Reversibility of strand invasion promoted by recA protein and its inhibition by Escherichia coli single-stranded DNA-binding protein or phage T4 gene 32 protein

New biochemistry in the Rhodanese-phosphatase superfamily: emerging roles in diverse metabolic processes, nucleic acid modifications, and biological conflicts

The protein-tyrosine/dual-specificity phosphatases and rhodanese domains constitute a sprawling superfamily of Rossmannoid domains that use a conserved active site with a cysteine to catalyze a range of phosphate-transfer, thiotransfer, selenotransfer and redox activities. While these enzymes have been extensively studied in the context of protein/lipid head group dephosphorylation and various thiotransfer reactions, their overall diversity and catalytic potential remain poorly understood. Using comparative genomics and sequence/structure analysis, we comprehensively investigate and develop a natural classification for this superfamily. As a result, we identified several novel clades, both those which retain the catalytic cysteine and those where a distinct active site has emerged in the same location (e.g. diphthine synthase-like methylases and RNA 2' OH ribosyl phosphate transferases). We also present evidence that the superfamily has a wider range of catalytic capabilities than previously known, including a set of parallel activities operating on various sugar/sugar alcohol groups in the context of NAD+-derivatives and RNA termini, and potential phosphate transfer activities involving sugars and nucleotides. We show that such activities are particularly expanded in the RapZ-C-DUF488-DUF4326 clade, defined here for the first time. Some enzymes from this clade are predicted to catalyze novel DNA-end processing activities as part of nucleic-acid-modifying systems that are likely to function in biological conflicts between viruses and their hosts.

Weaving DNA strands: structural insight on ATP hydrolysis in RecA-induced homologous recombination

Homologous recombination is a fundamental process in all living organisms that allows the faithful repair of DNA double strand breaks, through the exchange of DNA strands between homologous regions of the genome. Results of three decades of investigation and recent fruitful observations have unveiled key elements of the reaction mechanism, which proceeds along nucleofilaments of recombinase proteins of the RecA family. Yet, one essential aspect of homologous recombination has largely been overlooked when deciphering the mechanism: while ATP is hydrolyzed in large quantity during the process, how exactly hydrolysis influences the DNA strand exchange reaction at the structural level remains to be elucidated. In this study, we build on a previous geometrical approach that studied the RecA filament variability without bound DNA to examine the putative implication of ATP hydrolysis on the structure, position, and interactions of up to three DNA strands within the RecA nucleofilament. Simulation results on modeled intermediates in the ATP cycle bring important clues about how local distortions in the DNA strand geometries resulting from ATP hydrolysis can aid sequence recognition by promoting local melting of already formed DNA heteroduplex and transient reverse strand exchange in a weaving type of mechanism.

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.

The C terminus of the Escherichia coli RecA protein modulates the DNA binding competition with single-stranded DNA-binding protein

The nucleation step of Escherichia coli RecA filament formation on single-stranded DNA (ssDNA) is strongly inhibited by prebound E. coli ssDNA-binding protein (SSB). The capacity of RecA protein to displace SSB is dramatically enhanced in RecA proteins with C-terminal deletions. The displacement of SSB by RecA protein is progressively improved when 6, 13, and 17 C-terminal amino acids are removed from the RecA protein relative to the full-length protein. The C-terminal deletion mutants also more readily displace yeast replication protein A than does the full-length protein. Thus, the RecA protein has an inherent and robust capacity to displace SSB from ssDNA. However, the displacement function is suppressed by the RecA C terminus, providing another example of a RecA activity with C-terminal modulation. RecADeltaC17 also has an enhanced capacity relative to wild-type RecA protein to bind ssDNA containing secondary structure. Added Mg(2+) enhances the ability of wild-type RecA and the RecA C-terminal deletion mutants to compete with SSB and replication protein A. The overall binding of RecADeltaC17 mutant protein to linear ssDNA is increased further by the mutation E38K, previously shown to enhance SSB displacement from ssDNA. The double mutant RecADeltaC17/E38K displaces SSB somewhat better than either individual mutant protein under some conditions and exhibits a higher steady-state level of binding to linear ssDNA under all conditions.

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

We present biochemical evidence for the functional similarity of Escherichia coli RecO protein and bacteriophage T4 UvsY protein to eukaryotic Rad52 protein. Although Rad52 protein is conserved in eukaryotes, no sequence homologue has been found in prokaryotes or archeabacteria. Rad52 protein has two unique activities: facilitation of replication protein-A (RPA) displacement by Rad51 protein and annealing of RPA-single-stranded DNA (ssDNA) complexes. Both activities require species-specific interaction between Rad52 protein and RPA. Both RecO and UvsY proteins also possess the former property with regard to their cognate ssDNA-binding protein. Here, we report that RecO protein anneals ssDNA that is complexed with only its cognate ssDNA-binding protein, suggesting the involvement of species-specific interactions. Optimal activity for RecO protein occurs after formation of a 1:1 complex with SSB protein. RecR protein, which is known to stimulate RecO protein to facilitate SSB protein displacement by RecA protein, inhibits annealing by RecO protein, suggesting that RecR protein may regulate the choice between the DNA strand invasion versus annealing pathways. In addition, we show that UvsY protein anneals ssDNA; furthermore, ssDNA, which is complexed only with its cognate ssDNA-binding protein, is annealed in the presence of UvsY protein. These results indicate that RecO and possibly UvsY proteins are functional counterparts of Rad52 protein. Based on the conservation of these functions, we propose a modified double-strand break repair model that includes DNA annealing as an important intermediate step.

The simultaneous binding of two double-stranded DNA molecules by Escherichia coli RecA protein

We have characterized the double-stranded DNA (dsDNA) binding properties of RecA protein, using an assay based on changes in the fluorescence of 4',6-diamidino-2-phenylindole (DAPI)-dsDNA complexes. Here we use fluorescence, nitrocellulose filter-binding, and DNase I-sensitivity assays to demonstrate the binding of two duplex DNA molecules by the RecA protein filament. We previously established that the binding stoichiometry for the RecA protein-dsDNA complex is three base-pairs per RecA protein monomer, in the presence of ATP. In the presence of ATPgammaS, however, the binding stoichiometry depends on the MgCl2 concentration. The stoichiometry is 3 bp per monomer at low MgCl2 concentrations, but changes to 6 bp per monomer at higher MgCl2 concentrations, with the transition occurring at approximately 5 mM MgCl2. Above this MgCl2 concentration, the dsDNA within the RecA nucleoprotein complex becomes uncharacteristically sensitive to DNase I digestion. For these reasons we suggest that, at the elevated MgCl2 conditions, the RecA-dsDNA nucleoprotein filament can bind a second equivalent of dsDNA. These results demonstrate that RecA protein has the capacity to bind two dsDNA molecules, and they suggest that RecA or RecA-like proteins may effect homologous recognition between intact DNA duplexes.

The role of negative superhelicity and length of homology in the formation of paranemic joints promoted by RecA protein

Escherichia coli RecA protein pairs homologous DNA molecules to form paranemic joints when there is an absence of a free end in the region of homologous contact. Paranemic joints are a key intermediate in homologous recombination and are important in understanding the mechanism for a search of homology. The efficiency of paranemic joint formation depended on the length of homology and the topological forms of the duplex DNA. The presence of negative superhelicity increased the pairing efficiency and reduced the minimal length of homology required for paranemic joint formation. Negative superhelicity stimulated joint formation by favoring the initial unwinding of duplex DNA that occurred during the homology search and was not essential in the maintenance of the paired structure. Regardless of length of homology, formation of paranemic joints using circular duplex DNA required the presence of more than six negative supercoils. Above six negative turns, an increasing degree of negative superhelicity resulted in a linear increase in the pairing efficiency. These results support a model of two distinct kinds of DNA unwinding occurring in paranemic joint formation: an initial unwinding caused by heterologous contacts during synapsis and a later one during pairing of the homologous molecules.

A single-stranded DNA-binding protein is needed for efficient presynaptic complex formation by the Saccharomyces cerevisiae Rad51 protein

Protein-promoted DNA strand exchange requires formation of an active presynaptic complex between the DNA-pairing protein and single-stranded DNA (ssDNA). Formation of such a contiguous filament is stimulated by a ssDNA-binding protein. Here, the effects of replication protein A (RPA) on presynaptic complex formation and DNA strand exchange activities of Rad51 protein were examined. Presynaptic complex formation was assessed by measuring ATP hydrolysis. With phiX174 ssDNA, the ATPase activity of Rad51 protein is stimulated approximately 1.4-fold by RPA, provided that Rad51 protein is in excess of the ssDNA concentration; otherwise, RPA inhibits ATPase activity. In contrast, with ssDNA devoid of secondary structure (poly(dT), poly(dA), poly(dI), and etheno-M13 DNA), RPA does not stimulate the already elevated ATPase activity of Rad51 protein, but inhibits activity at low Rad51 protein concentrations. These results suggest that Rad51 protein and RPA exclude one another from ssDNA by competing for the same binding sites and that RPA exerts its effect on presynaptic complex formation by eliminating secondary structure to which Rad51 protein is bound nonproductively. DNA strand exchange catalyzed by Rad51 protein is also greatly stimulated by RPA. The optimal stoichiometry for stimulation is approximately 20-30 nucleotides of ssDNA/RPA heterotrimer. The ssDNA-binding protein of Escherichia coli can substitute for RPA, showing that the role of RPA is not specific. We conclude that RPA affects both presynaptic complex formation and DNA strand exchange via changes in DNA structure, employing the same mechanism used by the ssDNA-binding protein to effect change in E. coli RecA protein activity.

ATP-dependent DNA renaturation and DNA-dependent ATPase reactions catalyzed by the Ustilago maydis homologous pairing protein

Purification of the ATP-dependent homologous pairing activity from Ustilago maydis yields a protein preparation that is enriched for a 70-kDa polypeptide as determined by SDS-gel electrophoresis. The protein responsible for the ATP-dependent pairing activity, using renaturation of complementary single strands of DNA as an assay, has a Stokes radius of 3.6 nm and a sedimentation coefficient of 4.3 S consistent with the interpretation that the activity arises from a monomeric globular protein of 70 kDa. Including heparin-agarose and FPLC gel filtration chromatography steps in the previously published protocol improves the purification of the protein. ATP and Mg2+ are necessary cofactors for optimal DNA renaturation activity. ADP inhibits the reaction. Analysis of the ATP-dependent renaturation kinetics indicates the reaction proceeds through a first-order mechanism. The protein has an associated DNA-dependent ATPase as indicated by co-chromatography with the purified ATP-dependent renaturation activity through an FPLC gel-filtration column. Single-stranded DNA and Mg2+ are required for optimal ATP hydrolytic activity, although a number of other polynucleotides and divalent cations can substitute to varying degrees. Hydrolysis of ATP is activated in a sigmoidal manner with increasing amounts of the protein. At ATP concentrations below 0.1 mM the ATPase activity exhibits positive cooperativity as indicated from the Hill coefficient of 1.8 determined by steady-state kinetic analysis of the reaction. ADP and adenosine 5'-[beta,gamma-imido]triphosphate are inhibitors of the ATPase activity although they appear to exert their inhibitory effects through different modes. These results are interpreted as evidence for protein-protein interactions.

ATP hydrolysis and the displaced strand are two factors that determine the polarity of RecA-promoted DNA strand exchange

When the recA protein (RecA) of Escherichia coli promotes strand exchange between single-stranded DNA (ssDNA) circles and linear double-stranded DNAs (dsDNA) with complementary 5' or 3' ends a polarity is observed. This property of RecA depends on ATP hydrolysis and the ssDNA that is displaced in the reaction since no polarity is observed in the presence of the non-hydrolyzable ATP analog, ATP gamma S, or in the presence of single-strand specific exonucleases. Based on these results a model is presented in which both the 5' and 3' complementary ends of the linear dsDNA initiate pairing with the ssDNA circle but only one end remains stably paired. According to this model, the association/dissociation of RecA in the 5' to 3' direction on the displaced strand determines the polarity of strand exchange by favoring or blocking its reinvasion into the newly formed dsDNA. Reinvasion is favored when the displaced strand is coated with RecA whereas it is blocked when it lacks RecA, remains covered by single-stranded DNA binding protein or is removed by a single-strand specific exonuclease. The requirement for ATP hydrolysis is explained if the binding of RecA to the displaced strand occurs via the dissociation and/or transfer of RecA, two functions that depend on ATP hydrolysis. The energy for strand exchange derives from the higher binding constant of RecA for the newly formed dsDNA as compared with that for ssDNA and not from ATP hydrolysis.

RecA protein-promoted homologous pairing and strand exchange between intact and partially single-stranded duplex DNA

In the pairing reaction between circular gapped and fully duplex DNA, RecA protein first polymerizes on the gapped DNA to form a nucleoprotein filament. Conditions that removed the formation of secondary structure in the gapped DNA, such as addition of Escherichia coli single-stranded DNA binding protein or preincubation in 1 mM-MgCl2, optimized the binding of RecA protein and increased the formation of joint molecules. The gapped duplex formed stable joints with fully duplex DNA that had a 5' or 3' terminus complementary to the single-stranded region of the gapped molecule. However, the joints formed had distinct properties and structures depending on whether the complementary terminus was at the 5' or 3' end. Pairing between gapped DNA and fully duplex linear DNA with a 3' complementary terminus resulted in strand displacement, symmetric strand exchange and formation of complete strand exchange products. By contrast, pairing between gapped and fully duplex DNA with a 5' complementary terminus produced a joint that was restricted to the gapped region; there was no strand displacement or symmetric strand exchange. The joint formed in the latter reaction was likely a three-stranded intermediate rather than a heteroduplex with the classical Watson-Crick structure. We conclude that, as in the three-strand reaction, the process of strand exchange in the four-strand reaction is polar and progresses in a 5' to 3' direction with respect to the initiating strand. The present study provides further evidence that in both three-strand and four-strand systems the pairing and strand exchange reactions share a common mechanism.

The effects on strand exchange of 5' versus 3' ends of single-stranded DNA in RecA nucleoprotein filaments

Since the ends of DNA chains are thought to be important in homologous recombination, the way in which RecA protein and similar recombination enzymes process ends is important. We analyzed the effects of ends both on the formation of joints, and the progression of strand exchange. When the only homologous end was provided by a single strand, there was no significant difference between the formation of joints at a 5' end or a 3' end; but in agreement with the report of Konforti & Davis, Escherichia coli single-stranded DNA binding protein (SSB) selectively inhibited the activity of 5' ends. Complete strand exchange, assessed by study of linear single-stranded and double-stranded substrates, took place only in the 5' to 3' direction relative to DNA in the nucleoprotein filament. These observations pose a paradox: in the presence of SSB, of which there are about 800 tetramers per cell, the formation of homologous joints by RecA protein is favored at a 3' end, from which, however, authentic strand exchange appears not to occur. Since observations reported here and elsewhere show that joints have different properties when formed at a 5' versus a 3' end, we suggest that they may be processed differently in vivo.

Production of triple-stranded recombination intermediates by RecA protein, in vitro

During the directional strand exchange that is promoted by RecA protein between linear duplex DNA and circular single-stranded DNA, a triple-stranded DNA intermediate was formed and persisted even after the completion of strand transfer followed by deproteinization. In the deproteinized three-stranded DNA complexes, the sequestered linear third strand resisted digestion by E coli exonuclease I. In relation to polarity of strand exchange which defines the proximal and distal ends of the duplex DNA, when homology was restricted to the distal region of duplex substrate, the joints formed efficiently and were stable even upon complete deproteinization. Enzymatic probing of deproteinized distal joints with nuclease P1 revealed that the joints consist of long three-stranded structures that at neutral pH lack significant single-stranded character in any of the three strands. Instead of circular single-stranded DNA, when a linear single strand is recombined with partially homologous duplex DNA, in the presence of SSB, the formation of homologous joints by RecA protein, is significantly more efficient at distal end than at the proximal. Taken together, these observations suggest that with any single-stranded DNA (circular or linear), RecA protein efficiently promotes the formation of distal joints, from which, however, authentic strand exchange may not occur. Moreover, these joints might represent an intermediate which is trapped into a stable triple stranded state.

An overview of homologous pairing and DNA strand exchange proteins

Processes fundamental to all models of genetic recombination include the homologous pairing and subsequent exchange of DNA strands. Biochemical analysis of these events has been conducted primarily on the recA protein of Escherichia coli, although proteins which can promote such reactions have been purified from many sources, both prokaryotic and eukaryotic. The activities of these homologous pairing and DNA strand exchange proteins are either ATP-dependent, as predicted based on the recA protein paradigm, or, more unexpectedly, ATP-independent. This review examines the reactions promoted by both classes of proteins and highlights their similarities and differences. The mechanistic implications of the apparent existence of 2 classes of strand exchange protein are discussed.

RecA protein-promoted homologous pairing between duplex molecules: functional role of duplex regions of gapped duplex DNA

RecA protein promotes homologous pairing and symmetrical strand exchange between partially single-stranded duplex DNA and fully duplex molecules. We constructed circular gapped DNA with a defined gap length and studied the pairing reaction between the gapped substrate and fully duplex DNA. RecA protein polymerizes onto the single-stranded and duplex regions of the gapped DNA to form a nucleoprotein filament. The formation of such filaments requires a stoichiometric amount of RecA protein. Both the rate and yield of joint molecule formation were reduced when the pairing reaction was carried out in the presence of a sub-saturating amount of RecA protein. The amount of RecA protein required for optimal pairing corresponds to the binding site size of RecA protein at saturation on duplex DNA. The result suggests that in the 4-stranded system the single-stranded as well as the duplex regions are involved in pairing. By using fully duplex DNA that shares different lengths and regions of homology with the gapped molecule, we directly showed that the duplex region of the gapped DNA increased both the rate and yield of joint molecule formation. The present study indicates that even though strand exchange in the 4-stranded system must require the presence of a single-stranded region, the pairing that occurs in duplex regions between DNA molecules is functionally significant and contributes to the overall activity of the gapped DNA.

Monoclonal antibodies and mechanistic studies on recA protein

A protein has various epitopes, and a monoclonal antibody specifically binds to the protein by recognizing 1 of the epitopes. This characteristic of the monoclonal antibody has opened various new approaches in a wide variety of research works. In studies about recA protein and its promoted various reactions relating to genetic recombination, anti-recA protein-monoclonal antibodies are very useful to analyse reaction mechanisms and to detect transition in the higher order-structure of the protein, as well as to measure the amounts of recA protein in vitro or in vivo and to identify the related proteins. In this article, we will review studies on recA protein in which monoclonal antibodies were used as major tools. By using anti-recA protein-monoclonal IgGs as specific inhibitors, the partial reactions of the homologous pairing and strand exchange promoted by recA protein were separated, and by use of a set of anti-recA protein IgGs the stages of activation of recA protein in the above reactions were discriminated.

Biochemical and biological function of Escherichia coli RecA protein: behavior of mutant RecA proteins

The recA protein of E coli participates in several diverse biological processes and promotes a variety of complex in vitro reactions. A careful comparison of the phenotypic behavior of E coli recA mutations to the biochemical properties of the corresponding mutant proteins reveals a close parallel both between recombination phenotype and DNA strand exchange and renaturation activities, and between inducible phenomena and repressor cleavage activity. The biochemical alterations manifest by the mutant recA proteins are reflected in the strength of their interaction with ssDNA. The defective mutant recA proteins fail to properly assume the high-affinity DNA-binding state that is characteristic of the wild-type protein and, consequently, form less stable complexes with DNA. The mutant proteins displaying an 'enhanced' activity bind ssDNA with approximately the same affinity as the wild-type protein but, due to altered protein-protein interactions, they associate more rapidly with ssDNA. These changes proportionately affect the ability of recA protein to compete with SSB protein, to interact with dsDNA, and, perhaps, to bind repressor proteins. In turn, the DNA strand exchange, DNA renaturation, and repressor cleavage activities mirror these modifications.

The single-stranded DNA-binding protein of Escherichia coli

The single-stranded DNA-binding protein (SSB) of Escherichia coli is involved in all aspects of DNA metabolism: replication, repair, and recombination. In solution, the protein exists as a homotetramer of 18,843-kilodalton subunits. As it binds tightly and cooperatively to single-stranded DNA, it has become a prototypic model protein for studying protein-nucleic acid interactions. The sequences of the gene and protein are known, and the functional domains of subunit interaction, DNA binding, and protein-protein interactions have been probed by structure-function analyses of various mutations. The ssb gene has three promoters, one of which is inducible because it lies only two nucleotides from the LexA-binding site of the adjacent uvrA gene. Induction of the SOS response, however, does not lead to significant increases in SSB levels. The binding protein has several functions in DNA replication, including enhancement of helix destabilization by DNA helicases, prevention of reannealing of the single strands and protection from nuclease digestion, organization and stabilization of replication origins, primosome assembly, priming specificity, enhancement of replication fidelity, enhancement of polymerase processivity, and promotion of polymerase binding to the template. E. coli SSB is required for methyl-directed mismatch repair, induction of the SOS response, and recombinational repair. During recombination, SSB interacts with the RecBCD enzyme to find Chi sites, promotes binding of RecA protein, and promotes strand uptake.

Energetics of RecA-mediated recombination reactions. Without ATP hydrolysis RecA can mediate polar strand exchange but is unable to recycle

We demonstrate that the step of DNA strand exchange during RecA-mediated recombination reaction can occur equally efficiently in the presence or absence of ATP hydrolysis. The polarity of strand exchange is the same when instead of ATP its non-hydrolyzable analog adenosine-5'-O-(3-thiotriphosphate) is used. We show that the ATP dependence of recombination reaction is limited to the post-exchange stages of the reactions. The low DNA affinity state of RecA protomers, induced after ATP hydrolysis, is necessary for the dissociation of RecA-DNA complexes at the end of the reaction. This dissociation of RecA from DNA is necessary for the release of recombinant DNA molecules from the complexes formed with RecA and for the recycling of RecA protomers for another round of the recombination reaction.

RecA protein reinitiates strand exchange on isolated protein-free DNA intermediates. An ADP-resistant process

Efficient homologous pairing de novo of linear duplex DNA with a circular single strand (plus strand) coated with RecA protein requires saturation and extension of the single strand by the protein. However, strand exchange, the transfer of a strand from duplex DNA to the nucleoprotein filament, which follows homologous pairing, does not require the stable binding of RecA protein to single-stranded DNA. When RecA protein was added back to isolated protein-free DNA intermediates in the presence of sufficient ADP to inhibit strongly the binding of RecA protein to single-stranded DNA, strand exchange nonetheless resumed at the original rate and went to completion. Characterization of the protein-free DNA intermediate suggested that it has a special site or region to which RecA protein binds. Part of the nascent displaced plus strand of the deproteinized intermediate was unavailable as a cofactor for the ATPase activity of RecA protein, and about 30% resisted digestion by P1 endonuclease, which acts preferentially on single-stranded DNA. At the completion of strand exchange, when the distal 5' end of the linear minus strand had been fully incorporated into heteroduplex DNA, a nucleoprotein complex remained that contained all three strands of DNA from which the nascent displaced strand dissociated only over the next 50 to 60 minutes. Deproteinization of this intermediate yielded a complex that also contained three strands of DNA in which the nascent displaced strand was partially resistant to both Escherichia coli exonuclease I and P1 endonuclease. The deproteinized complex showed a broad melting transition between 37 degrees C and temperatures high enough to melt duplex DNA. These results show that strand exchange can be subdivided into two stages: (1) the exchange of base-pairs, which creates a new heteroduplex pair in place of a parental pair; and (2) strand separation, which is the physical displacement of the unpaired strand from the nucleoprotein filament. Between the creation of new heteroduplex DNA and the eventual separation of a third strand, there exists an unusual DNA intermediate that may contain three-stranded regions of natural DNA that are several thousand bases in length.

Biochemical properties of the Escherichia coli recA430 protein. Analysis of a mutation that affects the interaction of the ATP-recA protein complex with single-stranded DNA

The biochemical properties of the recA430 protein have been examined and compared to those of wild-type recA protein. We find that, while the recA430 protein possesses ssDNA-dependent rATP activity, this activity is inhibited by the Escherichia coli single-stranded DNA binding protein (SSB protein) under many conditions that enhance wild-type recA protein rATPase hydrolysis. Stimulation of rATPase activity by SSB protein is observed only at high concentrations of both rATP (greater than 1 mM) and recA430 protein (greater than 5 microM). In contrast, stimulation of ssDNA-dependent dATPase activity by SSB protein is less sensitive to protein and nucleotide concentration. Consistent with the nucleotide hydrolysis data, recA430 protein can carry out DNA strand exchange in the presence of either rATP or dATP. However, in the presence of rATP, both the rate and the extent of DNA strand exchange by recA430 protein are greatly reduced compared to wild-type recA protein and are sensitive to recA430 protein concentration. This reduction is presumably due to the inability of recA430 protein to compete with SSB protein for ssDNA binding sites under these conditions. The cleavage of lexA repressor protein by recA430 protein is also sensitive to the nucleotide cofactor present and is completely inhibited by SSB protein when rATP is the cofactor but not when dATP is used. Finally, the steady-state affinity and the rate of association of the recA430 protein-ssDNA complex are reduced, suggesting that the mutation affects the interaction of the ATP-bound form of recA protein with ssDNA. This alteration is the likely molecular defect responsible for inhibition of recA430 protein rATP-dependent function by SSB protein. The biochemical properties observed in the presence of dATP and SSB protein, i.e. the reduced levels of both DNA strand exchange activity and cleavage of lexA repressor protein, are consistent with the phenotypic behavior of recA430 mutations.

The mechanics of winding and unwinding helices in recombination: torsional stress associated with strand transfer promoted by RecA protein

Homologous recombination usually involves the production of heteroduplex DNA, DNA containing strands contributed from two different duplexes. RecA protein of E. coli can promote the formation of heteroduplex DNA in vitro by the exchange of DNA strands between two helical structures, duplex DNA and a helical recA nucleoprotein filament containing a single strand of DNA. Complete unwinding of the parental duplex and the rewinding of one strand with a new complement requires rotation of the helical structures about one another, or about their respective longitudinal axes. The observations described here demonstrate an association of torsional stress with strand exchange, and suggest that exchange is accomplished principally by concomitant rotation of duplex DNA and the recA nucleoprotein filament, each about its longitudinal axis.