Human Topoisomerase IIIα Is a Single-stranded DNA Decatenase That Is Stimulated by BLM and RMI1

Human topoisomerase IIIα is a type IA DNA topoisomerase that functions with BLM and RMI1 to resolve DNA replication and recombination intermediates. BLM, human topoisomerase IIIα, and RMI1 catalyze the dissolution of double Holliday junctions into noncrossover products via a strand-passage mechanism. We generated single-stranded catenanes that resemble the proposed dissolution intermediate recognized by human topoisomerase IIIα. We demonstrate that human topoisomerase IIIα is a single-stranded DNA decatenase that is specifically stimulated by the BLM-RMI1 pair. In addition, RMI1 interacts with human topoisomerase IIIα, and the interaction is required for the stimulatory effect of RMI1 on decatenase activity. Our data provide direct evidence that human topoisomerase IIIα functions as a decatenase with the assistance of BLM and RMI1 to facilitate the processing of homologous recombination intermediates without crossing over as a mechanism to preserve genome integrity. Human topoisomerase IIIα is a type IA DNA topoisomerase that functions with BLM and RMI1 to resolve DNA replication and recombination intermediates. BLM, human topoisomerase IIIα, and RMI1 catalyze the dissolution of double Holliday junctions into noncrossover products via a strand-passage mechanism. We generated single-stranded catenanes that resemble the proposed dissolution intermediate recognized by human topoisomerase IIIα. We demonstrate that human topoisomerase IIIα is a single-stranded DNA decatenase that is specifically stimulated by the BLM-RMI1 pair. In addition, RMI1 interacts with human topoisomerase IIIα, and the interaction is required for the stimulatory effect of RMI1 on decatenase activity. Our data provide direct evidence that human topoisomerase IIIα functions as a decatenase with the assistance of BLM and RMI1 to facilitate the processing of homologous recombination intermediates without crossing over as a mechanism to preserve genome integrity. IntroductionTopoisomerases are ubiquitous enzymes conserved from bacteria to humans. Their roles in modulating DNA topology in replication, transcription, and other cellular processes (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar, 2Wang J.C. Nat. Rev. Mol. Cell Biol. 2002; 3: 430-440Crossref PubMed Scopus (1870) Google Scholar) make them indispensable for cell viability. There are four subfamilies of topoisomerases as follows: IA, IB, IIA, and IIB. Type IA topoisomerases change DNA topological states in discrete steps of one via an enzyme-bridging mechanism (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar, 2Wang J.C. Nat. Rev. Mol. Cell Biol. 2002; 3: 430-440Crossref PubMed Scopus (1870) Google Scholar). The catalytic tyrosine residue initiates a transesterification reaction in a single-stranded region to generate a transient DNA break, allowing for the passage of the intact strand through the break. After religation of the broken strand by a reversal of the reaction, the enzyme is free to engage in another round of catalysis (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar, 2Wang J.C. Nat. Rev. Mol. Cell Biol. 2002; 3: 430-440Crossref PubMed Scopus (1870) Google Scholar). Members of the type IA topoisomerase family include Escherichia coli topoisomerase I (EcTop1) and III (EcTop3), yeast topoisomerase III (Top3), and two isoforms of topoisomerase III, α (Topo 2The abbreviations used are: TopotopoisomeraseDHJdouble Holliday junctionDTTdithiothreitolBSAbovine serum albuminhTopohuman topoisomeraseBSBloom syndromeSCEsister chromatid exchangeoligooligonucleotideGSTglutathione S-transferase. IIIα) and β (Topo IIIβ), in higher eukaryotes. These enzymes exhibit high sequence similarity in the N-terminal catalytic core domain, whereas the C-terminal tails are variable (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar). In addition to the ability to relax negatively supercoiled DNA, EcTop1 is capable of catalyzing knotting, unknotting, and interlinking of DNA substrates that contain exposed single-stranded regions (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar). Because single-stranded DNA gaps are a common feature found at DNA replication forks, replication termination sites, and replication and repair sites, it is believed that the main function of type IA topoisomerases is to unlink DNA catenanes.Type IA topoisomerases function in concert with RecQ helicases to control recombination events (3Mankouri H.W. Hickson I.D. Trends Biochem. Sci. 2007; 32: 538-546Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar). RecQ helicases are a highly conserved family of DNA helicases that are required for the maintenance of genome integrity (4Hickson I.D. Nat. Rev. Cancer. 2003; 3: 169-178Crossref PubMed Scopus (576) Google Scholar). Human topoisomerase IIIα (hTopo IIIα) physically interacts with BLM, one of the five RecQ helicases in humans (5Wu 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 (281) Google Scholar, 6Johnson F.B. Lombard D.B. Neff N.F. Mastrangelo M.A. Dewolf W. Ellis N.A. Marciniak R.A. Yin Y. Jaenisch R. Guarente L. Cancer Res. 2000; 60: 1162-1167PubMed Google Scholar). Biallelic mutations of BLM give rise to a clinically defined cancer predisposition disorder, Bloom syndrome (BS) (7German J. Dermatol. Clin. 1995; 13: 7-18Abstract Full Text PDF PubMed Google Scholar). BS cells display signs of genome instability, featuring an ∼10-fold elevation in the frequency of sister chromatid exchanges (SCE) (8Chaganti R.S. Schonberg S. German J. Proc. Natl. Acad. Sci. U.S.A. 1974; 71: 4508-4512Crossref PubMed Scopus (778) Google Scholar), events that arise from the processing of recombination intermediates (9Sonoda E. Sasaki M.S. Morrison C. Yamaguchi-Iwai Y. Takata M. Takeda S. Mol. Cell. Biol. 1999; 19: 5166-5169Crossref PubMed Scopus (365) Google Scholar). The hTopo IIIα interacting domain of BLM is required for suppression of SCE in BS cells (10Hu P. Beresten S.F. van Brabant A.J. Ye T.Z. Pandolfi P.P. Johnson F.B. Guarente L. Ellis N.A. Hum. Mol. Genet. 2001; 10: 1287-1298Crossref PubMed Google Scholar), suggesting that hTopo IIIα plays an anti-recombination role with BLM. Indeed, in vitro biochemical data show that BLM and hTopo IIIα catalyze the dissolution of double Holliday junctions (DHJs), a DNA structure that can arise as an intermediate during homologous recombination (11Wu L. Hickson I.D. Nature. 2003; 426: 870-874Crossref PubMed Scopus (868) Google Scholar). Dissolution occurs via a strand-passage mechanism that prevents genetic exchange between flanking sequences and is presumed to mimic the in vivo role of BLM-hTopo IIIα in suppressing SCE (11Wu L. Hickson I.D. Nature. 2003; 426: 870-874Crossref PubMed Scopus (868) Google Scholar). In the simplest case, the dissolution reaction is believed to have two components as follows: the helicase activity of BLM catalyzes branch migration of the Holliday junctions toward each other, resulting in collapse of the Holliday junctions, and generation of two duplex DNAs interlinked via catenated single strands. This structure, termed a hemicatenane, is then decatenated by hTopo IIIα to complete the dissolution of the DHJ (11Wu L. Hickson I.D. Nature. 2003; 426: 870-874Crossref PubMed Scopus (868) Google Scholar, 12Plank J.L. Wu J. Hsieh T.S. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 11118-11123Crossref PubMed Scopus (124) Google Scholar). Direct evidence that hTopo IIIα possesses the relevant decatenase is helicases and type IA topoisomerases to resolve replication with RecQ and the single-stranded catalyzes the of replication intermediates in vitro C. Mol. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). catalyzes of DNA during and replication in vitro H. J. Biol. Chem. Full Text PDF PubMed Google Scholar). have of evidence that topoisomerase III functions in a is required for C. Hickson I.D. Res. 1999; PubMed Scopus (98) Google Scholar). IIIα in cells of cells with gaps and M. S. Y. M. H. Y. Mol. Cell. Biol. 2006; PubMed Scopus Google Scholar). hTopo IIIα to DNA in a North P.S. Hickson I.D. J. 2007; PubMed Scopus Google Scholar). In each of the of III to and resolve replication to interlinked sister replication and sister chromatid in III functions in concert with S. P. PubMed Scopus Google Scholar, M. M. C. R. P. L. R. C. J. PubMed Scopus Google Scholar, Mol. Cell. Biol. PubMed Scopus Google Scholar, S. W. P. J. Biol. Chem. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar, L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar, J. C. M. L. W. J. PubMed Scopus Google Scholar, R. J. Hickson I.D. W. PubMed Scopus Google Scholar). In of the in to to and M. M. C. R. P. L. R. C. J. PubMed Scopus Google Scholar, Mol. Cell. Biol. PubMed Scopus Google Scholar), with and in the In RMI1 to hTopo IIIα via conserved N-terminal domain S. W. P. J. Biol. Chem. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar, L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar), and interaction to for hTopo IIIα in vivo J. C. M. L. W. J. PubMed Scopus Google Scholar). In RMI1 DHJ dissolution by hTopo IIIα and BLM S. W. P. J. Biol. Chem. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar, L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar). other type IA topoisomerases can function in DHJ dissolution in of dissolution by RMI1 specifically hTopo IIIα L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar). RMI1 with a via the domain at the to a R. J. Hickson I.D. W. PubMed Scopus Google Scholar). The is required for the of BLM in cells and for the of BLM to in to DNA S. P. PubMed Scopus Google Scholar, J. C. M. L. W. J. PubMed Scopus Google Scholar, R. J. Hickson I.D. W. PubMed Scopus Google Scholar). cells that are of the show an of SCE J. C. M. L. W. J. PubMed Scopus Google Scholar, R. J. Hickson I.D. W. PubMed Scopus Google Scholar), the of BS that the is for BLM the of BLM-hTopo the in higher and is termed the BLM core Y. PubMed Scopus Google the biochemical role of hTopo IIIα in DHJ dissolution and in the of replication forks, have generated a of single-stranded DNA to mimic the proposed of hTopo IIIα in the of an for the and of hTopo IIIα, demonstrate that hTopo IIIα as a single-stranded DNA decatenase in DHJ dissolution and that decatenase activity is specifically stimulated by that BLM and that BLM and RMI1 by hTopo IIIα. We that hTopo IIIα is the cellular decatenase that functions in dissolution of and of replication and that the decatenase activity on the of a with RMI1 as as BLM helicase DNA substrates that resemble replication and recombination in vitro have in the of of replication and recombination intermediates. the replication C. Mol. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar) and double Holliday junctions without crossing over (11Wu L. Hickson I.D. Nature. 2003; 426: 870-874Crossref PubMed Scopus (868) Google Scholar, 12Plank J.L. Wu J. Hsieh T.S. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 11118-11123Crossref PubMed Scopus (124) Google Scholar). In generated single-stranded catenanes that resemble the DNA structure that is recognized by hTopo IIIα in We provide direct evidence that hTopo IIIα is a single-stranded DNA decatenase and that the decatenase activity is stimulated by of the BLM core BLM and of and DHJ dissolution interaction between RMI1 and hTopo IIIα. The of single-stranded DNA and DHJ dissolution for type IA as as the of the RMI1 of that the relevant activity of hTopo IIIα in the dissolution of is single-stranded DNA and of of hTopo IIIα and for hTopo IIIα and RMI1 by the of S. as a for of evidence that S. is a E. coli for the and of hTopo IIIα and RMI1 is in E. coli L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar), the in S. In addition, RMI1 in of DHJ dissolution by BLM and hTopo IIIα with L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar). hTopo IIIα from S. in the and and with H. L. Riou J.F. Res. 1999; PubMed Scopus Google Scholar). in to with the yeast and of the and J. Biol. Chem. 2007; Full Text Full Text PDF PubMed Scopus Google Scholar), to of hTopo IIIα from hTopo IIIα and RMI1 in DNA and Our data that and hTopo IIIα and RMI1 in S. with biochemical IA DNA have an to the single-stranded DNA decatenase activity of type IA of type IA topoisomerases on substrates in the of RecQ activity of type IA topoisomerases in an in vitro replication in replication intermediates into H. J. Biol. Chem. Full Text PDF PubMed Google Scholar, H. J. Biol. Chem. Full Text PDF PubMed Google Scholar). the of it to the DNA on the enzyme and C. Mol. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar) used an to replication intermediates replication allowing for the of enzyme at a defined that RecQ and catalyze the of replication via a strand-passage mechanism to that in DHJ DNA is by to DNAs with single-stranded gaps C. Mol. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). the of the DHJ dissolution activity by Wu and Hickson (11Wu L. Hickson I.D. Nature. 2003; 426: 870-874Crossref PubMed Scopus (868) Google Scholar) is proposed to an hTopo of a The single-stranded DNA used the topoisomerase IA at the in the and the dissolution type IA and topoisomerases catalyze the of negatively supercoiled DNA by a single-stranded (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar), the of single-stranded DNA to type IA The in the is with that in the DHJ dissolution type IA, type IB, topoisomerases are to hTopo IIIα in catalyzing DHJ dissolution with BLM (11Wu L. Hickson I.D. Nature. 2003; 426: 870-874Crossref PubMed Scopus (868) Google Scholar). and DHJ dissolution a type IA and of by RMI1 the topoisomerase hTopo IIIα. of occurs at an hTopo the and is in RMI1 that hTopo IIIα data that is the relevant activity of hTopo IIIα in processing replication and recombination intermediates. to intermediates in vivo generate interlinked sister and homologous to and with hTopo IIIα to DNA during North P.S. Hickson I.D. J. 2007; PubMed Scopus Google Scholar), and of hTopo IIIα an in the of DNA L. E. C. E. M. Riou J.F. J. Google and RMI1 with hTopo IIIα in that BLM and RMI1 the decatenase activity of hTopo IIIα, the of by the two BLM the decatenase activity of hTopo IIIα and EcTop1 in an suggesting that of the by BLM activity to the the of EcTop1 decatenase activity is in with the of hTopo IIIα it that a interaction of BLM with hTopo IIIα to the on the other is to and hTopo of RMI1 to hTopo decatenase activity. This that the occurs via between hTopo IIIα and We two In the RMI1 hTopo IIIα to the single-stranded via a DNA activity of RMI1 a DNA DNA domain to the of human domain is for the of DHJ dissolution S. W. W. L. P. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar), and it that an activity for RMI1 function of the conserved N-terminal of RMI1 to DNA activity R. J. Hickson I.D. W. PubMed Scopus Google Scholar). yeast the C-terminal single-stranded DNA activity and can Holliday Mol. Cell. Biol. PubMed Scopus Google Scholar, J. Biol. Chem. 2007; Full Text Full Text PDF PubMed Scopus Google Scholar). is that the of RMI1 a DNA activity that is to hTopo IIIα that RMI1 catenated single-stranded DNA the interaction between hTopo IIIα and RMI1 in hTopo IIIα that decatenase activity. with the Holliday junctions and DHJ activity of III in yeast and human are stimulated by RMI1 L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar, J. Biol. Chem. 2007; Full Text Full Text PDF PubMed Scopus Google of evidence have that III and RMI1 a that then have a with and have with the J. Biol. Chem. 2007; Full Text Full Text PDF PubMed Scopus Google of RMI1 hTopo IIIα, whereas of BLM S. P. PubMed Scopus Google Scholar, R. J. Hickson I.D. W. PubMed Scopus Google the to M. M. C. R. P. L. R. C. J. PubMed Scopus Google Scholar, J. Biol. Chem. 2007; Full Text Full Text PDF PubMed Scopus Google of in yeast the of that of M. M. C. R. P. L. R. C. J. PubMed Scopus Google Scholar, Mol. Cell. Biol. PubMed Scopus Google of DHJ dissolution by RMI1 the type IA hTopo IIIα L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google and IIIα and RMI1 functions that are of a of BLM helicase in S. R. H. Genet. PubMed Scopus Google Scholar, L. M. Genet. PubMed Scopus Google and other type IA topoisomerases can for hTopo IIIα in DHJ a for the dissolution in the helicase and the topoisomerase roles proposed C. Wu L. Hickson I.D. Biochem. PubMed Scopus Google Scholar). is the at the BLM decatenase activity and hTopo IIIα helicase data that is an between BLM, hTopo IIIα, and RMI1 at the of the reaction decatenase activity is This is the are in the BLM and RMI1 display on hTopo IIIα decatenase activity the BLM-RMI1 the activity by at on the interaction between RMI1 and IIIα. the of of the The for the for that is in Holliday by BLM, hTopo for activity W. S. P. J. Biol. Chem. 2007; Full Text Full Text PDF PubMed Scopus Google Scholar). Because and to DHJ it that activity in DHJ dissolution is components are S. W. P. J. Biol. Chem. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar, L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar). in a the two junctions are by the activity of hTopo IIIα required at to generated by branch the steps in catalyzing DHJ dissolution to the of the in the BLM core are required for activity in suppressing in to an in by III in eukaryotes. with replication intermediates between RecQ and III in E. coli C. Mol. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar), data that in the and to strand passage activity of BLM-hTopo is for a of cellular processes in Holliday junctions are a feature These include the repair of DNA the processing of replication forks, the of via the of and the of replication forks, are to Holliday of processes a common We that hTopo IIIα catalyzes the of in vivo and that the is in with BLM and a role is with the of two in the hTopo IIIα with of cancer E. J. M. C. H. M. Cancer. PubMed Scopus Google Scholar). The of by hTopo IIIα on the role of hTopo IIIα and BLM and RMI1 in the maintenance of genome integrity. IntroductionTopoisomerases are ubiquitous enzymes conserved from bacteria to humans. Their roles in modulating DNA topology in replication, transcription, and other cellular processes (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar, 2Wang J.C. Nat. Rev. Mol. Cell Biol. 2002; 3: 430-440Crossref PubMed Scopus (1870) Google Scholar) make them indispensable for cell viability. There are four subfamilies of topoisomerases as follows: IA, IB, IIA, and IIB. Type IA topoisomerases change DNA topological states in discrete steps of one via an enzyme-bridging mechanism (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar, 2Wang J.C. Nat. Rev. Mol. Cell Biol. 2002; 3: 430-440Crossref PubMed Scopus (1870) Google Scholar). The catalytic tyrosine residue initiates a transesterification reaction in a single-stranded region to generate a transient DNA break, allowing for the passage of the intact strand through the break. After religation of the broken strand by a reversal of the reaction, the enzyme is free to engage in another round of catalysis (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar, 2Wang J.C. Nat. Rev. Mol. Cell Biol. 2002; 3: 430-440Crossref PubMed Scopus (1870) Google Scholar). Members of the type IA topoisomerase family include Escherichia coli topoisomerase I (EcTop1) and III (EcTop3), yeast topoisomerase III (Top3), and two isoforms of topoisomerase III, α (Topo 2The abbreviations used are: TopotopoisomeraseDHJdouble Holliday junctionDTTdithiothreitolBSAbovine serum albuminhTopohuman topoisomeraseBSBloom syndromeSCEsister chromatid exchangeoligooligonucleotideGSTglutathione S-transferase. IIIα) and β (Topo IIIβ), in higher eukaryotes. These enzymes exhibit high sequence similarity in the N-terminal catalytic core domain, whereas the C-terminal tails are variable (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar). In addition to the ability to relax negatively supercoiled DNA, EcTop1 is capable of catalyzing knotting, unknotting, and interlinking of DNA substrates that contain exposed single-stranded regions (1Champoux J.J. Annu. Rev. Biochem. 2001; 70: 369-413Crossref PubMed Scopus (2153) Google Scholar). Because single-stranded DNA gaps are a common feature found at DNA replication forks, replication termination sites, and replication and repair sites, it is believed that the main function of type IA topoisomerases is to unlink DNA catenanes.Type IA topoisomerases function in concert with RecQ helicases to control recombination events (3Mankouri H.W. Hickson I.D. Trends Biochem. Sci. 2007; 32: 538-546Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar). RecQ helicases are a highly conserved family of DNA helicases that are required for the maintenance of genome integrity (4Hickson I.D. Nat. Rev. Cancer. 2003; 3: 169-178Crossref PubMed Scopus (576) Google Scholar). Human topoisomerase IIIα (hTopo IIIα) physically interacts with BLM, one of the five RecQ helicases in humans (5Wu 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 (281) Google Scholar, 6Johnson F.B. Lombard D.B. Neff N.F. Mastrangelo M.A. Dewolf W. Ellis N.A. Marciniak R.A. Yin Y. Jaenisch R. Guarente L. Cancer Res. 2000; 60: 1162-1167PubMed Google Scholar). Biallelic mutations of BLM give rise to a clinically defined cancer predisposition disorder, Bloom syndrome (BS) (7German J. Dermatol. Clin. 1995; 13: 7-18Abstract Full Text PDF PubMed Google Scholar). BS cells display signs of genome instability, featuring an ∼10-fold elevation in the frequency of sister chromatid exchanges (SCE) (8Chaganti R.S. Schonberg S. German J. Proc. Natl. Acad. Sci. U.S.A. 1974; 71: 4508-4512Crossref PubMed Scopus (778) Google Scholar), events that arise from the processing of recombination intermediates (9Sonoda E. Sasaki M.S. Morrison C. Yamaguchi-Iwai Y. Takata M. Takeda S. Mol. Cell. Biol. 1999; 19: 5166-5169Crossref PubMed Scopus (365) Google Scholar). The hTopo IIIα interacting domain of BLM is required for suppression of SCE in BS cells (10Hu P. Beresten S.F. van Brabant A.J. Ye T.Z. Pandolfi P.P. Johnson F.B. Guarente L. Ellis N.A. Hum. Mol. Genet. 2001; 10: 1287-1298Crossref PubMed Google Scholar), suggesting that hTopo IIIα plays an anti-recombination role with BLM. Indeed, in vitro biochemical data show that BLM and hTopo IIIα catalyze the dissolution of double Holliday junctions (DHJs), a DNA structure that can arise as an intermediate during homologous recombination (11Wu L. Hickson I.D. Nature. 2003; 426: 870-874Crossref PubMed Scopus (868) Google Scholar). Dissolution occurs via a strand-passage mechanism that prevents genetic exchange between flanking sequences and is presumed to mimic the in vivo role of BLM-hTopo IIIα in suppressing SCE (11Wu L. Hickson I.D. Nature. 2003; 426: 870-874Crossref PubMed Scopus (868) Google Scholar). In the simplest case, the dissolution reaction is believed to have two components as follows: the helicase activity of BLM catalyzes branch migration of the Holliday junctions toward each other, resulting in collapse of the Holliday junctions, and generation of two duplex DNAs interlinked via catenated single strands. This structure, termed a hemicatenane, is then decatenated by hTopo IIIα to complete the dissolution of the DHJ (11Wu L. Hickson I.D. Nature. 2003; 426: 870-874Crossref PubMed Scopus (868) Google Scholar, 12Plank J.L. Wu J. Hsieh T.S. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 11118-11123Crossref PubMed Scopus (124) Google Scholar). Direct evidence that hTopo IIIα possesses the relevant decatenase is helicases and type IA topoisomerases to resolve replication with RecQ and the single-stranded catalyzes the of replication intermediates in vitro C. Mol. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). catalyzes of DNA during and replication in vitro H. J. Biol. Chem. Full Text PDF PubMed Google Scholar). have of evidence that topoisomerase III functions in a is required for C. Hickson I.D. Res. 1999; PubMed Scopus (98) Google Scholar). IIIα in cells of cells with gaps and M. S. Y. M. H. Y. Mol. Cell. Biol. 2006; PubMed Scopus Google Scholar). hTopo IIIα to DNA in a North P.S. Hickson I.D. J. 2007; PubMed Scopus Google Scholar). In each of the of III to and resolve replication to interlinked sister replication and sister chromatid in III functions in concert with S. P. PubMed Scopus Google Scholar, M. M. C. R. P. L. R. C. J. PubMed Scopus Google Scholar, Mol. Cell. Biol. PubMed Scopus Google Scholar, S. W. P. J. Biol. Chem. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar, L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar, J. C. M. L. W. J. PubMed Scopus Google Scholar, R. J. Hickson I.D. W. PubMed Scopus Google Scholar). In of the in to to and M. M. C. R. P. L. R. C. J. PubMed Scopus Google Scholar, Mol. Cell. Biol. PubMed Scopus Google Scholar), with and in the In RMI1 to hTopo IIIα via conserved N-terminal domain S. W. P. J. Biol. Chem. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar, L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar), and interaction to for hTopo IIIα in vivo J. C. M. L. W. J. PubMed Scopus Google Scholar). In RMI1 DHJ dissolution by hTopo IIIα and BLM S. W. P. J. Biol. Chem. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar, L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar). other type IA topoisomerases can function in DHJ dissolution in of dissolution by RMI1 specifically hTopo IIIα L. J. C. Yin J. M. W. L. Hickson I.D. Proc. Natl. Acad. Sci. U.S.A. 2006; 103: PubMed Scopus Google Scholar). RMI1 with a via the domain at the to a R. J. Hickson I.D. W. PubMed Scopus Google Scholar). The is required for the of BLM in cells and for the of BLM to in to DNA S. P. PubMed Scopus Google Scholar, J. C. M. L. W. J. PubMed Scopus Google Scholar, R. J. Hickson I.D. W. PubMed Scopus Google Scholar). cells that are of the show an of SCE J. C. M. L. W. J. PubMed Scopus Google Scholar, R. J. Hickson I.D. W. PubMed Scopus Google Scholar), the of BS that the is for BLM the of BLM-hTopo the in higher and is termed the BLM core Y. PubMed Scopus Google the biochemical role of hTopo IIIα in DHJ dissolution and in the of replication forks, have generated a of single-stranded DNA to mimic the proposed of hTopo IIIα in the of an for the and of hTopo IIIα, demonstrate that hTopo IIIα as a single-stranded DNA decatenase in DHJ dissolution and that decatenase activity is specifically stimulated by that BLM and that BLM and RMI1 by hTopo IIIα. We that hTopo IIIα is the cellular decatenase that functions in dissolution of and of replication and that the decatenase activity on the of a with RMI1 as as BLM helicase activity.

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