recA protein promoted DNA strand exchange

recA protein and circular single-stranded DNA form a stable complex in the presence of single-stranded DNA binding protein (SSB), in which one recA protein monomer is bound per two nucleotides of DNA. These complexes are kinetically significant intermediates in the exchange of strands between the single-stranded DNA and an homologous linear duplex. After completion of strand exchange, the recA protein remains tightly associated with the circular duplex product of the reaction and the SSB is bound to the displaced linear single strand. Upon addition of ADP, the recA protein-duplex DNA complex dissociates. RecA protein also interacts with single-stranded DNA in the absence of SSB; however, the amount of recA protein bound is substantially reduced. These findings provide direct physical evidence for the participation of SSB in the formation of the recA protein-single-stranded DNA complexes inferred earlier from kinetic analysis. Moreover, they confirm the ability of recA protein to equilibrate between bound and free forms in the absence of SSB.     

Patterns of nuclease protection during strand exchange. recA protein forms heteroduplex DNA by binding to strands of the same polarity

recA protein, in the presence of ATP, polymerizes on single-stranded DNA (plus strand) to form a presynaptic nucleoprotein filament that pairs with linear duplex DNA and actively displaces the plus strand from the recipient molecule in a polarized fashion to form a new heteroduplex molecule. The interaction between recA protein and DNA during strand exchange was studied by labeling different strands and probing the intermediate with pancreatic deoxyribonuclease I (DNase I) or restriction endonuclease. The incoming single strand was resistant to DNase I in the original nucleoprotein filament and remained resistant even after extensive strand exchange had occurred. Both strands of the parental duplex molecule were sensitive to DNase I in the absence of joint molecule formation; but as strand exchange progressed following homologous pairing, increasing stretches of the parental plus strand became resistant, whereas the complementary parental minus strand remained sensitive to DNase I throughout the reaction. Except for a region of 50-100 base pairs at the end of the newly formed heteroduplex DNA where strand exchange was initiated, the rest of the heteroduplex region was resistant to cleavage by restriction endonucleases. The data suggest that recA protein promotes strand exchange by binding both the incoming and outgoing strands of the same polarity, whereas the complementary strand, which must switch pairing partners, is unhindered by direct contact with the protein.     

Concerted strand exchange and formation of Holliday structures by E. coli RecA protein

RecA protein makes stable joint molecules from fully duplex DNA and molecules that are partially single-stranded; the latter may be either duplex molecules with an internal gap in one strand or molecules with single-stranded ends. Stable joint molecules form only when the end of at least one strand is in a homologous region. When RecA protein pairs linear duplex molecules and tailed molecules that share the same sequence end to end, the joints, which are located away from the single-stranded tails in most instances, have the electron microscopic appearance associated with the Holliday structure resulting from the reciprocal exchange of strands. The reaction leading to reciprocal strand exchange involves the concerted displacement of a strand from the end of the duplex molecule. These observations support the view that RecA protein makes stable joint molecules only by transferring strands and not by the side-by-side pairing of duplex regions.     

Enhancement of recA protein-promoted DNA strand exchange activity by volume-occupying agents.

To investigate the in vivo effects of macromolecular crowding we examined the effect of inert macromolecules such as polyvinyl alcohol and polyethylene glycol on the in vitro activity of recA protein. The addition of either of these volume-occupying agents enables recA protein to promote homologous pairing and exchange of DNA strands at an otherwise nonpermissive magnesium ion concentration. In the presence of these macromolecules, both the rate of recA protein association with single-stranded DNA (ssDNA) and the steady-state affinity of recA protein for ssDNA are increased. Consequently, the ability of recA protein to compete with ssDNA-binding protein (SSB protein) is enhanced, and the inhibitory effects of SSB protein on the formation of recA protein-ssDNA presynaptic complexes are eliminated. Because the ability of recA protein to bind to ssDNA-containing secondary structures is also enhanced in volume-occupied solution, joint molecule formation is not greatly reduced when SSB protein is omitted from the reaction. Thus, increased recA protein interactions with ssDNA contribute to enhanced presynaptic complex formation. In addition, polyvinyl alcohol and polyethylene glycol must also affect another property of recA protein, i.e. self-association, which is required for synapsis and DNA strand exchange. Our examination of DNA strand exchange in the presence of volume-occupying agents helps to reconcile the requirement for elevated magnesium ion concentrations in recA protein-promoted recombination reactions in vitro, with a presumably low magnesium ion concentration in vivo.     

A postsynaptic role for single-stranded DNA-binding protein in recA protein-promoted DNA strand exchange.

The single-stranded DNA-binding protein (SSB protein) is required for efficient genetic recombination in vivo. One function for SSB protein in DNA strand exchange in vitro is to remove secondary structure from single-stranded DNA (ssDNA) and thereby aid in the formation of recA protein-saturated presynaptic complexes. In the preceding paper (Lavery, P. E., and Kowalczykowski, S. C. (1992) J. Biol. Chem. 267, 9307-9314) we demonstrated that DNA strand exchange can occur in the presence of volume-occupying agents at low magnesium ion concentration, where secondary structures are reduced. Our results suggest that SSB protein is not acting during presynapsis under these conditions, yet the DNA strand exchange reaction is stimulated by the addition of SSB protein. In this study we present biochemical evidence which suggests that SSB protein stimulates DNA strand exchange by binding to the ssDNA displaced from joint molecules, thereby stabilizing them and allowing branch migration to extend the region of heteroduplex DNA. Therefore, our results indicate dual roles for SSB protein at elevated magnesium ion concentration; it functions during presynapsis, removing secondary structure from ssDNA, as indicated previously, and it also functions postsynaptically, binding to the ssDNA displaced from joint molecules.     

Homology-dependent changes in adenosine 5'-triphosphate hydrolysis during recA protein promoted DNA strand exchange: evidence for long paranemic complexes

As a first step in DNA strand exchange, recA protein forms a filamentous complex on single-stranded DNA (ssDNA), which contains stoichiometric (one recA monomer per four nucleotides) amounts of recA protein. recA protein monomers within this complex hydrolyze ATP with a turnover number of 25 min-1. Upon introduction of linear homologous duplex DNA to initiate strand exchange, this rate of ATP hydrolysis drops by 33%. The decrease in rate is complete in less than 2 min, and the rate of ATP hydrolysis then remains constant during and subsequent to the strand exchange reaction. This drop is completely dependent upon homology in the duplex DNA. In addition, the magnitude of the drop is linearly dependent upon the length of the homologous region in the linear duplex DNA. Linear DNA substrates in which pairing is topologically restricted to a paranemic joint also follow this relationship. Taken together, these properties imply that all of the available homology in the incoming duplex DNA is detected very early in the DNA strand exchange reaction, with the linear duplex DNA paired paranemically with the homologous ssDNA in the complex throughout its length. The results indicate that paranemic joints can extend over thousands of base pairs. We note elsewhere [Pugh, B. F., & Cox, M. M. (1987b) J. Biol. Chem. 262, 1337-1343] that this duplex acquires resistance to digestion by DNase with a much slower time course (30 min), which parallels the progress of strand exchange. Together these results imply that the duplex DNA is paired with the ssDNA but remains outside the nucleoprotein filament. Finally, the results also support the notion that ATP hydrolysis occurs throughout the recA nucleoprotein filament.     

The function of the secondary DNA-binding site of RecA protein during DNA strand exchange.

RecA protein features two distinct DNA-binding sites. During DNA strand exchange, the primary site binds to single-stranded DNA (ssDNA), forming the helical RecA nucleoprotein filament. The weaker secondary site binds double-stranded DNA (dsDNA) during the homology search process. Here we demonstrate that this site has a second important function. It binds the ssDNA strand that is displaced from homologous duplex DNA during DNA strand exchange, stabilizing the initial heteroduplex DNA product. Although the high affinity of the secondary site for ssDNA is essential for DNA strand exchange, it renders DNA strand exchange sensitive to an excess of ssDNA which competes with dsDNA for binding. We further demonstrate that single-stranded DNA-binding protein can sequester ssDNA, preventing its binding to the secondary site and thereby assisting at two levels: it averts the inhibition caused by an excess of ssDNA and prevents the reversal of DNA strand exchange by removing the displaced strand from the secondary site.     

Purification and properties of the recA protein of Proteus mirabilis. Comparison with Escherichia coli recA protein; specificity of interaction with single strand binding protein

The product of the cloned recA+ gene of Proteus mirabilis substitutes for a defective recA protein in Escherichia coli recA- mutants and restores recombination, repair, and prophage induction functions to near normal levels (Eitner, G., Adler, B., Lanzov, V. A., and Hofemeister, J. (1982) Mol. Gen. Genet. 185, 481-486). In this paper, we report the purification to near homogeneity of the P. mirabilis recA protein (recApm). The polypeptide has a molecular weight similar to that of E. coli recA protein (recAec) and shows partial identity with recAec when reacted against antibodies specific for the E. coli recA protein. recApm catalyzes the hydrolysis of ATP in the presence of single-stranded but not double-stranded DNA. We have compared the recombination-like activities of recApm with those of recAec and found them to be similar. In the presence of ATP and Mg2+, stoichiometric amounts of recApm promote the complete reciprocal exchange of strands between gapped circular and linear duplex DNA molecules. The enzyme also efficiently promotes the formation of D-loops from circular duplex DNA and homologous single-stranded fragments. However, although recApm and recAec share the above physical and functional similarities, they differ in their ability to interact with the E. coli single strand binding protein to catalyze the transfer of one DNA strand from a linear duplex to a single-stranded circle.     

E. coli recA protein possesses a strand separating activity on short duplex DNAs.

RecA protein was found to catalyze the dissociation of the strands of a DNA substrate consisting of a 20-nucleotide primer annealed to circular single-stranded M13mp DNA. The strand separation reaction requires ATP hydrolysis and the presence of single-stranded DNA flanking the duplex DNA region to be unwound. RecA-catalyzed strand separation is effective only for very short duplexes, not exceeding 30 bp, and is not stimulated by single-stranded DNA-binding protein. These results are consistent with the ability of recA protein to disrupt regions of secondary structure in single-stranded DNA and to incorporate large non-homologies into heteroduplex DNA.     

The direction of RecA protein assembly onto single strand DNA is the same as the direction of strand assimilation during strand exchange

The RecA protein of Escherichia coli optimally promotes DNA strand exchange reactions in the presence of the single strand DNA-binding protein of E. coli (SSB protein). Under these conditions, assembly of RecA protein onto single-stranded DNA (ssDNA) occurs in three steps. First, the ssDNA is rapidly covered by SSB protein. The binding of RecA protein is then initiated by nucleation of a short tract of RecA protein onto the ssDNA. Finally, cooperative polymerization of additional RecA protein accompanied by displacement of SSB protein results in a ssDNA-RecA protein filament (Griffith, J. D., Harris, L. D., and Register, J. C. (1984) Cold Spring Harbor Symp. Quant. Biol. 49, 553-559). We report here that RecA protein assembly onto circular ssDNA yields RecA protein-covered circles in which greater than 85% are completely covered by RecA protein with no remaining SSB protein-covered segments (as detected by electron microscopy). However, when linear ssDNA is used, 90% of the filaments contain a short segment at one end complexed with SSB protein. This suggests that RecA protein assembly is unidirectional. Visualization of the assembly of RecA protein onto either long ssDNA tails (containing either 5' or 3' termini) or ssDNA gaps generated in double strand DNA allowed us to determine that the RecA protein polymerizes in the 5' to 3' direction on ssDNA and preferentially nucleates at ssDNA-double strand DNA junctions containing 5' termini.     

The inactive form of recA protein: the 'compact' structure.

When recA protein is enzymatically inactive in vitro, it adopts a more compact helical polymer form than that of the active protein polymerized onto DNA in the presence of ATP. Here we describe some aspects of this structure. By cryo-electron microscopy, a pitch of 76 A is found for both the self-polymer and the inactive complex with ssDNA. A smaller pitch of 64 A is observed in conventional electron micrographs. The contour length of complexes with ssDNA was used to estimate the binding stoichiometry in the compact complex, 6 +/- 1 nt/recA. In addition, the compact structure was observed in vivo in Escherichia coli: inclusion bodies produced upon induction of recA expression in an overproducing strain have a fibrous morphology with the structural parameters of the compact polymer.     

Mechanism of inhibition of site-specific recombination by the Holliday junction-trapping peptide WKHYNY: insights into phage lambda integrase-mediated strand exchange

Holliday junctions are central intermediates in site-specific recombination reactions mediated by tyrosine recombinases. Because these intermediates are extremely transient, only artificially assembled Holliday junctions have been available for study. We have recently identified hexapeptides that cause the accumulation of natural Holliday junctions of bacteriophage lambda Integrase (Int)-mediated reactions. We now show that one of these peptides acts after the first DNA cleavage event to stabilize protein-bound junctions and to prevent their resolution. The peptide acts before the step affected by site affinity (saf) mutations in the core region, in agreement with a model that the peptide stabilizes the products of strand exchange (i.e. Holliday junctions) while saf mutations reduce ligation of exchanged strands.Strand exchange events leading to Holliday junctions in phage lambda integration and excision are asymmetric, presumably because interactions between Int and some of its core-binding sites determine the order of strand cleavage. We have compared the structure of Holliday junctions in one unidirectional and in two bidirectional Int-mediated pathways and show that the strand cleavage steps are much more symmetric in the bidirectional pathways. Thus Int-DNA interactions which determine the order of top and bottom strand cleavage and exchange are unique in each recombination pathway.     

Properties of recA441 protein-catalyzed DNA strand exchange can be attributed to an enhanced ability to compete with SSB protein

We have investigated the recombinase activity of recA441 protein by comparing its in vitro DNA strand exchange activity to that of wild-type recA protein. Consistent with its proficiency in recombination in vivo, recA441 protein is able to catalyze the in vitro exchange of a circular single-stranded DNA molecule for a homologous strand in a linear double-stranded DNA molecule. Under conditions optimal for wild-type recA protein, the rates of joint molecule formation are the same for the two recA proteins, but the wild-type protein converts these intermediate species to gapped circular heteroduplex DNA product molecules more rapidly than recA441 protein. In the recA441 protein reaction, joint molecules are instead converted to extensive homology-dependent DNA networks via presumed reinitiation reactions. Under some conditions, the DNA strand exchange activity of recA441 protein is enhanced relative to the wild-type. These conditions include when single-stranded DNA.SSB protein (where SSB is Escherichia coli single-stranded DNA-binding protein) complexes are formed prior to the addition of recA protein, at low magnesium ion concentration in the presence of spermidine, and at low ATP concentrations. Under the conditions examined, recA441 protein competes more effectively with SSB protein for DNA-binding sites; thus, the differences between the strand exchange activities of the wild-type and recA441 proteins can be attributed to this enhanced ability in SSB protein competition.     

Escherichia coli recA protein protects single-stranded DNA or gapped duplex DNA from degradation by RecBC DNase

RecA- mutants of Escherichia coli extensively degrade their DNA following UV irradiation. Most of this degradation is due to the recBC DNase, which suggests that the recA gene is involved in the control of recBC DNase in vivo. We have shown that purified recA protein inhibits the endonuclease and exonuclease activities of recBC DNase on single-stranded DNA. The extent of inhibition is dependent on the relative concentration of recA protein, recBC DNase, and the DNA substrate; inhibition is greatest when the concentrations of DNA and recBC DNase are low and the concentrations of recA protein is high. At fixed concentrations of recA protein and recBC DNase, inhibition is eliminated at high concentrations of DNA. In the presence of adenosine 5'-O-(3-thiotriphosphate), an ATP analog which stabilizes the binding of recA protein to both single- and double-stranded DNA, recA protein is a more potent inhibitor of the nuclease activities on single-stranded DNA and is a weak inhibitor of the exonuclease activity on double-stranded DNA. Inhibition of the latter is enhanced by oligodeoxynucleotides, which stimulate the binding of recA protein to double-stranded DNA. In the presence of adenosine 5'-O-(3-thiotriphosphate), recA protein also inhibits the action of exonuclease I on single-stranded DNA and of lambda exonuclease on double-stranded DNA. These observations are most consistent with the idea that recA protein protects DNA from recBC DNase by binding to DNA. RecA protein also blocks the endonucleolytic cleavage of gapped circular DNA by recBC DNase. Since both recA protein and recBC DNase have the ability under certain conditions to unwind duplex DNA and to displace strands, we looked for evidence that their combined action would enlarge gaps but found no extensive enlargement. D-loops, a putative intermediate in genetic recombination, are effectively protected against the action of recBC DNase by the E. coli single strand binding protein and by recA protein in the presence of adenosine 5'-O-(3-thiotriphosphate).     

Duplex-duplex interactions catalyzed by recA protein allow strand exchanges to pass double-strand breaks in DNA

Using gapped circular DNA and homologous duplex DNA cut with restriction nucleases, we show that E. coli RecA protein promotes strand exchanges past double-strand breaks. The products of strand exchange are heteroduplex DNA molecules that contain nicks, which can be sealed by DNA ligase, thereby effecting the repair of double-strand breaks in vitro. These results show that RecA protein can promote pairing interactions between homologous DNA molecules at regions where both are duplex. Moreover, pairing leads to strand exchanges and the formation of heteroduplex DNA. In contrast, strand exchanges are unable to pass a double-strand break in the gapped substrate. This apparent paradox is discussed in terms of a model for RecA-DNA interactions in which we propose that each RecA monomer contains two nonequivalent DNA-binding sites.     

Characterization of strand exchange activity of yeast Rad51 protein.

The Saccharomyces cerevisiae RAD51 gene product takes part in genetic recombination and repair of DNA double strand breaks. Rad51, like Escherichia coli RecA, catalyzes strand exchange between homologous circular single-stranded DNA (ssDNA) and linear double-stranded DNA (dsDNA) in the presence of ATP and ssDNA-binding protein. The formation of joint molecules between circular ssDNA and linear dsDNA is initiated at either the 5' or the 3' overhanging end of the complementary strand; joint molecules are formed only if the length of the overhanging end is more than 1 nucleotide. Linear dsDNAs with recessed complementary or blunt ends are not utilized. The polarity of strand exchange depends upon which end is used to initiate the formation of joint molecules. Joint molecules formed via the 5' end are processed by branch migration in the 3'-to-5' direction with respect to ssDNA, and joint molecules formed with a 3' end are processed in the opposite direction.     

Escherichia coli RuvC protein is an endonuclease that resolves the Holliday structure.

Genetic evidence suggests that the Escherichia coli ruvC gene is involved in DNA repair and in the late step of RecE and RecF pathway recombination. To study the biochemical properties of RuvC protein, we overproduced and highly purified the protein. By employing model substrates, we examined the possibility that RuvC protein is an endonuclease that resolves the Holliday structure, an intermediate in genetic recombination in which two double-stranded DNA molecules are linked by single-stranded crossover. RuvC protein cleaves cruciform junctions, which are formed by the extrusion of inverted repeat sequences from a supercoiled plasmid and which are structurally analogous to Holliday junctions, by introducing nicks into strands with the same polarity. The nicked ends are ligated by E.coli or T4 DNA ligases. Analysis of the cleavage sites suggests that DNA topology rather than a particular sequence determines the cleavage site. RuvC protein also cleaves Holliday junctions which are formed between gapped circular and linear duplex DNA by the function of RecA protein. However, it does not cleave a synthetic four-way junction that does not possess homology between arms. The active form of RuvC protein, as studied by gel filtration, is a dimer. This is mechanistically suited for an endonuclease involved in swapping DNA strands at the crossover junctions. From these properties of RuvC protein and the phenotypes of the ruvC mutants, we infer that RuvC protein is an endonuclease that resolves Holliday structures in vivo.     

Genetic recombination: recA protein promotes homologous pairing between duplex DNA molecules without strand unwinding.

The recA protein of Escherichia coli promotes pairing in vitro between covalent circular duplex DNA and homologous circular duplex DNA containing a single stranded region. We have used a filter binding assay to investigate the frequency of homologous pairing between gapped and intact duplex DNA when unwinding of the free 3' and 5' ends of the gapped molecules was blocked. In order to obtain DNA without free ends, the gapped DNA was treated with trimethylpsoralen and 360 nm light so as to introduce about 6 crosslinks per DNA molecule and the double stranded regions on either side of the gaps were then digested up to the first crosslinks with exonuclease III and lambda exonuclease. This treatment did not diminish the frequency of homologous pairing, an observation which is difficult to reconcile with models for recombination requiring strand unwinding before pairing.     

The tertiary structure of the four-way DNA junction affords protection against DNase I cleavage.

The accessibility of phosphodiester bonds in the DNA of four-way helical junctions has been probed with the nuclease DNase I. Regions of protection were observed on all four strands of the junctions, that tended to be longer on the strands that are exchanged between the coaxially stacked pairs of helices. The protected regions on the continuous strands of the stacked helices were not located exactly at the junction, but were displaced towards the 3' side of the strand. This is the region of backbone that becomes located in the major groove of the opposed helix in the non-crossed, right-handed structure for the junction, and might therefore be predicted to be protected against cleavage by an enzyme. However, the major grooves of the structure remain accessible to the much smaller probe dimethyl sulphate.