Magnesium Ion-dependent Activation of the RecA Protein Involves the C Terminus

Optimal conditions for RecA protein-mediated DNA strand exchange include 6–8 mm Mg2+ in excess of that required to form complexes with ATP. We provide evidence that the free magnesium ion is required to mediate a conformational change in the RecA protein C terminus that activates RecA-mediated DNA strand exchange. In particular, a “closed” (low Mg2+) conformation of a RecA nucleoprotein filament restricts DNA pairing by incoming duplex DNA, although single-stranded overhangs at the ends of a duplex allow limited DNA pairing to occur. The addition of excess Mg2+ results in an “open” conformation, which can promote efficient DNA pairing and strand exchange regardless of DNA end structure. The removal of 17 amino acid residues at theEscherichia coli RecA C terminus eliminates a measurable requirement for excess Mg2+ and permits efficient DNA pairing and exchange similar to that seen with the wild-type protein at high Mg2+ levels. Thus, the RecA C terminus imposes the need for the high magnesium ion concentrations requisite in RecA reactions in vitro. We propose that the C terminus acts as a regulatory switch, modulating the access of double-stranded DNA to the presynaptic filament and thereby inhibiting homologous DNA pairing and strand exchange at low magnesium ion concentrations. Optimal conditions for RecA protein-mediated DNA strand exchange include 6–8 mm Mg2+ in excess of that required to form complexes with ATP. We provide evidence that the free magnesium ion is required to mediate a conformational change in the RecA protein C terminus that activates RecA-mediated DNA strand exchange. In particular, a “closed” (low Mg2+) conformation of a RecA nucleoprotein filament restricts DNA pairing by incoming duplex DNA, although single-stranded overhangs at the ends of a duplex allow limited DNA pairing to occur. The addition of excess Mg2+ results in an “open” conformation, which can promote efficient DNA pairing and strand exchange regardless of DNA end structure. The removal of 17 amino acid residues at theEscherichia coli RecA C terminus eliminates a measurable requirement for excess Mg2+ and permits efficient DNA pairing and exchange similar to that seen with the wild-type protein at high Mg2+ levels. Thus, the RecA C terminus imposes the need for the high magnesium ion concentrations requisite in RecA reactions in vitro. We propose that the C terminus acts as a regulatory switch, modulating the access of double-stranded DNA to the presynaptic filament and thereby inhibiting homologous DNA pairing and strand exchange at low magnesium ion concentrations. single-stranded DNA double-stranded DNA adenosine 5′-O-(thiotriphosphate) single-stranded DNA-binding protein 4-morpholineethanesulfonic acid The RecA protein of Escherichia coli plays a central role in the processes of homologous DNA recombination and DNA repair. RecA is a DNA-dependent ATPase that catalyzes an in vitro DNA strand exchange reaction between single-stranded (ssDNA)1 and homologous double-stranded DNA (dsDNA) molecules. The DNA strand exchange reaction takes place in several stages (Fig. 1). The RecA protein forms a nucleoprotein filament that completely encompasses the circular ssDNA. This filament then aligns the bound single strand with a homologous duplex DNA to form a DNA pairing intermediate often referred to as a joint molecule. 1000 base pairs of DNA can be aligned and exchanged in a joint molecule under the empirically defined optimal reaction conditions, which typically include 1–3 mm ATP and about 10 mm magnesium ion. All steps to this point, including the formation of joint molecules, require ATP but not ATP hydrolysis. ATP hydrolysis is needed only to complete the late stages of strand exchange of long DNA substrates, often derived from bacteriophage DNAs. Whereas DNA pairing, leading to joint molecule formation, can occur at either end of a linear duplex, the subsequent and ATP hydrolysis-dependent extension of the nascently paired regions is unidirectional, proceeding 5′ to 3′ relative to ssDNA initially bound in the filament. Thus, exchange proceeds in one direction along the linear duplex, and joints formed at the “wrong” end in the pairing phase are eliminated. In a DNA strand exchange involving quite long DNAs that leads to nicked circular product formation, the ends of the duplex where the exchange begins and ends are referred to as proximal and distal, respectively (Fig.1) (1Lusetti S.L. Cox M.M. Annu. Rev. Biochem. 2002; 71: 71-100Crossref PubMed Scopus (356) Google Scholar, 2Cox M.M. Prog. Nucleic Acids Res. Mol. Biol. 1999; 63: 310-366Google Scholar). Examination of the conditions for an optimal RecA protein-catalyzed DNA strand exchange reaction reveals an unexplored enigma. Magnesium forms a relatively strong 1:1 complex with ATP (3Phillips R.C. George P. Rutman R.J. J. Am. Chem. Soc. 1966; 88: 2631-2640Crossref PubMed Scopus (152) Google Scholar) and should be required at concentrations equal to the ATP added to the reaction. However, optimal rates and yields in RecA-mediated DNA strand exchange require an additional 6–8 mm of “free” Mg2+ (4Cox M.M. Lehman I.R. J. Biol. Chem. 1982; 257: 8523-8532Abstract Full Text PDF PubMed Google Scholar, 5Shibata T. Das Gupta C. Cunningham R.P. Radding C.M. Proc. Natl. Acad. Sci. U. S. A. 1979; 76: 1638-1642Crossref PubMed Scopus (279) Google Scholar, 6Roman L.J. Kowalczykowski S.C. Biochemistry. 1986; 25: 7375-7385Crossref PubMed Scopus (44) Google Scholar). Some of this magnesium ion is associated with the DNA, but the DNA concentration in most experiments is on the order of a few (1–20) μm in total nucleotides (and backbone phosphate) and thus could not complex more than a small fraction of the available magnesium ion. Lower magnesium concentrations, more stoichiometric with the added ATP, are sufficient for primary DNA binding. Some homologous DNA pairing occurs in the presence of ATPγS, an analog that is not appreciably hydrolyzed, but the higher magnesium ion concentrations are required for the generation of long hybrid DNA products with ATP (7Honigberg S.M. Gonda D.K. Flory J. Radding C.M. J. Biol. Chem. 1985; 260: 11845-11851Abstract Full Text PDF PubMed Google Scholar). The extra Mg2+ is contributing something significant to the reaction, but the effects have not been explained. The question is intriguing, since the free magnesium ion requirements for RecA protein-mediated DNA strand exchange in vitroappear to exceed what is available in vivo. Mg2+is at about mm in a A. J. 1985; PubMed Google Scholar, J. PubMed Google Scholar, R.J. J. Biol. Chem. Full Text PDF PubMed Google but of this is bound in and The of free magnesium ion is to be on the order of mm at most T. A. J. 1985; PubMed Google Scholar). on in vitro the RecA protein should be in the although the of the effects of low available Mg2+ Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). is a requirement for magnesium ion in the presence of as to the of the that an conformation of RecA protein can be by either excess magnesium ion the presence of at low magnesium Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). The RecA protein a small from to the protein terminus at the of which not been (1Lusetti S.L. Cox M.M. Annu. Rev. Biochem. 2002; 71: 71-100Crossref PubMed Scopus (356) Google Scholar, S.C. Annu. Rev. Biochem. 63: PubMed Scopus Google Scholar). The amino acid residues of the RecA protein are in the RecA protein PubMed Scopus Google Scholar, PubMed Scopus Google Scholar, S. M.M. Nucleic Acids Res. PubMed Google Scholar). We to this as the RecA C terminus to the this of amino acid residues are In addition to a of about the RecA C been that this of the protein of the coli RecA protein have been from the C terminus of either amino acid residues S. T. T. J. Mol. Biol. PubMed Scopus Google a about of the RecA R.C. Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google in leading to filament formation on that the could be to the of between the residues in the C terminus and the in the DNA S. T. T. J. Mol. Biol. PubMed Scopus Google Scholar, R.C. Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). DNA strand exchange under at conditions, and the an DNA strand exchange in the of single-stranded DNA-binding protein S. T. T. J. Mol. Biol. PubMed Scopus Google Scholar, R.C. Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). The ATPase and ssDNA similar to wild-type R.C. Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). The bound to ssDNA more in S. T. T. J. Mol. Biol. PubMed Scopus Google Scholar). of the RecA protein of to P. J. Biol. Chem. 1985; 260: Full Text PDF PubMed Google Scholar). of RecA protein have been and but that most of the with wild-type with DNA and Mol. 1982; PubMed Scopus Google Scholar). not to of the on Mol. PubMed Scopus Google Scholar). small on recombination the and wild-type in Mol. PubMed Scopus Google Scholar). of residues from the C terminus in a conformational change in RecA bound to an between the C terminus and the RecA J. Biol. PubMed Scopus Google Scholar). In the S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google a of RecA to the role of the amino in the C terminus of the RecA of the 17 amino acid residues the of to The a on the of RecA DNA strand exchange reactions relative to the wild-type in the and in the protein to to the reaction In the that the C terminus of the RecA protein a modulating on the DNA strand exchange of RecA In the of free magnesium the C terminus the protein in a conformation in which the of DNA strand exchange is the free magnesium ion concentration is to 6–8 the DNA strand exchange of RecA is The results that magnesium ion with RecA the conformation of the C The wild-type and as S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the The RecA protein and as for the wild-type RecA protein S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). The concentrations of the RecA from the at the of J. Biol. Chem. Full Text PDF PubMed Google Scholar). from The concentration of the protein from the at the of J. Biol. Chem. 1985; 260: Full Text PDF PubMed Google Scholar). from from from ATP, and from and from single-stranded and circular duplex DNAs from bacteriophage Scholar, J. PubMed Scopus Google Scholar, Cox M.M. J. Biol. Chem. 1986; Full Text PDF PubMed Google Scholar). where linear duplex DNA by the of bacteriophage DNA with the conditions by the The DNA with by The concentrations of ssDNA and by at and as All DNA concentrations are in μm exchange reactions in mm mm and the addition of reaction mm and the concentration of an ATP of 10 and All are at The wild-type with μm circular ssDNA for 10 protein and the of ATP then by The reactions by the addition of linear to to as a point, the reaction and at the and the reaction by the addition of of a mm and to at in with mm and mm with and to with a DNA with the from The of this to the magnesium ion of DNA strand exchange reactions by RecA We on the and in the S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the reaction conditions for DNA strand exchange by wild-type RecA the promote a reaction S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). We to reaction conditions that the of the protein-catalyzed DNA strand exchange in an to the role of the C terminus in the DNA strand exchange reaction. In an to the optimal strand exchange conditions for the magnesium DNA strand exchange reactions in at mm ATP and magnesium ion concentrations from to mm (Fig. the of strand exchange to an and then as the concentration of magnesium ion is the wild-type RecA this occurs at about 10 mm magnesium in with results a of (4Cox M.M. Lehman I.R. J. Biol. Chem. 1982; 257: 8523-8532Abstract Full Text PDF PubMed Google Scholar, 5Shibata T. Das Gupta C. Cunningham R.P. Radding C.M. Proc. Natl. Acad. Sci. U. S. A. 1979; 76: 1638-1642Crossref PubMed Scopus (279) Google Scholar, 6Roman L.J. Kowalczykowski S.C. Biochemistry. 1986; 25: 7375-7385Crossref PubMed Scopus (44) Google Scholar). the 17 amino optimal at magnesium ion concentrations that are to the ATP magnesium ion concentrations are The optimal magnesium ion concentrations for the and between the that the magnesium ion requirements for the reaction as more of the C terminus is reactions are at strand exchange reactions to similar for of the wild-type and RecA as the magnesium ion concentration needed for an optimal reaction as the C terminus of RecA is the magnesium ion concentrations needed to of the strand exchange reaction magnesium ion concentrations strand exchange products by the wild-type RecA protein include DNA complexes that not the complexes are not are not in and at magnesium ion concentrations for the The of DNA strand exchange products by protein at magnesium ion concentrations between and 10 mm (Fig. The products in this include the to the complex in the joint molecules, and nicked circular products of DNA strand exchange. and by the of the linear in the The at the in the are and are needed to for of the DNA in the The magnesium ion concentration required for of DNA products is mm for wild-type as been by several (4Cox M.M. Lehman I.R. J. Biol. Chem. 1982; 257: 8523-8532Abstract Full Text PDF PubMed Google Scholar, 5Shibata T. Das Gupta C. Cunningham R.P. Radding C.M. Proc. Natl. Acad. Sci. U. S. A. 1979; 76: 1638-1642Crossref PubMed Scopus (279) Google Scholar, 6Roman L.J. Kowalczykowski S.C. Biochemistry. 1986; 25: 7375-7385Crossref PubMed Scopus (44) Google Scholar). amino acid residues are from the the promote an optimal DNA strand exchange reaction at concentrations of magnesium ion (Fig. In order to the optimal strand exchange conditions on the ATP strand exchange reactions at several ATP concentrations and with the reaction as a of magnesium either mm ATP the an optimal strand exchange reaction at a magnesium equal to the ATP. at mm ATP, wild-type RecA protein 10 mm magnesium ion and an higher magnesium ion concentration mm ATP is The reactions that only the nicked circular product of strand exchange as a of the duplex DNA is that the wild-type RecA protein excess what is needed to complex with ATP. the ATP concentration the concentration of magnesium ion needed for an optimal reaction at 6–8 mm the ATP the 17 amino are from the the optimal DNA strand exchange reaction is at a magnesium ion concentration that the concentration of effects of on DNA strand as a of ATP by wild-type RecA and the are as a of magnesium and ATP The as under The of duplex to reaction product circular at is magnesium ion concentration for the wild-type RecA and mm ATP from mm ATP from mm ATP from is in the DNA strand exchange of the results in with a from P. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google in which the wild-type RecA protein the formation of significant of nicked circular product in DNA strand exchange reactions a of mm the optimal reaction seen with the reaction significant but at in of the the ATP concentration This to not only the results but results that wild-type RecA protein not promote DNA strand exchange reactions at low magnesium ion concentrations (7Honigberg S.M. Gonda D.K. Flory J. Radding C.M. J. Biol. Chem. 1985; 260: 11845-11851Abstract Full Text PDF PubMed Google Scholar). We this in The DNA strand exchange reaction results in this The DNA in the derived from and the linear by of circular with the P. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). This 3′ overhangs at the DNA which are from the 5′ overhangs in the duplex DNA in the overhangs a role in the reaction, DNA strand exchange reactions with linear by circular with the to 3′ 5′ respectively (Fig. and 10 mm magnesium the results of wild-type RecA reactions the linear ends are at mm magnesium of the nicked circular product of strand exchange can be seen only 3′ overhangs are We that the wild-type reaction with the DNA in more reaction than the DNA, although not to The 5′ overhangs be to the circular and thus DNA pairing, at the end of the linear The protein to not require regardless of the magnesium ion DNA strand exchange long DNA not the formation of joint ATP is not However, the of DNA pairing in the presence of should occur in a similar to the more DNA strand exchange. is the for the DNA strand exchange reaction with ATP, the optimal conditions for RecA formation of joint molecule in the presence of include magnesium ion concentrations in significant excess relative to the that is Kowalczykowski S.C. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). We the of DNA overhangs on wild-type RecA and DNA strand exchange The linear for with mm and either 10 mm a magnesium ion concentration of 10 joint formed by wild-type the linear are However, at mm of joint molecule products can be seen only 3′ 5′ overhangs are The is that a single-stranded is needed for reactions to be with the wild-type protein in the of free magnesium ion. The reaction seen with 5′ overhangs is with the of in the pairing reactions by RecA on the end of the linear duplex DNA be ATP hydrolyzed, as in In to not require end at mm but is to form joint at 10 mm The of joint in this reaction at the higher concentration of magnesium although a more complete DNA strand exchange reaction is seen under conditions, is under In an S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) the of the RecA that the in to nicked circular products in DNA strand exchange reactions at In of the results that the optimal magnesium ion concentration for the reaction is the magnesium ion concentrations in DNA strand exchange reactions with ATP at (Fig. The protein is to promote a complete strand exchange reaction with ATP promote joint molecule formation with at at mm magnesium ion. Thus, the effects of magnesium ion are from the effects S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google at by this However, the wild-type RecA reaction at low magnesium is by the to the residues of the RecA C terminus are contributing to the requirement for excess magnesium in wild-type RecA-mediated DNA strand exchange a that the at with a in the protein that is in the but is in the DNA strand exchange reactions with wild-type RecA and the RecA at mm ATP and magnesium ion concentrations from to mm (Fig. The the strand exchange reaction at magnesium ion concentrations than the wild-type the for the that the optimal reaction occurs for the at about similar to that of This with one of the residues in the C terminus to a thus an similar to the of residues from the C The primary of this is that the C terminus of the RecA protein the DNA strand exchange The 17 amino acid residues are for the requirement for excess magnesium that to form complexes with ATP, in a wild-type RecA DNA strand exchange reaction in vitro M.M. Lehman I.R. J. Biol. Chem. Full Text PDF PubMed Google Scholar, J. Gonda C. S. Radding C.M. Biol. PubMed Google Scholar, Radding C.M. Biochemistry. 1985; PubMed Scopus Google Scholar, J. Cox M.M. Biochemistry. 1986; 25: PubMed Scopus Google Scholar). of residues eliminates the measurable requirement for the excess results that the RecA C terminus a regulatory role in RecA protein low magnesium ion concentrations, the C terminus access of duplex DNA ends to the RecA nucleoprotein although single single strand of duplex can a limited DNA pairing reaction. in the protein a for DNA strand exchange in the presence of excess magnesium ion. the excess Mg2+ is for the that a conformational change involving the C terminus is a of the The primary of the C terminus for with magnesium ion. to the magnesium ion could be by one more of the residues and in one more of the residues and in and in We that the 17 amino acid residues of RecA protein include and is that could be in magnesium ion Biochemistry. PubMed Scopus Google Scholar, A. S. PubMed Scopus Google Scholar). of is in of the and acid However, the need not be in the C the not Mg2+ at the C only that Mg2+ the conformation of the C terminus as to about for DNA strand exchange. and have that the of the bound at the ATP can have a on the conformation of the of RecA S. C. Full Text Full Text PDF PubMed Scopus Google Scholar, S.C. T. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). the of Mg2+ to a in the in the could a conformational that the of the C In an S.L. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google that high Mg2+ have a significant on of the The results are with a that acts as a protein to access of the duplex DNA to the filament This as a of the the conformational change by the of magnesium ion. the could conformational in of the protein that of the C terminus on the conformation of the RecA protein been by and J. Biol. PubMed Scopus Google Scholar, S. C. Full Text Full Text PDF PubMed Scopus Google Scholar). The wild-type RecA protein is in homologous pairing of duplex DNA at low magnesium in the presence of ATP. pairing can be at low magnesium either the 17 residues of the protein are as are added to the reaction Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). the of DNA pairing can be to a is of ATP, is a single strand on the duplex DNA the is to the results that the C terminus is to DNA pairing and strand exchange at low We propose that the of RecA bound to ssDNA at low Mg2+ Mg2+ with the ATP be The of RecA on single-stranded DNA in the presence of ATP been referred to as Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar, J. Mol. Biol. PubMed Scopus Google Scholar, Google the The at low Mg2+is relatively to with incoming duplex DNA, the change in is needed to DNA pairing and strand exchange. In most with the coli RecA the change is about by Mg2+ in excess to the ATP We propose that the be the to that the protein is to DNA pairing with duplex DNA regardless of end structure. is that in the presence of ATP. and J. Biol. PubMed Scopus Google Scholar, S. C. Full Text Full Text PDF PubMed Scopus Google Scholar, S.C. T. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar) have that the hydrolysis of ATP can about a of conformational in the of the can ATP with equal as The of DNA pairing by overhangs under the low Mg2+ conditions that the to DNA pairing is for ssDNA relative to duplex single at the end of a duplex can provide sufficient of the pairing to the and allow DNA pairing to occur. Magnesium ion could DNA strand exchange at but is the stages the formation of nucleoprotein that to be most have that the wild-type RecA protein not require excess magnesium ion to to ssDNA. The rates of ATP hydrolysis by the wild-type RecA protein with mm magnesium ion and mm ATP and protein are to the rates at 10 mm magnesium in the presence of protein S.C. J. A. J. Mol. Biol. PubMed Scopus Google Scholar, Cox M.M. Biochemistry. PubMed Scopus Google Scholar). the in ssDNA and ATP the between the and be The is that the low Mg2+ conditions not of a DNA strand exchange reaction. is additional evidence in the that RecA nucleoprotein have been in the in the presence of mm the of to be relative to duplex DNA in the presence of mm (7Honigberg S.M. Gonda D.K. Flory J. Radding C.M. J. Biol. Chem. 1985; 260: 11845-11851Abstract Full Text PDF PubMed Google Scholar) and relative to duplex DNA in the presence of mm ATP J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google which to filament extension than the extension the are formed with 10 mm magnesium ion S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, A. J. Mol. Biol. 1986; PubMed Scopus Google J. Mol. Biol. PubMed Scopus Google Scholar). is that this filament extension seen at low Mg2+ concentrations is a of an conformation of the C Mg2+ in the formation of RecA filament by A. J. Mol. Biol. 1986; PubMed Scopus Google Scholar, A. J. Mol. Biol. PubMed Scopus Google Scholar, J. Mol. Biol. PubMed Scopus Google Scholar). with RecA protein by S.C. J. Mol. Biol. PubMed Scopus Google Scholar). The protein a reaction in DNA strand exchange reactions S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). We have that this is not by at the protein is in homologous pairing with and in the formation of nicked circular product with ATP at low (Fig. high magnesium S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the from this and the S.L. J. Cox M.M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) to the protein for the of the of RecA We propose that the amino acid residues of wild-type RecA protein are not only to the primary of RecA protein to a filament on S. T. T. J. Mol. Biol. PubMed Scopus Google Scholar, R.C. Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar) but to DNA pairing of duplex DNA with a single strand bound a RecA filament. be quite since the to of RecA protein to a duplex DNA should be from the of a duplex DNA to a RecA filament formed on a single The to of RecA protein to is at with wild-type the of one more residues in the C but is not by excess magnesium ion at This that the and magnesium in protein are to the C terminus in Kowalczykowski and R.C. Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar) that the a role in modulating DNA and strand exchange. However, more to an of DNA strand exchange under conditions similar to in which the the of strand exchange. and S. T. A. Radding C.M. T. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) have a for homologous pairing in which the for to the presynaptic filament in the filament that is on one the of the RecA of of the residues in this have been to homologous The residues include S. A. T. Biochem. 1999; PubMed Scopus Google Scholar, S. T. PubMed Scopus Google and S. T. A. Radding C.M. T. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) (Fig. residues of the of the RecA protein seen in the PubMed Scopus Google Scholar, PubMed Scopus Google Scholar). are residues the that have in the that in is that the of the C terminus can form with residues in this thereby the access of to the presynaptic filament. be of a of to the in the requirement for magnesium seen with the removal in the of the C could the of the a the effects of the RecA which a similar to although that is not in the with Magnesium could by homologous In this the of the RecA protein could form a that to the nucleoprotein filament. The of protein to DNA is a by DNA-binding and been Biol. 2002; PubMed Scopus Google Scholar). in the the of free in the are to about the seen in vitro. is that the in the for the effects of high Mg2+ Kowalczykowski S.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). molecule Mg2+ in the as an We and for