is either blunt-ended or terminated with 3'-ssDNA; however, such DNA is unwound by RecQ to create ssDNA for RecJ exonuclease. RecJ requires interaction with SSB for exonucleolytic degradation of ssDNA but not dsDNA. Thus, complementary action by RecJ and RecQ permits initiation of recombinational repair from all dsDNA ends: 5'-overhangs, blunt, or 3'-overhangs. Such helicase-nuclease coordination is a common mechanism underlying resection in all organisms.

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.

Phage ORF family recombinases: conservation of activities and involvement of the central channel in DNA binding

Genetic and biochemical evidence suggests that λ Orf is a recombination mediator, promoting nucleation of either bacterial RecA or phage Redβ recombinases onto single-stranded DNA (ssDNA) bound by SSB protein. We have identified a diverse family of Orf proteins that includes representatives implicated in DNA base flipping and those fused to an HNH endonuclease domain. To confirm a functional relationship with the Orf family, a distantly-related homolog, YbcN, from Escherichia coli cryptic prophage DLP12 was purified and characterized. As with its λ relative, YbcN showed a preference for binding ssDNA over duplex. Neither Orf nor YbcN displayed a significant preference for duplex DNA containing mismatches or 1-3 nucleotide bulges. YbcN also bound E. coli SSB, although unlike Orf, it failed to associate with an SSB mutant lacking the flexible C-terminal tail involved in coordinating heterologous protein-protein interactions. Residues conserved in the Orf family that flank the central cavity in the λ Orf crystal structure were targeted for mutagenesis to help determine the mode of DNA binding. Several of these mutant proteins showed significant defects in DNA binding consistent with the central aperture being important for substrate recognition. The widespread conservation of Orf-like proteins highlights the importance of targeting SSB coated ssDNA during lambdoid phage recombination.

The key residue for SSB-RecO interaction is dispensable for Deinococcus radiodurans DNA repair in vivo

The RecFOR DNA repair pathway is one of the major RecA-dependent recombinatorial repair pathways in bacteria and plays an important role in double-strand breaks repair. RecO, one of the major recombination mediator proteins in the RecFOR pathway, has been shown to assist RecA loading onto single-stranded binding protein (SSB) coated single-stranded DNA (ssDNA). However, it has not been characterized whether the protein-protein interaction between RecO and SSB contributes to that process in vivo. Here, we identified the residue arginine-121 of Deinococcus radiodurans RecO (drRecO-R121) as the key residue for RecO-SSB interaction. The substitution of drRecO-R121 with alanine greatly abolished the binding of RecO to SSB but not the binding to RecR. Meanwhile, SSB-coated ssDNA annealing activity was also compromised by the mutation of the residue of drRecO. However, the drRecO-R121A strain showed only modest sensitivity to DNA damaging agents. Taking these data together, arginine-121 of drRecO is the key residue for SSB-RecO interaction, which may not play a vital role in the SSB displacement and RecA loading process of RecFOR DNA repair pathway in vivo.

Early steps of double-strand break repair in Bacillus subtilis

All organisms rely on integrated networks to repair DNA double-strand breaks (DSBs) in order to preserve the integrity of the genetic information, to re-establish replication, and to ensure proper chromosomal segregation. Genetic, cytological, biochemical and structural approaches have been used to analyze how Bacillus subtilis senses DNA damage and responds to DSBs. RecN, which is among the first responders to DNA DSBs, promotes the ordered recruitment of repair proteins to the site of a lesion. Cells have evolved different mechanisms for efficient end processing to create a 3'-tailed duplex DNA, the substrate for RecA binding, in the repair of one- and two-ended DSBs. Strand continuity is re-established via homologous recombination (HR), utilizing an intact homologous DNA molecule as a template. In the absence of transient diploidy or of HR, however, two-ended DSBs can be directly re-ligated via error-prone non-homologous end-joining. Here we review recent findings that shed light on the early stages of DSB repair in Firmicutes.

A dual role for mycobacterial RecO in RecA-dependent homologous recombination and RecA-independent single-strand annealing

Mycobacteria have two genetically distinct pathways for the homology-directed repair of DNA double-strand breaks: homologous recombination (HR) and single-strand annealing (SSA). HR is abolished by deletion of RecA and reduced in the absence of the AdnAB helicase/nuclease. By contrast, SSA is RecA-independent and requires RecBCD. Here we examine the function of RecO in mycobacterial DNA recombination and repair. Loss of RecO elicits hypersensitivity to DNA damaging agents similar to that caused by deletion of RecA. We show that RecO participates in RecA-dependent HR in a pathway parallel to the AdnAB pathway. We also find that RecO plays a role in the RecA-independent SSA pathway. The mycobacterial RecO protein displays a zinc-dependent DNA binding activity in vitro and accelerates the annealing of SSB-coated single-stranded DNA. These findings establish a role for RecO in two pathways of mycobacterial DNA double-strand break repair and suggest an in vivo function for the DNA annealing activity of RecO proteins, thereby underscoring their similarity to eukaryal Rad52.

Architecture and conservation of the bacterial DNA replication machinery, an underexploited drug target

New antibiotics with novel modes of action are required to combat the growing threat posed by multi-drug resistant bacteria. Over the last decade, genome sequencing and other high-throughput techniques have provided tremendous insight into the molecular processes underlying cellular functions in a wide range of bacterial species. We can now use these data to assess the degree of conservation of certain aspects of bacterial physiology, to help choose the best cellular targets for development of new broad-spectrum antibacterials.

DNA replication is a conserved and essential process, and the large number of proteins that interact to replicate DNA in bacteria are distinct from those in eukaryotes and archaea; yet none of the antibiotics in current clinical use acts directly on the replication machinery. Bacterial DNA synthesis thus appears to be an underexploited drug target. However, before this system can be targeted for drug design, it is important to understand which parts are conserved and which are not, as this will have implications for the spectrum of activity of any new inhibitors against bacterial species, as well as the potential for development of drug resistance. In this review we assess similarities and differences in replication components and mechanisms across the bacteria, highlight current progress towards the discovery of novel replication inhibitors, and suggest those aspects of the replication machinery that have the greatest potential as drug targets.

A RAD52-like single-stranded DNA binding protein affects mitochondrial DNA repair by recombination

The plant mitochondrial DNA-binding protein ODB1 was identified from a mitochondrial extract after DNA-affinity purification. ODB1 (organellar DNA-binding protein 1) co-purified with WHY2, a mitochondrial member of the WHIRLY family of plant-specific proteins involved in the repair of organellar DNA. The Arabidopsis thaliana ODB1 gene is identical to RAD52-1, which encodes a protein functioning in homologous recombination in the nucleus but additionally localizing to mitochondria. We confirmed the mitochondrial localization of ODB1 by in vitro and in vivo import assays, as well as by immunodetection on Arabidopsis subcellular fractions. In mitochondria, WHY2 and ODB1 were found in large nucleo-protein complexes. Both proteins co-immunoprecipitated in a DNA-dependent manner. In vitro assays confirmed DNA binding by ODB1 and showed that the protein has higher affinity for single-stranded than for double-stranded DNA. ODB1 showed no sequence specificity in vitro. In vivo, DNA co-immunoprecipitation indicated that ODB1 binds sequences throughout the mitochondrial genome. ODB1 promoted annealing of complementary DNA sequences, suggesting a RAD52-like function as a recombination mediator. Arabidopsis odb1 mutants were morphologically indistinguishable from the wild-type, but following DNA damage by genotoxic stress, they showed reduced mitochondrial homologous recombination activity. Under the same conditions, the odb1 mutants showed an increase in illegitimate repair bypasses generated by microhomology-mediated recombination. These observations identify ODB1 as a further component of homologous recombination-dependent DNA repair in plant mitochondria.

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.

RecOR complex including RecR N-N dimer and RecO monomer displays a high affinity for ssDNA

RecR is an important recombination mediator protein in the RecFOR pathway. RecR together with RecO and RecF facilitates RecA nucleoprotein filament formation and homologous pairing. Structural and biochemical studies of Thermoanaerobacter tengcongensis RecR (TTERecR) and its series mutants revealed that TTERecR uses the N-N dimer as a basic functional unit to interact with TTERecO monomer. Two TTERecR N-N dimers form a ring-shaped tetramer via an interaction between their C-terminal regions. The tetramer is a result of crystallization only. Hydrophobic interactions between the entire helix-hairpin-helix domains within the N-terminal regions of two TTERecR monomers are necessary for formation of a RecR functional N-N dimer. The TTERecR N-N dimer conformation also affects formation of a hydrophobic patch, which creates a binding site for TTERecO in the TTERecR Toprim domain. In addition, we demonstrate that TTERecR does not bind single-stranded DNA (ssDNA) and binds double-stranded DNA very weakly, whereas TTERecOR complex can stably bind DNA, with a higher affinity for ssDNA than double-stranded DNA. Based on these results, we propose an interaction model for the RecOR:ssDNA complex.

RecFOR proteins target RecA protein to a DNA gap with either DNA or RNA at the 5' terminus: implication for repair of stalled replication forks

The repair of single-stranded gaps in duplex DNA by homologous recombination requires the proteins of the RecF pathway. The assembly of RecA protein onto gapped DNA (gDNA) that is complexed with the single-stranded DNA-binding protein is accelerated by the RecF, RecO, and RecR (RecFOR) proteins. Here, we show the RecFOR proteins specifically target RecA protein to gDNA even in the presence of a thousand-fold excess of single-stranded DNA (ssDNA). The binding constant of RecF protein, in the presence of the RecOR proteins, to the junction of ssDNA and dsDNA within a gap is 1-2 nm, suggesting that a few RecF molecules in the cell are sufficient to recognize gDNA. We also found that the nucleation of a RecA filament on gDNA in the presence of the RecFOR proteins occurs at a faster rate than filament elongation, resulting in a RecA nucleoprotein filament on ssDNA for 1000-2000 nucleotides downstream (5' → 3') of the junction with duplex DNA. Thus, RecA loading by RecFOR is localized to a region close to a junction. RecFOR proteins also recognize RNA at the 5'-end of an RNA-DNA junction within an ssDNA gap, which is compatible with their role in the repair of lagging strand gaps at stalled replication forks.

Unwinding and rewinding: double faces of helicase?

Helicases are enzymes that use ATP-driven motor force to unwind double-stranded DNA or RNA. Recently, increasing evidence demonstrates that some helicases also possess rewinding activity—in other words, they can anneal two complementary single-stranded nucleic acids. All five members of the human RecQ helicase family, helicase PIF1, mitochondrial helicase TWINKLE, and helicase/nuclease Dna2 have been shown to possess strand-annealing activity. Moreover, two recently identified helicases—HARP and AH2 have only ATP-dependent rewinding activity. These findings not only enhance our understanding of helicase enzymes but also establish the presence of a new type of protein: annealing helicases. This paper discusses what is known about these helicases, focusing on their biochemical activity to zip and unzip double-stranded DNA and/or RNA, their possible regulation mechanisms, and biological functions.

Mgm101 is a Rad52-related protein required for mitochondrial DNA recombination

Homologous recombination is a conserved molecular process that has primarily evolved for the repair of double-stranded DNA breaks and stalled replication forks. However, the recombination machinery in mitochondria is poorly understood. Here, we show that the yeast mitochondrial nucleoid protein, Mgm101, is related to the Rad52-type recombination proteins that are widespread in organisms from bacteriophage to humans. Mgm101 is required for repeat-mediated recombination and suppression of mtDNA fragmentation in vivo. It preferentially binds to single-stranded DNA and catalyzes the annealing of ssDNA precomplexed with the mitochondrial ssDNA-binding protein, Rim1. Transmission electron microscopy showed that Mgm101 forms large oligomeric rings of ∼14-fold symmetry and highly compressed helical filaments. Specific mutations affecting ring formation reduce protein stability in vitro. The data suggest that the ring structure may provide a scaffold for stabilization of Mgm101 by preventing the aggregation of the otherwise unstable monomeric conformation. Upon binding to ssDNA, Mgm101 is remobilized from the rings to form distinct nucleoprotein filaments. These studies reveal a recombination protein of likely bacteriophage origin in mitochondria and support the notion that recombination is indispensable for mtDNA integrity.

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.

SSB functions as a sliding platform that migrates on DNA via reptation

SSB proteins bind to and control the accessibility of single-stranded DNA (ssDNA), likely facilitated by their ability to diffuse on ssDNA. Using a hybrid single-molecule method combining fluorescence and force, we probed how proteins with large binding site sizes can migrate rapidly on DNA and how protein-protein interactions and tension may modulate the motion. We observed force-induced progressive unraveling of ssDNA from the SSB surface between 1 and 6 pN, followed by SSB dissociation at ∼10 pN, and obtained experimental evidence of a reptation mechanism for protein movement along DNA wherein a protein slides via DNA bulge formation and propagation. SSB diffusion persists even when bound with RecO and at forces under which the fully wrapped state is perturbed, suggesting that even in crowded cellular conditions SSB can act as a sliding platform to recruit and carry its interacting proteins for use in DNA replication, recombination and repair.

Role of the single-stranded DNA-binding protein SsbB in pneumococcal transformation: maintenance of a reservoir for genetic plasticity

Bacteria encode a single-stranded DNA (ssDNA) binding protein (SSB) crucial for genome maintenance. In Bacillus subtilis and Streptococcus pneumoniae, an alternative SSB, SsbB, is expressed uniquely during competence for genetic transformation, but its precise role has been disappointingly obscure. Here, we report our investigations involving comparison of a null mutant (ssbB⁻) and a C-ter truncation (ssbBΔ7) of SsbB of S. pneumoniae, the latter constructed because SSBs' acidic tail has emerged as a key site for interactions with partner proteins. We provide evidence that SsbB directly protects internalized ssDNA. We show that SsbB is highly abundant, potentially allowing the binding of ∼1.15 Mb ssDNA (half a genome equivalent); that it participates in the processing of ssDNA into recombinants; and that, at high DNA concentration, it is of crucial importance for chromosomal transformation whilst antagonizing plasmid transformation. While the latter observation explains a long-standing observation that plasmid transformation is very inefficient in S. pneumoniae (compared to chromosomal transformation), the former supports our previous suggestion that SsbB creates a reservoir of ssDNA, allowing successive recombination cycles. SsbBΔ7 fulfils the reservoir function, suggesting that SsbB C-ter is not necessary for processing protein(s) to access stored ssDNA. We propose that the evolutionary raison d'être of SsbB and its abundance is maintenance of this reservoir, which contributes to the genetic plasticity of S. pneumoniae by increasing the likelihood of multiple transformation events in the same cell.

Natural genetic transformation can compensate for the absence of sexual reproduction in bacteria, allowing genetic diversification by frequent recombination. In many species, transformability is a transient property relying on a specialized membrane-associated machinery for binding exogenous double-stranded DNA and internalization of single-stranded DNA (ssDNA) fragments extracted from exogenous DNA. Subsequent physical integration of internalized ssDNA into the recipient chromosome by homologous recombination requires dedicated cytosolic ssDNA–processing proteins. Here, we document the roles in the model transformable species Streptococcus pneumoniae of one of these processing proteins, SsbB, a paralogue of SsbA the ssDNA–binding protein essential for genome maintenance in bacteria, which is expressed uniquely in cells competent for genetic transformation. We show that SsbB is highly abundant, potentially allowing the binding of ∼1.15 Mb ssDNA (half a genome equivalent); that it participates in the processing of ssDNA into recombinants; that it protects and stabilizes internalized ssDNA; and that, at high DNA concentration, it is of crucial importance for chromosomal transformation whilst antagonizing plasmid transformation. We conclude that SsbB creates a reservoir of ssDNA, presumably allowing multiple transformations in the same cell, and that S. pneumoniae has evolved SsbB to optimize chromosomal transformation, thereby contributing to its remarkable genetic plasticity.

Novel, fluorescent, SSB protein chimeras with broad utility

The Escherichia coli single-stranded DNA binding protein (SSB) is a central player in DNA metabolism where it organizes genome maintenance complexes and stabilizes single-stranded DNA (ssDNA) intermediates generated during DNA processing. Due to the importance of SSB and to facilitate real-time studies, we developed a dual plasmid expression system to produce novel, chimeric SSB proteins. These chimeras, which contain mixtures of histidine-tagged and fluorescent protein(FP)-fusion subunits, are easily purified in milligram quantities and used without further modification, a significant enhancement over previous methods to produce fluorescent SSB. Chimeras retain the functionality of wild type in all assays, demonstrating that SSB function is unaffected by the FPs. We demonstrate the power and utility of these chimeras in single molecule studies providing a great level of insight into the biochemical mechanism of RecBCD. We also utilized the chimeras to show for the first time that RecG and SSB interact in vivo. Consequently, we anticipate that the chimeras described herein will facilitate in vivo, in vitro and single DNA molecule studies using proteins that do not require further modification prior to use.

Structures of a key interaction protein from the Trypanosoma brucei editosome in complex with single domain antibodies

Several major global diseases are caused by single-cell parasites called trypanosomatids. These organisms exhibit many unusual features including a unique and essential U-insertion/deletion RNA editing process in their single mitochondrion. Many key RNA editing steps occur in ∼20S editosomes, which have a core of 12 proteins. Among these, the "interaction protein" KREPA6 performs a central role in maintaining the integrity of the editosome core and also binds to ssRNA. The use of llama single domain antibodies (VHH domains) accelerated crystal growth of KREPA6 from Trypanosoma brucei dramatically. All three structures obtained are heterotetramers with a KREPA6 dimer in the center, and one VHH domain bound to each KREPA6 subunit. Two of the resultant heterotetramers use complementarity determining region 2 (CDR2) and framework residues to form a parallel pair of beta strands with KREPA6 - a mode of interaction not seen before in VHH domain-protein antigen complexes. The third type of VHH domain binds in a totally different manner to KREPA6. Intriguingly, while KREPA6 forms tetramers in solution adding either one of the three VHH domains results in the formation of a heterotetramer in solution, in perfect agreement with the crystal structures. Biochemical solution studies indicate that the C-terminal tail of KREPA6 is involved in the dimerization of KREPA6 dimers to form tetramers. The implications of these crystallographic and solution studies for possible modes of interaction of KREPA6 with its many binding partners in the editosome are discussed.

RecA protein assures fidelity of DNA repair and genome stability in Deinococcus radiodurans

Deinococcus radiodurans is one of the most radiation-resistant organisms known. It can repair hundreds of radiation-induced double-strand DNA breaks without loss of viability. Genome reassembly in heavily irradiated D. radiodurans is considered to be an error-free process since no genome rearrangements were detected after post-irradiation repair. Here, we describe for the first time conditions that frequently cause erroneous chromosomal assemblies. Gross chromosomal rearrangements have been detected in recA mutant cells that survived exposure to 5kGy γ-radiation. The recA mutants are prone also to spontaneous DNA rearrangements during normal exponential growth. Some insertion sequences have been identified as dispersed genomic homology blocks that can mediate DNA rearrangements. Whereas the wild-type D. radiodurans appears to repair accurately its genome shattered by 5kGy γ-radiation, extremely high γ-doses, e.g., 25kGy, produce frequent genome rearrangements among survivors. Our results show that the RecA protein is quintessential for the fidelity of repair of both spontaneous and γ-radiation-induced DNA breaks and, consequently, for genome stability in D. radiodurans. The mechanisms of decreased genome stability in the absence of RecA are discussed.

The C-terminus of the phage λ Orf recombinase is involved in DNA binding

Phage λ Orf substitutes for the activities of the Escherichia coli RecFOR proteins in vivo and is therefore implicated as a recombination mediator, encouraging the assembly of bacterial RecA onto single-stranded DNA (ssDNA) coated with SSB. Orf exists as a dimer in solution, associates with E. coli SSB and binds preferentially to ssDNA. To help identify interacting domains we analysed Orf and SSB proteins carrying mutations or truncations in the C-terminal region. A cluster of acidic residues at the carboxy-terminus of SSB is known to attract multiple protein partners to assist in DNA replication and repair. In this case an alternative domain must be utilized since Orf association with SSB was unaffected by an SSB113 point mutant (P176S) or removal of the last ten residues (ΔC10). Structurally the Orf C-terminus consists of a helix with a flexible tail that protrudes from each side of the dimer and could serve as a binding site for either SSB or DNA. Eliminating the six residue flexible tail (ΔC6) or the entire helix (ΔC19) had no significant impact on the Orf-SSB interaction. However, the OrfΔC6 protein exhibited reduced DNA binding, a feature shared by single amino acid substitutions within (W141F) or adjacent (R140A) to this region. The OrfΔC19 mutant bound poorly to DNA and secondary structure analysis in solution revealed that this truncation induces protein misfolding and aggregation. The results show that the carboxy-terminus of Orf is involved in nucleic acid recognition and also plays an unexpected role in maintaining structural integrity.

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.