Potential Role for the BLM Helicase in Recombinational Repair via a Conserved Interaction with RAD51

Abstract

Bloom's syndrome (BS) is an autosomal recessive disorder that predisposes individuals to a wide range of cancers. The gene mutated in BS, BLM, encodes a member of the RecQ family of DNA helicases. The precise role played by these enzymes in the cell remains to be determined. However, genome-wide hyper-recombination is a feature of many RecQ helicase-deficient cells. In eukaryotes, a central step in homologous recombination is catalyzed by the RAD51 protein. In response to agents that induce DNA double-strand breaks, RAD51 accumulates in nuclear foci that are thought to correspond to sites of recombinational repair. Here, we report that purified BLM and human RAD51 interact in vitroand in vivo, and that residues in the N- and C-terminal domains of BLM can independently mediate this interaction. Consistent with these observations, BLM localizes to a subset of RAD51 nuclear foci in normal human cells. Moreover, the number of BLM foci and the extent to which BLM and RAD51 foci co-localize increase in response to ionizing radiation. Nevertheless, the formation of RAD51 foci does not require functional BLM. Indeed, in untreated BS cells, an abnormally high proportion of the cells contain RAD51 nuclear foci. Exogenous expression of BLM markedly reduces the fraction of cells containing RAD51 foci. The interaction between BLM and RAD51 appears to have been evolutionarily conserved since the C-terminal domain of Sgs1, theSaccharomyces cerevisiae homologue of BLM, interacts with yeast Rad51. Furthermore, genetic analysis reveals that theSGS1 and RAD51 genes are epistatic indicating that they operate in a common pathway. Potential roles for BLM in the RAD51 recombinational repair pathway are discussed. Bloom's syndrome (BS) is an autosomal recessive disorder that predisposes individuals to a wide range of cancers. The gene mutated in BS, BLM, encodes a member of the RecQ family of DNA helicases. The precise role played by these enzymes in the cell remains to be determined. However, genome-wide hyper-recombination is a feature of many RecQ helicase-deficient cells. In eukaryotes, a central step in homologous recombination is catalyzed by the RAD51 protein. In response to agents that induce DNA double-strand breaks, RAD51 accumulates in nuclear foci that are thought to correspond to sites of recombinational repair. Here, we report that purified BLM and human RAD51 interact in vitroand in vivo, and that residues in the N- and C-terminal domains of BLM can independently mediate this interaction. Consistent with these observations, BLM localizes to a subset of RAD51 nuclear foci in normal human cells. Moreover, the number of BLM foci and the extent to which BLM and RAD51 foci co-localize increase in response to ionizing radiation. Nevertheless, the formation of RAD51 foci does not require functional BLM. Indeed, in untreated BS cells, an abnormally high proportion of the cells contain RAD51 nuclear foci. Exogenous expression of BLM markedly reduces the fraction of cells containing RAD51 foci. The interaction between BLM and RAD51 appears to have been evolutionarily conserved since the C-terminal domain of Sgs1, theSaccharomyces cerevisiae homologue of BLM, interacts with yeast Rad51. Furthermore, genetic analysis reveals that theSGS1 and RAD51 genes are epistatic indicating that they operate in a common pathway. Potential roles for BLM in the RAD51 recombinational repair pathway are discussed. Bloom's syndrome glutathione S-transferase hydroxyurea homologous recombination methyl methanesulfonate sister chromatid exchange double-strand break polymerase chain reaction bromodeoxyuridine Germline mutations in the BLM gene give rise to Bloom's syndrome (BS),1 a rare disorder associated with stunted growth, facial sun sensitivity, immunodeficiency, fertility defects, and a greatly elevated increase in the occurrence of a wide range of cancers (1German J. Medicine. 1993; 72: 393-406Crossref PubMed Scopus (460) Google Scholar). BLM encodes a 159-kDa protein that is a member of the RecQ family of DNA helicases (2Ellis N.A. Groden J. Ye T.Z. Straughen J. Lennon D.J. Ciocci S. Proytcheva M. German J. Cell. 1995; 83: 655-666Abstract Full Text PDF PubMed Scopus (1221) Google Scholar). This highly conserved family of proteins is required for the maintenance of genomic stability in all organisms (3Karow J.K. Wu L. Hickson I.D. Curr. Opin. Genet. Dev. 2000; 10: 32-38Crossref PubMed Scopus (160) Google Scholar). In humans, five RecQ helicases have been identified. In addition to BLM, mutations in two other genes encoding RecQ helicases in humans have been associated with disease conditions: WRN andRECQ4, being defective in Werner's syndrome and Rothmund-Thomson syndrome, respectively (4Yu C. Oshima J. Fu Y. Wijsman E.M. Hisama F. Alisch R. Matthews S. Najura J. Miki T. Ouais S. Martin G.M. Mulligan J. Schellenberg G.D. Science. 1996; 272: 258-262Crossref PubMed Scopus (1496) Google Scholar, 5Kitao S. Shimamoto A. Goto M. Miller R.W. Smithson W.A. Lindor N.M. Furuichi Y. Nat. Genet. 1999; 22: 82-84Crossref PubMed Scopus (579) Google Scholar). Werner's syndrome is primarily associated with premature aging, and Rothmund-Thomson syndrome with skin and skeletal abnormalities, but both disorders also give rise to an elevated incidence of cancers (6Shen J.C. Loeb L.A. Trends Genet. 2000; 16: 213-220Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar, 7Vennos E.M. James W.D. Dermatol. Clin. 1995; 13: 143-150Abstract Full Text PDF PubMed Google Scholar). All RecQ family members contain a catalytic helicase domain that comprises seven highly conserved motifs found in many DNA and RNA helicases (3Karow J.K. Wu L. Hickson I.D. Curr. Opin. Genet. Dev. 2000; 10: 32-38Crossref PubMed Scopus (160) Google Scholar). Outside of this helicase domain, the RecQ family proteins show little sequence conservation. In BLM, these non-conserved domains are located both N- and C-terminal to the helicase domain and comprise ∼650 and 450 amino acids, respectively. It is likely that these non-conserved domains are important in functionally differentiating the roles of the different RecQ helicases within the cell by either providing additional enzymatic functions, such as the exonuclease activity dependent upon the N-terminal domain of WRN (8Huang S. Li B. Gray M.D. Oshima J. Mian I.S. Campisi J. Nat. Genet. 1998; 20: 114-116Crossref PubMed Scopus (376) Google Scholar, 9Shen J.C. Gray M.D. Oshima J. Kamath-Loeb A.S. Fry M. Loeb L.A. J. Biol. Chem. 1998; 273: 34139-34144Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar), or by mediating interactions with other proteins (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar). Cells from BS patients display genomic instability, the diagnostic feature being an increase in the frequency of sister chromatid exchanges (SCEs) (11German J. Crippa L.P. Bloom D. Chromosoma. 1974; 48: 361-366Crossref PubMed Scopus (109) Google Scholar). Sonoda et al. (12Sonoda E. Sasaki M.S. Morrison C. Yamaguchi-Iwai Y. Takata M. Takeda S. Mol. Cell. Biol. 1999; 19: 5166-5169Crossref PubMed Scopus (368) Google Scholar) recently demonstrated that SCE formation requires homologous recombination (HR). Moreover, chicken BLM −/−cells display elevated SCE levels that are partially dependent onRAD54 (13Wang W. Seki M. Narita Y. Sonoda E. Takeda S. Yamada K. Masuko T. Katada T. Enomoto T. EMBO J. 2000; 19: 3428-3435Crossref PubMed Scopus (126) Google Scholar). BLM therefore seems to function in the regulation of HR events during replication. Consistent with this notion, mutations in SGS1 orrqh1 + , the budding and fission yeast RecQ homologues, respectively, also give rise to excessive recombination events (14Gangloff S. McDonald J.P. Bendixen C. Arthur L. Rothstein R. Mol. Cell. Biol. 1994; 14: 8391-8398Crossref PubMed Scopus (617) Google Scholar, 15Watt P.M. Hickson I.D. Borts R.H. Louis E.J. Genetics. 1996; 144: 935-945Crossref PubMed Google Scholar, 16Stewart E. Chapman C.R. Al-Khodairy F. Carr A.M. Enoch T. EMBO J. 1997; 16: 2682-2692Crossref PubMed Scopus (329) Google Scholar). In rqh1 mutants, inhibition of DNA replication, in particular, stimulates this excessive recombination (16Stewart E. Chapman C.R. Al-Khodairy F. Carr A.M. Enoch T. EMBO J. 1997; 16: 2682-2692Crossref PubMed Scopus (329) Google Scholar). Further evidence of a role for RecQ helicases in HR comes from the finding that RecQ, Sgs1, BLM, and WRN can all disrupt four-way junctions, a structural mimic for the Holliday junction intermediate formed during HR (17Harmon F.G. Kowalczykowski S.C. Genes Dev. 1998; 12: 1134-1144Crossref PubMed Scopus (239) Google Scholar, 18Bennett R.J. Keck J.L. Wang J.C. J. Mol. Biol. 1999; 289: 235-248Crossref PubMed Scopus (114) Google Scholar, 19Karow J.K. Constantinou A. Li J.-L. West S.C. Hickson I.D. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 6504-6508Crossref PubMed Scopus (423) Google Scholar, 20Constantinou A. Tarsounas M. Karow J.K. Brosh R.M. Bohr V.A. Hickson I.D. West S.C. EMBO Rep. 2000; 1: 80-84Crossref PubMed Scopus (337) Google Scholar). Moreover, both BLM and WRN can promote branch migration of Holliday J.K. Constantinou A. Li J.-L. West S.C. Hickson I.D. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 6504-6508Crossref PubMed Scopus (423) Google Scholar, 20Constantinou A. Tarsounas M. Karow J.K. Brosh R.M. Bohr V.A. Hickson I.D. West S.C. EMBO Rep. 2000; 1: 80-84Crossref PubMed Scopus (337) Google Scholar). The for repair of DNA double-strand by HR been conserved in The central step the of the with homologous to the exchange of DNA In this is by which a DNA formed as a of of the to DNA This the for homologous and a within which DNA exchange In eukaryotes, the reaction is by which is to A. Science. 1993; PubMed Scopus Google Scholar, Science. 1994; PubMed Scopus Google Scholar, A. West S.C. EMBO J. 1994; 13: PubMed Scopus Google Scholar, West S.C. Cell. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). In the is a member of the display in and recombination and to ionizing the role that RAD51 in both HR and recombinational repair of DNA F. Mol. Biol. 1999; PubMed Google Scholar). RAD51 in have been by the that with a of gene during Mol. Cell. Biol. 1996; 16: PubMed Scopus Google Scholar, T. Y. K. Y. K. M. A. Y. T. Proc. Natl. Acad. Sci. U. S. A. 1996; PubMed Scopus Google Scholar). However, with that of cells to ionizing Mol. Cell. Biol. 1996; 16: PubMed Scopus Google Scholar). chicken cells, by expression of human RAD51 the of a of E. Sasaki M.S. A. H. Takata M. Yamaguchi-Iwai Y. Takeda S. EMBO J. 1998; PubMed Scopus Google Scholar). This a role for in the repair of DNA in cells, likely during DNA replication. of cells with agents that induce such as ionizing of to nuclear foci that are to correspond to in recombinational repair T. Proc. Natl. Acad. Sci. U. S. A. 1995; PubMed Scopus Google Scholar). However, the precise of these recombinational repair is In this we have the that the Bloom's syndrome gene in the RAD51 recombinational repair pathway the in recombination by BS cells. BLM and found to interact in and from nuclear Consistent with these BLM nuclear a subset of co-localize with Furthermore, the to which these two proteins co-localize to nuclear foci in response to ionizing radiation. The interaction between BLM and appears to have been evolutionarily conserved since with yeast and genetic analysis reveals that the genes encoding these two proteins are The normal human cell from as a normal human cell The cell is an cell from a BS from of and a in premature of the protein (2Ellis N.A. Groden J. Ye T.Z. Straughen J. Lennon D.J. Ciocci S. Proytcheva M. German J. Cell. 1995; 83: 655-666Abstract Full Text PDF PubMed Scopus (1221) Google Scholar). cells from a of cells with an (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar), to that BLM by this of the BS by SCE frequency analysis which that cells have a SCE and of these cells be S. et in All cell in with cells in with The from in the S. cerevisiae with as P.M. Hickson I.D. Borts R.H. Louis E.J. Genetics. 1996; 144: 935-945Crossref PubMed Google Scholar). The RAD51 with The in The of and been (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar). The for the expression of of that N- and C-terminal of BLM by and the seven and respectively, of The an additional and sites also the and respectively, to of the the the DNA domain the and the which the from R. A.S. J. R. Cell. 1993; 72: Full Text PDF PubMed Scopus Google Scholar) and by R. and R. are from and contain the of human and residues of respectively. of or SGS1 that C-terminal of protein by and and the seven and respectively, of The an additional and sites also the and respectively, to of the The S. cerevisiae RAD51 by from yeast genomic DNA and and sites that the and respectively. and have been (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar). a from West from BLM and proteins in yeast from the expression and (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar), respectively, and purified as J.K. Hickson I.D. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus (329) Google Scholar). a from in and purified from cells as (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar). as (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar) from cells or cells that been in for The in which and a of et al. D. A. R.H. 20: PubMed Scopus Google Scholar). for and being to containing from and of BLM and the (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar) and respectively, in cells or cells that been with or with for of nuclear foci from cells for the of sites of DNA and cells for in in in for with for by with Cells in containing for and for with and for with by a step of in in by a in in Cells in for by five in and with DNA by nuclear from cells for as (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar) with the that to the of BLM This as (10Wu L. Davies S.L. North P.S. Goulaouic H. Riou J.F. Turley H. Gatter K.C. Hickson I.D. J. Biol. Chem. 2000; 275: 9636-9644Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar) and either BLM, or containing N- or C-terminal of BLM. of yeast either or containing and for yeast or containing and the cell number a a of inhibition by the in cell number between the in the in a of two of The helicase domain in BLM, in common with the human WRN and is by N- and C-terminal domains (3Karow J.K. Wu L. Hickson I.D. Curr. Opin. Genet. Dev. 2000; 10: 32-38Crossref PubMed Scopus (160) Google Scholar) that are likely important in functionally these members of the RecQ helicase the role of these domains in BLM, we a yeast of a the C-terminal domain of BLM as independently found to The interaction since not interact with two proteins Further that the C-terminal amino of BLM to mediate an interaction with a of BLM been in the and interactions this in be by we analysis with purified BLM and to the proteins to interact with analysis that the proteins interact and and that the interaction since not interact with either of two proteins in this and and not containing of the C-terminal domain of BLM, the amino of BLM, as being to mediate an interaction with providing of the of the interaction domain BLM by the yeast of the N-terminal domain of BLM to analysis of purified and a containing residues of BLM is a the purified as the are the proteins as in the to and with or as The for the of an BLM but not a of BLM residues to analysis of purified BLM and to which BLM and with as and for the of by an a of an and the of protein The of BLM and the are the the of additional interaction domains BLM, analysis between and a protein containing the amino of BLM. also to interact with this protein indicating that BLM two domains with which can independently with and In to BLM, a purified which of residues of BLM and therefore both the N- and C-terminal interaction domains BLM, to This that additional of BLM interact with In response to DNA by ionizing localizes to nuclear foci that are thought to correspond to sites of repair T. Proc. Natl. Acad. Sci. U. S. A. 1995; PubMed Scopus Google Scholar). BLM also nuclear foci in normal cells that have been to correspond to nuclear A.M. D. T. J.F. J. Biol. 1999; PubMed Scopus Google Scholar, S. Ye R. N.A. 1999; PubMed Scopus Google Scholar, L. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: PubMed Scopus Google Scholar). In an of cells, a human cell from a normal the proportion of cells containing BLM foci and the number of BLM found to increase in response to ionizing therefore the that BLM co-localize with foci in response to ionizing radiation. In a of cells, of the cells a of BLM and and of these such that of the a of BLM and and the proportion of cells containing BLM and nuclear foci to the in the untreated However, the number of nuclear foci that in the untreated of the proportion of cells containing BLM and foci in or cells and of nuclear foci containing both BLM and in or cells containing BLM containing of BLM of of BLM and of BLM and in are the number of of in the range of the number of foci in in a in are the number of of in the range of the number of foci in The that BLM and interact in and co-localize to nuclear foci that these two proteins as of a in this we BLM and be from nuclear In from or cells, we to BLM not for this is that a of of these cells to contain BLM and foci therefore a that for the number of cells containing BLM and foci. BLM is and is highly in the and of the cell and is from cells S. M. R. J. B. C. M. 2000; 19: PubMed Scopus Google Scholar). of cells in in an increase in the number of cells containing BLM and foci a increase in the proportion of cells containing BLM and foci nuclear from these cells, we to BLM that BLM and as a in The precise by which nuclear foci is therefore BLM required for to to nuclear foci. In the of of cells, a cell from a BS nuclear foci In in an of cells, a from cells that the BLM of the cells nuclear foci proportion of nuclear cells also in cells In normal cells, RAD51 foci have been to in S. A. K. K. 1996; 12: Google Scholar). for the in the proportion of and cells containing foci be an cell However, cell analysis of and cells that the two cell of cells in and of the cell Furthermore, the high proportion of cells containing foci that foci can we and cells with with and that foci in cells in both and cells these that foci can in the of BLM and in BS cells, DNA are formed that the response in a high of foci. both RecQ helicases and have been conserved and that RecQ helicase in yeast and humans give rise to in genetic we the of Sgs1, the S. cerevisiae homologue of BLM, interacts with yeast in the Sgs1, BLM, also N- and C-terminal domains that the helicase domain (3Karow J.K. Wu L. Hickson I.D. Curr. Opin. Genet. Dev. 2000; 10: 32-38Crossref PubMed Scopus (160) Google Scholar). the C-terminal domains of and BLM little sequence the C-terminal domain of found to interact with yeast within this domain containing residues to mediate an interaction with yeast RAD51 a containing residues of not to interact with Rad51. This that amino residues of are important for the interaction with Rad51. the that the SGS1 genes are that either or are but are to agents that DNA or DNA In are to the hydroxyurea and the methyl methanesulfonate and as been found by other K. J. Shimamoto A. Goto M. Furuichi Y. H. Proc. Natl. Acad. Sci. U. S. A. 1998; PubMed Scopus Google Scholar, Genetics. 2000; PubMed Google Scholar). also a of to and and However, of both theSGS1 and RAD51 genes not to have or either or sensitivity, that these two genes are these that and function as a in a common pathway. The that give rise to BS of BS cells that either homologous recombination an frequency or that recombination events are In this we have that BLM, the of the gene defective in BS, a with the recombination protein This the between BLM and the pathway for HR in human cells. have of evidence that BLM and interact residues in the N- and C-terminal domains of BLM, with the finding that BLM and a in nuclear the of sequence between BLM and Sgs1, of the helicase domain, the C-terminal domain of found to interact with yeast indicating that the of functional domains within these two RecQ helicases been conserved to The of an interaction between RecQ helicases and RAD51 that these two of proteins a important role during DNA The finding that SGS1 and RAD51 are epistatic is in with the of et al. S. C. F. Nat. Genet. 2000; PubMed Scopus Google Scholar), demonstrated that of RAD51 can the of an of these is that RecQ helicases of which is with the enzymatic of these two of RAD51 the of the of homologous and exchange of DNA to a Holliday junction recombination intermediate Science. 1994; PubMed Scopus Google Scholar, West S.C. Cell. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). RecQ BLM and Sgs1, can to and disrupt such recombination (17Harmon F.G. Kowalczykowski S.C. Genes Dev. 1998; 12: 1134-1144Crossref PubMed Scopus (239) Google Scholar, 18Bennett R.J. Keck J.L. Wang J.C. J. Mol. Biol. 1999; 289: 235-248Crossref PubMed Scopus (114) Google Scholar, 19Karow J.K. Constantinou A. Li J.-L. West S.C. Hickson I.D. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 6504-6508Crossref PubMed Scopus (423) Google Scholar, 20Constantinou A. Tarsounas M. Karow J.K. Brosh R.M. Bohr V.A. Hickson I.D. West S.C. EMBO Rep. 2000; 1: 80-84Crossref PubMed Scopus (337) Google Scholar). we have that BLM can promote branch migration of Holliday J.K. Constantinou A. Li J.-L. West S.C. Hickson I.D. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 6504-6508Crossref PubMed Scopus (423) Google Scholar). an activity either promote the formation of of DNA or recombination the in which the Holliday junction is but formation have been to be a feature of the E. RecQ protein (17Harmon F.G. Kowalczykowski S.C. Genes Dev. 1998; 12: 1134-1144Crossref PubMed Scopus (239) Google Scholar). The interaction between and BLM therefore to BLM to sites of recombinational repair. the role of BLM is to disrupt recombination by branch of BLM give rise to excessive the normal role of BLM is to promote branch the of BLM to or of Holliday events give rise to the in BS cells. It therefore be of to is the branch migration activity catalyzed by BLM in is that BLM Holliday in a that the of junction The response pathway to nuclear formation is in BS cells, but can be by expression of BLM. both that BLM is not to induce the response and that a of in the of BLM. This is with the that BS cells are not to ionizing M. M.S. J.F. Google Scholar, Cell. 1993; Full Text PDF PubMed Scopus Google Scholar) and that the for the of are in BS cells. cells not to ionizing S. C. F. Nat. Genet. 2000; PubMed Scopus Google Scholar). However, in DNA be either the sister chromatid or the homologous of S. C. F. Nat. Genet. 2000; PubMed Scopus Google Scholar), in the HR pathway for repair of DNA all foci levels of BLM, that BLM upon a subset of DNA by is that BLM that since BLM been to co-localize to foci Davies Hickson Biol. in Scholar). a role be with the expression of BLM the cell and the in BS cells. DNA during for the a DNA in the DNA The formation of a in of the Indeed, been that such events cell in the of DNA D.J. 2000; PubMed Scopus Google Scholar, S.C. Trends Sci. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar, Trends Sci. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). The by which is formation of a remains to be However, is that is to recombination D.J. 2000; PubMed Scopus Google Scholar, S.C. Trends Sci. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar, Trends Sci. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). In of the of the the to a is thought to a for the of by a pathway the helicase Trends Sci. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). In eukaryotes, RAD51 a in to The Holliday junction to be that BLM to this which be by either branch the junction the of the or of the junction of Holliday junction In BS cells show DNA recombination defects, and we have an evolutionarily conserved between RecQ helicases and that BLM with the pathway of Further purified BLM and in addition to other of the recombinational repair we the role that BLM during this conserved and recombination can to of the and members of the for