Synaptic complex revisited; a homologous recombinase flips and switches bases

Sulfur-substitution-induced base flipping in the DNA duplex

Base flipping is widely observed in a number of important biological processes. The genetic codes deposited inside the DNA duplex become accessible to external agents upon base flipping. The sulfur substitution of guanine leads to thioguanine, which alters the thermodynamic stability of the GC base pairs and the GT mismatches. Experimental studies conclude that the sulfur substitution decreases the lifetime of the GC base pair. In this work, under three AMBER force fields for nucleotide systems, we firstly performed equilibrium and nonequilibrium free energy simulations to investigate the variation of the thermodynamic profiles in base flipping upon sulfur substitution. It is found that the bsc0 modification, the bsc1 modification and the OL15 modification of AMBER force fields are able to qualitatively describe the sulfur-substitution dependent behavior of the thermodynamics. However, only the two last-generation AMBER force fields are able to provide quantitatively correct predictions. The second computational study on the sulfur substitutions focused on the relative stability of the S6G-C base pair and the S6G-T mismatch. Two conflicting experimental observations were reported by the same authors. One suggested that the S6G-C base pair was more stable, while the other concludes that the S6G-T mismatch was more stable. We answered this question by constructing the free energy profiles along the base flipping pathway computationally.

Cooperative RecA clustering: the key to efficient homology searching

The mechanism by which pre-synaptic RecA nucleoprotein filaments efficiently locate sequence homology across genomic DNA remains unclear. Here, using atomic force microscopy, we directly investigate the intermediates of the RecA-mediated homologous recombination process and find it to be highly cooperative, involving multiple phases. Initially, the process is dominated by a rapid ‘association’ phase, where multiple filaments interact on the same dsDNA simultaneously. This cooperative nature is reconciled by the observation of localized dense clusters of pre-synaptic filaments interacting with the observed dsDNA molecules. This confinement of reactive species within the vicinity of the dsDNA, is likely to play an important role in ensuring that a high interaction rate between the nucleoprotein filaments and the dsDNA can be achieved. This is followed by a slower ‘resolution’ phase, where the synaptic joints either locate sequence homology and progress to a post-synaptic joint, or dissociate from the dsDNA. Surprisingly, the number of simultaneous synaptic joints decreases rapidly after saturation of the dsDNA population, suggesting a reduction in interaction activity of the RecA filaments. We find that the time-scale of this decay is in line with the time-scale of the dispersion of the RecA filament clusters, further emphasising the important role this cooperative phenomena may play in the RecA-facilitated homology search.

Binding energies of nucleobase complexes: Relevance to homology recognition of DNA

The binding energies of complexes of DNA nucleobase pairs are evaluated using quantum mechanical calculations at the level of dispersion corrected density functional theory. We begin with Watson-Crick base pairs of singlets, duplets, and triplets and calculate their binding energies. At a second step, mismatches are incorporated into the Watson-Crick complexes in order to evaluate the variation in the binding energy with respect to the canonical Watson-Crick pairs. A linear variation of this binding energy with the degree of mismatching is observed. The binding energies for the duplets and triplets containing mismatches are further compared to the energies of the respective singlets in order to assess the degree of collectivity in these complexes. This study also suggests that mismatches do not considerably affect the energetics of canonical base pairs. Our work is highly relevant to the recognition process in DNA promoted through the RecA protein and suggests a clear distinction between recognition in singlets, and recognition in duplets or triplets. Our work assesses the importance of collectivity in the homology recognition of DNA.

Roles of Bacillus subtilis DprA and SsbA in RecA-mediated genetic recombination

Bacillus subtilis competence-induced RecA, SsbA, SsbB, and DprA are required to internalize and to recombine single-stranded (ss) DNA with homologous resident duplex. RecA, in the ATP · Mg(2+)-bound form (RecA · ATP), can nucleate and form filament onto ssDNA but is inactive to catalyze DNA recombination. We report that SsbA or SsbB bound to ssDNA blocks the RecA filament formation and fails to activate recombination. DprA facilitates RecA filamentation; however, the filaments cannot engage in DNA recombination. When ssDNA was preincubated with SsbA, but not SsbB, DprA was able to activate DNA strand exchange dependent on RecA · ATP. This work demonstrates that RecA · ATP, in concert with SsbA and DprA, catalyzes DNA strand exchange, and SsbB is an accessory factor in the reaction. In contrast, RecA · dATP efficiently catalyzes strand exchange even in the absence of single-stranded binding proteins or DprA, and addition of the accessory factors marginally improved it. We proposed that the RecA-bound nucleotide (ATP and to a lesser extent dATP) might dictate the requirement for accessory factors.

Mechanisms and principles of homology search during recombination

Homologous recombination is crucial for genome stability and for genetic exchange. Although our knowledge of the principle steps in recombination and its machinery is well advanced, homology search, the critical step of exploring the genome for homologous sequences to enable recombination, has remained mostly enigmatic. However, recent methodological advances have provided considerable new insights into this fundamental step in recombination that can be integrated into a mechanistic model. These advances emphasize the importance of genomic proximity and nuclear organization for homology search and the critical role of homology search mediators in this process. They also aid our understanding of how homology search might lead to unwanted and potentially disease-promoting recombination events.

Mechanism of homology recognition in DNA recombination from dual-molecule experiments

In E. coli homologous recombination, a filament of RecA protein formed on DNA searches and pairs a homologous sequence within a second DNA molecule with remarkable speed and fidelity. Here, we directly probe the strength of the two-molecule interactions involved in homology search and recognition using dual-molecule manipulation, combining magnetic and optical tweezers. We find that the filament's secondary DNA-binding site interacts with a single strand of the incoming double-stranded DNA during homology sampling. Recognition requires opening of the helix and is strongly promoted by unwinding torsional stress. Recognition is achieved upon binding of both strands of the incoming dsDNA to each of two ssDNA-binding sites in the filament. The data indicate a physical picture for homology recognition in which the fidelity of the search process is governed by the distance between the DNA-binding sites.

Real-time observation of strand exchange reaction with high spatiotemporal resolution

RecA binds to single-stranded (ss) DNA to form a helical filament that catalyzes strand exchange with a homologous double-stranded (ds) DNA. The study of strand exchange in ensemble assays is limited by the diffusion limited homology search process, which masks the subsequent strand exchange reaction. We developed a single-molecule fluorescence assay with a few base-pair and millisecond resolution that can separate initial docking from the subsequent propagation of joint molecule formation. Our data suggest that propagation occurs in 3 bp increments with destabilization of the incoming dsDNA and concomitant pairing with the reference ssDNA. Unexpectedly, we discovered the formation of a dynamic complex between RecA and the displaced DNA that remains bound transiently after joint molecule formation. This finding could have important implications for the irreversibility of strand exchange. Our model for strand exchange links structural models of RecA to its catalytic function.

Structural, thermodynamic, and kinetic basis for the activities of some nucleic acid repair enzymes

X-ray structural analysis provides no quantitative estimate of the relative contribution of specific and nonspecific or strong and weak interactions to the total affinity of enzymes for nucleic acids. We have shown that the interaction between enzymes and long nucleic acids at the molecular level can be successfully analyzed by the method of stepwise increase in ligand complexity (SILC). In the present review we summarize our studies of human uracil DNA glycosylase and apurinic/apyrimidinic endonuclease, E. coli 8-oxoguanine DNA glycosylase and RecA protein using the SILC approach. The relative contribution of structural (X-ray analysis data), thermodynamic, and catalytic factors to the discrimination of specific and nonspecific DNA by these enzymes at the stages of complex formation, the following changes in DNA and enzyme conformations and especially the catalysis of the reactions is discussed.

Reversibility, equilibration, and fidelity of strand exchange reaction between short oligonucleotides promoted by RecA protein from escherichia coli and human Rad51 and Dmc1 proteins

We demonstrate the reversibility of RecA-promoted strand exchange reaction between short oligonucleotides in the presence of adenosine 5'-O-(thiotriphosphate). The reverse reaction proceeds without the dissociation of RecA from DNA. The reaction reaches equilibrium and its yield depends on the homology between the reaction substrates. We estimate the tolerance of the RecA-promoted strand exchange to individual base substitutions for a comprehensive set of possible base combinations in a selected position along oligonucleotide substrates for strand exchange and find, in agreement with previously reported estimations, that this tolerance is higher than in the case of free DNA. It is demonstrated that the short oligonucleotide-based approach can be applied to the human recombinases Rad51 and Dmc1 when strand exchange is performed in the presence of calcium ions and ATP. Remarkably, despite the commonly held belief that the eukaryotic recombinases have an inherently lower strand exchange activity, in our system their efficiencies in strand exchange are comparable with that of RecA. Under our experimental conditions, the human recombinases exhibit a significantly higher tolerance to interruptions of homology due to point base substitutions than RecA. Finding conditions where a chemical reaction is reversible and reaches equilibrium is critically important for its thermodynamically correct description. We believe that the experimental system described here will substantially facilitate further studies on different aspects of the mechanisms of homologous recombination.

Maintaining replication fork integrity in UV-irradiated Escherichia coli cells

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

Homology-driven chromatin remodeling by human RAD54

Human RAD51 and RAD54 are key players in homologous recombination, a process that requires homology recognition and strand invasion by a RAD51-single-stranded DNA (ssDNA) nucleoprotein filament and chromatin remodeling by RAD54. Here we use in vitro chromatin reconstitution systems to show that RAD51-ssDNA stimulates RAD54-dependent chromatin remodeling in a homology-dependent, polarity-independent manner. This stimulation was not seen with RAD54B or other remodelers. Chromatin remodeling by RAD54 enabled strand invasion by RAD51-ssDNA on nucleosomal templates, which was homology- and polarity-dependent. Three natural RAD54 mutants found in primary cancer cells showed specific defects in remodeling or in the RAD54-RAD51 interaction. We propose that RAD54 is recruited by RAD51-ssDNA filament to the chromatin of the intact chromosome and that it remodels that chromatin to facilitate accessibility for strand exchange.

Replication fork stalling and cell cycle arrest in UV-irradiated Escherichia coli

Faithful duplication of the genome relies on the ability to cope with an imperfect template. We investigated replication of UV-damaged DNA in Escherichia coli and found that ongoing replication stops for at least 15-20 min before resuming. Undamaged origins of replication (oriC) continue to fire at the normal rate and in a DnaA-dependent manner. UV irradiation also induces substantial DnaA-independent replication. These two factors add substantially to the DNA synthesis detected after irradiation and together mask the delay in the progression of pre-existing forks in assays measuring net synthesis. All DNA synthesis after UV depends on DnaC, implying that replication restart of blocked forks requires DnaB loading and possibly the entire assembly of new replisomes. Restart appears to occur synchronously when most lesions have been removed. This raises the possibility that restart and lesion removal are coupled. Both restart and cell division suffer long delays if lesion removal is prevented, but restart can occur. Our data fit well with models invoking the stalling of replication forks and their extensive processing before replication can restart. Delayed restart avoids the dangers of excessive recombination that might result if forks skipped over lesion after lesion, leaving many gaps in their wake.

Direct evaluation of a kinetic model for RecA-mediated DNA-strand exchange: the importance of nucleic acid dynamics and entropy during homologous genetic recombination

The Escherichia coli RecA protein is the prototype of a class of proteins that play central roles in genomic repair and recombination in all organisms. The unresolved mechanistic strategy by which RecA aligns a single strand of DNA with a duplex DNA and mediates a DNA strand switch is central to understanding homologous recombination. We explored the mechanism of RecA-mediated DNA-strand exchange using oligonucleotide substrates with the intrinsic fluorophore 6-methylisoxanthopterin. Pre-steady-state spectrofluorometric analysis elucidated the earliest transient intermediates formed during recombination and delineated the mechanistic strategy by which RecA facilitates this process. The structural features of the first detectable intermediate and the energetic characteristics of its formation were consistent with interactions between a few bases of the single-stranded DNA and the minor groove of a locally melted or stretched duplex DNA. Further analysis revealed RecA to be an unusual enzyme in that entropic rather than enthalpic contributions dominate its catalytic function, and no unambiguously active role for the protein was detected in the earliest molecular events of recombination. The data best support the conclusion that the mechanistic strategy of RecA likely relies on intrinsic DNA dynamics.

Origins of sequence selectivity in homologous genetic recombination: insights from rapid kinetic probing of RecA-mediated DNA strand exchange

Despite intense effort over the past 30 years, the molecular determinants of sequence selectivity in RecA-mediated homologous recombination have remained elusive. Here, we describe when and how sequence homology is recognized between DNA strands during recombination in the context of a kinetic model for RecA-mediated DNA strand exchange. We characterized the transient intermediates of the reaction using pre-steady-state kinetic analysis of strand exchange using oligonucleotide substrates containing a single fluorescent G analog. We observed that the reaction system was sensitive to heterology between the DNA substrates; however, such a "heterology effect" was not manifest when functional groups were added to or removed from the edges of the base-pairs facing the minor groove of the substrate duplex. Hence, RecA-mediated recombination must occur without the involvement of a triple helix, even as a transient intermediate in the process. The fastest detectable reaction phase was accelerated when the structure or stability of the substrate duplex was perturbed by internal mismatches or the replacement of G.C by I.C base-pairs. These findings indicate that the sequence specificity in recombination is achieved by Watson-Crick pairing in the context of base-pair dynamics inherent to the extended DNA structure bound by RecA during strand exchange.

Construction and evaluation of a kinetic scheme for RecA-mediated DNA strand exchange

The Escherichia coli RecA protein is the prototype of a class of proteins playing a central role in genomic repair and recombination in all organisms. The unresolved mechanistic strategy by which RecA aligns a single strand of DNA with a duplex DNA and mediates a DNA strand switch is central to understanding its recombinational activities. Toward a molecular-level understanding of RecA-mediated DNA strand exchange, we explored its mechanism using oligonucleotide substrates and the intrinsic fluorescence of 6-methylisoxanthopterin (6MI). Steady- and presteady-state spectrofluorometric data demonstrate that the reaction proceeds via a sequential four-step mechanism comprising a rapid, bimolecular association step followed by three slower unimolecular steps. Previous authors have proposed multistep mechanisms involving two or three steps. Careful analysis of the differences among the experimental systems revealed a previously undiscovered intermediate (N1) whose formation may be crucial in the kinetic discrimination of homologous and heterologous sequences. This observation has important implications for probing the fastest events in DNA strand exchange using 6MI to further elucidate the molecular mechanisms of recombination and recombinational repair.

Formation of nucleoprotein RecA filament on single-stranded DNA. Analysis by stepwise increase in ligand complexity

RecA protein plays a pivotal role in homologous recombination in Escherichia coli. RecA polymerizes on single-stranded (ss) DNA forming a nucleoprotein filament. Then double-stranded (ds) DNA is bound and searched for segments homologous to the ssDNA. Finally, homologous strands are exchanged, a new DNA duplex is formed, and ssDNA is displaced. We report a quantitative analysis of RecA interactions with ss d(pN)n of various structures and lengths using these oligonucleotides as inhibitors of RecA filamentation on d(pT)20. DNA recognition appears to be mediated by weak interactions between its structural elements and RecA monomers within a filament. Orthophosphate and dNMP are minimal inhibitors of RecA filamentation (I50 = 12-20 mM). An increase in homo-d(pN)2-40 length by one unit improves their affinity for RecA (f factor) approximately twofold through electrostatic contacts of RecA with internucleoside phosphate DNA moieties (f approximately = 1.56) and specific interactions with T or C bases (f approximately = 1.32); interactions with adenine bases are negligible. RecA affinity for d(pN)n containing normal or modified nucleobases depends on the nature of the base, features of the DNA structure. The affinity considerably increases if exocyclic hydrogen bond acceptor moieties are present in the bases. We analyze possible reasons underlying RecA preferences for DNA sequence and length and propose a model for recognition of ssDNA by RecA.