The conserved BTR complex, composed of the Bloom's syndrome helicase (BLM), topoisomerase IIIα, RMI1, and RMI2, regulates homologous recombination in favor of non-crossover formation via the dissolution of the double Holliday Junction (dHJ). Here we show enhancement of the BTR-mediated dHJ dissolution reaction by the heterotrimeric single-stranded DNA binding protein replication protein A (RPA). Our results suggest that RPA acts by sequestering a single-stranded DNA intermediate during dHJ dissolution. We provide evidence that RPA physically interacts with RMI1. The RPA interaction domain in RMI1 has been mapped, and RMI1 mutants impaired for RPA interaction have been generated. Examination of these mutants ascertains the significance of the RMI1-RPA interaction in dHJ dissolution. Our results thus implicate RPA as a cofactor of the BTR complex in dHJ dissolution.

Homologous recombination (HR) provides an efficient mechanism for error-free repair of DNA double-strand breaks (DSBs). However, HR can be also harmful as inappropriate or untimely HR events can give rise to lethal recombination intermediates and chromosome rearrangements. A critical step of HR is the formation of a RAD51 filament on single-stranded (ss)DNA, which mediates the invasion of a homologous DNA molecule. In mammalian cells, several DNA helicases have been implicated in the regulation of this process. RECQ5, a member of the RecQ family of DNA helicases, interacts physically with the RAD51 recombinase and disrupts RAD51 presynaptic filaments in a reaction dependent on ATP hydrolysis. Here, we have precisely mapped the RAD51-interacting domain of RECQ5 and generated mutants that fail to interact with RAD51. We show that although these mutants retain normal ATPase activity, they are impaired in their ability to displace RAD51 from ssDNA. Moreover, we show that ablation of RECQ5-RAD51 complex formation by a point mutation alleviates the inhibitory effect of RECQ5 on HR-mediated DSB repair. These findings provide support for the proposal that interaction with RAD51 is critical for the anti-recombinase attribute of RECQ5.

DNA end resection is critical for chromosome break repair by homologous recombination and influences the efficiency of repair by nonhomologous DNA end joining. An elegant study by Sinha and colleagues (pp. 1423-1437) published in the June 15, 2009, issue of Genes & Development identified a novel mycobacterial DNA end resection protein complex, AdnAB, that harbors dual DNA motor and dual nuclease functions. Sinha and colleagues also demonstrated that the DNA end-binding protein complex Ku regulates the activity of AdnAB.

Homologous recombination is an important mechanism for the repair of damaged chromosomes, for preventing the demise of damaged replication forks, and for several other aspects of chromosome metabolism and maintenance. The homologous recombination reaction is mediated by the Rad51 recombinase. In the presence of ATP, Rad51 polymerizes on single-stranded DNA (ssDNA) to form a nucleoprotein filament that is commonly referred to as the "presynaptic filament." The presynaptic filament is capable of locating a homologous duplex DNA molecule and catalyzing invasion of the duplex to form a DNA displacement loop called the "D-loop." This protocol describes an in vitro D-loop assay that uses a radiolabeled ssDNA oligonucleotide and a nonlabeled homologous supercoiled duplex DNA as substrates, and agarose gel electrophoresis together with PhosphorImaging for product analysis. To enhance the efficiency of the D-loop reaction, an ancillary factor (the Hop2-Mnd1 complex or Rad54) is included in the reaction. This reconstituted system provides researchers a biochemical means to dissect the mechanisms of the homologous recombination machinery.

The BLAP75 protein combines with the BLM helicase and topoisomerase (Topo) IIIalpha to form an evolutionarily conserved complex, termed the BTB complex, that functions to regulate homologous recombination. BLAP75 binds DNA, associates with both BLM and Topo IIIalpha, and enhances the ability of the BLM-Topo IIIalpha pair to branch migrate the Holliday junction (HJ) or dissolve the double Holliday junction (dHJ) structure to yield non-crossover recombinants. Here we seek to understand the relevance of the biochemical attributes of BLAP75 in HJ processing. With the use of a series of BLAP75 protein fragments, we show that the evolutionarily conserved N-terminal third of BLAP75 mediates complex formation with BLM and Topo IIIalpha and that the DNA binding activity resides in the C-terminal third of this novel protein. Interestingly, the N-terminal third of BLAP75 is just as adept as the full-length protein in the promotion of dHJ dissolution and HJ unwinding by BLM-Topo IIIalpha. Thus, the BLAP75 DNA binding activity is dispensable for the ability of the BTB complex to process the HJ in vitro. Lastly, we show that a BLAP75 point mutant (K166A), defective in Topo IIIalpha interaction, is unable to promote dHJ dissolution and HJ unwinding by BLM-Topo IIIalpha. This result provides proof that the functional integrity of the BTB complex is contingent upon the interaction of BLAP75 with Topo IIIalpha.

Bloom Syndrome is an autosomal recessive cancer-prone disorder caused by mutations in the BLM gene. BLM encodes a DNA helicase of the RECQ family, and associates with Topo III alpha and BLAP75/RMI1 ( BLAP for BLM-associated polypeptide/RecQ-mediated genome instability) to form the BTB ( BLM-Topo III alpha-BLAP75/RMI1) complex. This complex can resolve the double Holliday junction ( dHJ), a DNA intermediate generated during homologous recombination, to yield noncrossover recombinants exclusively. This attribute of the BTB complex likely serves to prevent chromosomal aberrations and rearrangements. Here we report the isolation and characterization of a novel member of the BTB complex termed BLAP18/RMI2. BLAP18/RMI2 contains a putative OB-fold domain, and several lines of evidence suggest that it is essential for BTB complex function. First, the majority of BLAP18/RMI2 exists in complex with Topo III alpha and BLAP75/RMI1. Second, depletion of BLAP18/RMI2 results in the destabilization of the BTB complex. Third, BLAP18/RMI2-depleted cells show spontaneous chromosomal breaks and are sensitive to methyl methanesulfonate treatment. Fourth, BLAP18/RMI2 is required to target BLM to chromatin and for the assembly of BLM foci upon hydroxyurea treatment. Finally, BLAP18/RMI2 stimulates the dHJ resolution capability of the BTB complex. Together, these results establish BLAP18/RMI2 as an essential member of the BTB dHJ dissolvasome that is required for the maintenance of a stable genome.

Nucleolytic processing of DNA double-strand breaks (DSBs) generates 3' ssDNA tails that are essential for the assembly of DNA damage checkpoint signaling and DNA repair protein complexes. Genetic studies have provided evidence that multiple nuclease activities are involved in DSB end resection. Three recent studies, including work by Jackson and colleagues (pp. 2767- 2772) in the October 15, 2008, issue of Genes & Development, have begun to shed some light on the intricacy of this process.

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Members of the RecQ helicase family play critical roles in genome maintenance. There are five RecQ homologs in mammals, and defects in three of these (BLM, WRN, and RECQL4) give rise to cancer predisposition syndromes in humans. RECQL and RECQL5 have not been associated with a human disease. Here we show that deletion of Recql5 in mice results in cancer susceptibility. Recql5-deficient cells exhibit elevated frequencies of spontaneous DNA double-strand breaks and homologous recombination (HR) as scored using a reporter that harbors a direct repeat, and are prone to gross chromosomal rearrangements in response to replication stress. To understand how RECQL5 regulates HR, we use purified proteins to demonstrate that human RECQL5 binds the Rad51 recombinase and inhibits Rad51-mediated D-loop formation. By biochemical means and electron microscopy, we show that RECQL5 displaces Rad51 from single-stranded DNA (ssDNA) in a reaction that requires ATP hydrolysis and RPA. Together, our results identify RECQL5 as an important tumor suppressor that may act by preventing inappropriate HR events via Rad51 presynaptic filament disruption.

BLM, the protein mutated in Bloom's syndrome, possesses a helicase activity that can dissociate DNA structures, including the Holliday junction, expected to arise during homologous recombination. BLM is stably associated with topoisomerase III alpha (Topo III alpha) and the BLAP75 protein. The BLM-Topo III alpha-BLAP75 (BTB) complex can efficiently resolve a DNA substrate that harbors two Holliday junctions (the double Holliday junction) in a non-crossover manner. Here we show that the Holliday junction unwinding activity of BLM is greatly enhanced as a result of its association with Topo III alpha and BLAP75. Enhancement of this BLM activity requires both Topo III alpha and BLAP75. Importantly, Topo III alpha cannot be substituted by Escherichia coli Top3, and the Holliday junction unwinding activity of BLM-related helicases WRN and RecQ is likewise impervious to Topo III alpha and BLAP75. However, the topoisomerase activity of Topo III alpha is dispensable for the enhancement of the DNA unwinding reaction. We have also ascertained the requirement for the BLM ATPase activity in double Holliday junction dissolution and DNA unwinding by constructing, purifying, and characterizing specific mutant variants that lack this activity. These results provide valuable information concerning how the functional integrity of the BTB complex is governed by specific protein-protein interactions among the components of this complex and the enzymatic activities of BLM and Topo III alpha.

Bloom syndrome (BS), an autosomal recessive disorder, is marked by a high incidence of cancer early in life. Cells derived from BS patients are unstable genetically and exhibit frequent sister chromatid exchanges, reflective of homologous recombination (HR) deregulation. BLM, the RecQ-like helicase mutated in BS, is found in several cellular protein complexes, all of which contain topoisomerase III alpha (Topo III alpha) and a novel protein BLAP75. Here, using highly purified human proteins, we show that BLAP75 associates independently with both Topo III alpha and BLM. Even though BLM and Topo III alpha can dissolve the double Holliday junction (DHJ) to yield non-crossover recombinants (1), under physiological conditions, DHJ dissolution becomes completely dependent on BLAP75. The effect of BLAP75 on BLM-Topo III alpha is highly specific, as it is not seen with the combination of Topo III alpha and Escherichia coli RecQ helicase or another human RecQ-like helicase WRN. Thus, BLM, Topo III alpha, and BLAP75 constitute a dissolvasome complex that processes HR intermediates to limit DNA crossover formation. This function of the BLM-Topo III alpha-BLAP75 dissolvasome is likely indispensable for genome maintenance and cancer avoidance.

Homologous recombination (HR) is important in repairing errors of replication and other forms of DNA damage. In mammalian cells, potential templates include the homologous chromosome, and after DNA replication, the sister chromatid. Previous work has shown that the mammalian recombination machinery is organized to suppress interchromosomal recombination while preserving intrachromosomal HR. In the present study, we investigated spontaneous intrachromosomal HR in mouse hybridoma cell lines in which variously numbered tandem repeats of the mu heavy chain constant (C mu) region reside at the haploid, chromosomal immunoglobulin mu heavy chain locus. This organization provides the opportunity to investigate recombination between homologous gene repeats in a well-defined chromosomal locus under conditions in which recombinants are conveniently recovered. This system revealed several features about the mammalian intrachromosomal HR process: (i) the frequency of HR was high (recombinants represented as much as several percent of the total of recombinants and non-recombinants); (ii) the recombination process appeared to be predominantly non-reciprocal, consistent with the possibility of gene conversion; (iii) putative gene conversion tracts were long (up to 13.4 kb); (iv) the recombination process occurred with precision, initiating and terminating within regions of shared homology. The results are discussed with respect to mammalian intrachromosomal HR involving interactions both within and between sister chromatids.

In mammalian cells, little is known about the nature of recombination-prone regions of the genome. Previously, we reported that the immunoglobulin heavy chain (IgH) mu locus behaved as a hotspot for mitotic, intrachromosomal gene conversion (GC) between repeated mu constant (Cmu) regions in mouse hybridoma cells. To investigate whether elements within the mu gene regulatory region were required for hotspot activity, gene targeting was used to delete a 9.1 kb segment encompassing the mu gene promoter (Pmu), enhancer (Emu) and switch region (Smu) from the locus. In these cell lines, GC between the Cmu repeats was significantly reduced, indicating that this 'recombination-enhancing sequence' (RES) is necessary for GC hotspot activity at the IgH locus. Importantly, the RES fragment stimulated GC when appended to the same Cmu repeats integrated at ectopic genomic sites. We also show that deletion of Emu and flanking matrix attachment regions (MARs) from the RES abolishes GC hotspot activity at the IgH locus. However, no stimulation of ectopic GC was observed with the Emu/MARs fragment alone. Finally, we provide evidence that no correlation exists between the level of transcription and GC promoted by the RES. We suggest a model whereby Emu/MARS enhances mitotic GC at the endogenous IgH mu locus by effecting chromatin modifications in adjacent DNA.

Homologous recombination accomplishes the exchange of genetic information between two similar or identical DNA duplexes. It can occur either by gene conversion, a process of unidirectional genetic exchange, or by reciprocal crossing over. Homologous recombination is well known for its role in generating genetic diversity, in meiosis and, in mitosis, as a DNA repair mechanism. In the immune system, the evidence suggests a role for homologous recombination in Ig gene evolution and in the diversification of Ab function. Previously, we reported the occurrence of homologous recombination between repeated, donor and recipient alleles of the Ig H chain mu gene C (Cmu) region residing at the Ig mu locus in mouse hybridoma cells. In this study, we constructed mouse hybridoma cell lines bearing Cmu region heteroalleles to learn more about the intrachromosomal homologous recombination process. A high frequency of homologous recombination (gene conversion) was observed for markers spanning the entire recipient Cmu region, suggesting that recombination might initiate at random sites within the Cmu region. The Cmu region heteroalleles were equally proficient as either conversion donors or recipients. Remarkably, when the same Cmu heteroalleles were tested for recombination in ectopic genomic positions, the mean frequency of gene conversion was reduced by at least 65-fold. These results are consistent with the murine IgH mu locus behaving as a hot spot for intrachromosomal homologous recombination.

In this study, the formation and repair of large (>1 kb) insertion/deletion (I/D) heterologies during double-strand-break repair (DSBR) was investigated using a gene-targeting assay that permits efficient recovery of sequence insertion events at the haploid chromosomal immunoglobulin (Ig) mu-locus in mouse hybridoma cells. The results revealed that (i) large I/D heterologies were generated on one or both sides of the DSB and, in some cases, formed symmetrically in both homology regions; (ii) large I/D heterologies did not negatively affect the gene targeting frequency; and (iii) prior to DNA replication, the large I/D heterologies were rectified.