Rad51 expression and localization in B cells carrying out class switch recombination

Biomarkers for Homologous Recombination Deficiency in Cancer

Defective DNA repair is a common hallmark of cancer. Homologous recombination is a DNA repair pathway of clinical interest due to the sensitivity of homologous recombination-deficient cells to poly-ADP ribose polymerase (PARP) inhibitors. The measurement of homologous recombination deficiency (HRD) in cancer is therefore vital to the appropriate design of clinical trials incorporating PARP inhibitors. However, methods to identify HRD in tumors are varied and controversial. Understanding existing and new methods to measure HRD is important to their appropriate use in clinical trials and practice. The aim of this review is to summarize the biology and clinical validation of current methods to measure HRD, to aid decision-making for patient stratification and translational research in PARP inhibitor trials. We discuss the current clinical development of PARP inhibitors, along with established indicators for HRD such as germline BRCA1/2 mutation status and clinical response to platinum-based therapy. We then examine newer assays undergoing clinical validation, including 1) somatic mutations in homologous recombination genes, 2) "genomic scar" assays using array-based comparative genomic hybridization (aCGH), single nucleotide polymorphism (SNP) analysis or mutational signatures derived from next-generation sequencing, 3) transcriptional profiles of HRD, and 4) phenotypic or functional assays of protein expression and localization. We highlight the strengths and weaknesses of each of these assays, for consideration during the design of studies involving PARP inhibitors.

Meiosis-specific gene discovery in plants: RNA-Seq applied to isolated Arabidopsis male meiocytes

BACKGROUND

Meiosis is a critical process in the reproduction and life cycle of flowering plants in which homologous chromosomes pair, synapse, recombine and segregate. Understanding meiosis will not only advance our knowledge of the mechanisms of genetic recombination, but also has substantial applications in crop improvement. Despite the tremendous progress in the past decade in other model organisms (e.g., Saccharomyces cerevisiae and Drosophila melanogaster), the global identification of meiotic genes in flowering plants has remained a challenge due to the lack of efficient methods to collect pure meiocytes for analyzing the temporal and spatial gene expression patterns during meiosis, and for the sensitive identification and quantitation of novel genes.

RESULTS

A high-throughput approach to identify meiosis-specific genes by combining isolated meiocytes, RNA-Seq, bioinformatic and statistical analysis pipelines was developed. By analyzing the studied genes that have a meiosis function, a pipeline for identifying meiosis-specific genes has been defined. More than 1,000 genes that are specifically or preferentially expressed in meiocytes have been identified as candidate meiosis-specific genes. A group of 55 genes that have mitochondrial genome origins and a significant number of transposable element (TE) genes (1,036) were also found to have up-regulated expression levels in meiocytes.

CONCLUSION

These findings advance our understanding of meiotic genes, gene expression and regulation, especially the transcript profiles of MGI genes and TE genes, and provide a framework for functional analysis of genes in meiosis.

Widespread genomic breaks generated by activation-induced cytidine deaminase are prevented by homologous recombination

Activation induced cytidine deaminase (AID) is required for somatic hypermutation and immunoglobulin class switching in activated B cells. Because AID possesses no known target site specificity, there have been efforts to identify non-immunoglobulin AID targets. We show that AID acts promiscuously, generating widespread DNA double strand breaks (DSB), genomic instability and cytotoxicity in B cells with diminished homologous recombination (HR) capability. We demonstrate that the HR factor XRCC2 suppresses AID-induced off-target DSBs, promoting B cell survival. Finally, we suggest that aberrations affecting human chromosome 7q36, including XRCC2, correlate with genomic instability in B cell cancers. Our findings demonstrate that AID has promiscuous genomic DSB-inducing activity, identify HR as a safeguard against off-target AID action, and have implications for genomic instability in B cell cancers.

Vitamin C acts indirectly to modulate isotype switching in mouse B cells

Vitamin C, one of essential micronutrients, has been reported to modulate the humoral immune responses in some mammals. We investigated whether vitamin C might modulate this response in mice by directly affecting B cells. Splenic B cells were isolated and activated by CD40- and B cell receptor-ligation in vitro. The cells were cultured with a pretreatment of vitamin C from 0 to 1 mM of concentrations. Vitamin C slightly increased apoptosis of B cells dose-dependently and behaved as an antioxidant. We found that in vivo administration of vitamin C by intraperitoneal injection affected isotype switching as previously reported: the titer of antigen-specific IgG1 antibody was decreased, while that of IgG2a was unaffected. Somewhat different from those observed in vivo, in vitro exposure to vitamin C slightly decreased isotype switching to IgG1 and increased isotype switching to IgG2a. Pretreatment with vitamin C in the safe range did not affect either proliferation of cultured B cells or the expression of CD80 and CD86 in those cells. Taken together, in vivo results suggest that vitamin C acts to modulate isotype switching in the mouse. However, because of our in vitro results, we suggest that the modulation exerted by vitamin C in vivo is by indirectly affecting B cells, perhaps by directly influencing other immune cells such as dendritic cells.

Non-conservative homologous recombination in human B lymphocytes is promoted by activation-induced cytidine deaminase and transcription

During secondary immunoglobulin (Ig) diversification in vertebrates, the sequence of the variable region of Ig genes may be altered by templated or non-templated mechanisms. In both cases, cytidine deamination by activation-induced cytidine deaminase (AID) in the transcribed Ig loci leads to DNA lesions, which are repaired by conservative homologous recombination (HR) during Ig gene conversion, or by non-templated mutagenesis during somatic hypermutation. The molecular basis for the differential use of these two pathways in different species is unclear. While experimental ablation of HR in avian cells performing Ig gene conversion may promote a switch to somatic hypermutation, the activity of HR processes in intrinsically hypermutating mammalian cells has not been measured to date. Employing a functional HR assay in human germinal centre like B cell lines, we detect elevated HR activity that can be enhanced by transcription and AID. Products of such recombination events mostly arise through non-conservative HR pathways, while the activity of conservative HR is low to absent. Our results identify non-conservative HR as a novel DNA transaction pathway promoted by AID and suggest that somatic hypermutation in germinal centre B cells may be based on a physiological suppression of conservative HR.

Class switch recombination: a comparison between mouse and human

Humans and mice separated more than 60 million years ago. Since then, evolution has led to a multitude of changes in their genomic sequences. The divergence of genes has resulted in differences both in the innate and adaptive immune systems. In this chapter, we focus on species difference with regard to immunoglobulin class switch recombination (CSR). We have compared the immunoglobulin constant region gene loci from human and mouse, with an emphasis on the switch regions, germ line transcription promoters, and 3' enhancers. We have also compared pathways/factors that are involved in CSR. Although there are remarkable similarities in the cellular machinery involved in CSR, there are also a number of unique features in each species.

Identification and characterization of rad51 of Pneumocystis

Rad51, a eukaryotic homolog of RecA, is an important protein involved in DNA recombination and repair. We have characterized rad51 of Pneumocystis carinii and Pneumocystis murina. rad51 is a single copy gene that encodes a 1.2 kb mRNA, which contains an open reading frame encoding 343 amino acids. Rad51 from Pneumocystis showed high homology to those from yeast. ATP binding motifs GEFRTGKS and LLIVD, similar to those of Saccharomyces cerevisiae and Schizosaccharomyces pombe, are conserved in Pneumocystis Rad51. The recombinant protein when expressed in E. coli showed DNA-dependent ATPase activity. Since Rad51 is a key enzyme in DNA repair and recombination, it potentially plays an important role in the recombination process leading to antigenic variation and thereby resistance to host immune responses in Pneumocystis.

HP1 proteins are essential for a dynamic nuclear response that rescues the function of perturbed heterochromatin in primary human cells

Cellular information is encoded genetically in the DNA nucleotide sequence and epigenetically by the "histone code," DNA methylation, and higher-order packaging of DNA into chromatin. Cells possess intricate mechanisms to sense and repair damage to DNA and the genetic code. However, nothing is known of the mechanisms, if any, that repair and/or compensate for damage to epigenetically encoded information, predicted to result from perturbation of DNA and histone modifications or other changes in chromatin structure. Here we show that primary human cells respond to a variety of small molecules that perturb DNA and histone modifications by recruiting HP1 proteins to sites of altered pericentromeric heterochromatin. This response is essential to maintain the HP1-binding kinetochore protein hMis12 at kinetochores and to suppress catastrophic mitotic defects. Recruitment of HP1 proteins to pericentromeres depends on histone H3.3 variant deposition, mediated by the HIRA histone chaperone. These data indicate that defects in pericentromeric epigenetic heterochromatin modifications initiate a dynamic HP1-dependent response that rescues pericentromeric heterochromatin function and is essential for viable progression through mitosis.

The Repair of DNA Damages/Modifications During the Maturation of the Immune System: Lessons from Human Primary Immunodeficiency Disorders and Animal Models

The immune system is the site of various genotoxic stresses that occur during its maturation as well as during immune responses. These DNA lesions/modifications are primarily the consequences of specific physiological processes such as the V(D)J recombination, the immunoglobulin class switch recombination (CSR), and the generation of somatic hypermutations (SHMs) within Ig variable domains. The DNA lesions can be introduced either by specific factors (RAG1 and RAG2 in the case of V(D)J recombination and AID in the case of CSR and SHM) or during the various phases of cellular proliferation and cellular activation. All these DNA lesions are taken care of by the diverse DNA repair machineries of the cell. Several animal models as well as human conditions have established the critical importance of these DNA lesions/modifications and their repair in the physiology of the immune system. Indeed their defects have consequences ranging from immune deficiency to development of immune malignancy. The survey of human pathology has been highly instrumental in the past in identifying key factors involved in the generation of DNA modifications (AID for the Ig CSR and generation of SHM) or the repair of specific DNA damages (Artemis for V(D)J recombination). Defects in factors involved in the cell cycle checkpoints following DNA damage also have deleterious consequences on the immune system. The continuous survey of human diseases characterized by primary immunodeficiency associated with increased sensitivity to ionizing radiation should help identify other important DNA repair factors essential for the development and maintenance of the immune system.

Accessibility control and machinery of immunoglobulin class switch recombination

Immunoglobulin (Ig) class switching is a process by which B lymphocytes shift from production of IgM to other Ig classes and subclasses via Ig class switch recombination (CSR). Multiple cellular and molecular processes are involved in CSR. Induction of a given IgH germline transcription initiates CSR processes. Ig germline transcription is selectively activated and induced by specific cytokine(s) via cytokine-specific signal pathways, synergized by CD40 signaling, and optimized by the 3' Ig alpha enhancers through locus control region function. Following Ig germline transcription, the switch-region DNA undergoes conformational changes so that it can serve as an appropriate substrate for nicking and cleavage by switch recombination machinery. Finally, the double-strand breaks in donor and acceptor switch DNAs are processed, repaired, and ligated through a general nonhomologous end join pathway. CSR generates a new transcriptional unit for production of a class-switched Ig isotype.

Molecular mechanism of class switch recombination: linkage with somatic hypermutation

Class switch recombination (CSR) and somatic hypermutation (SHM) have been considered to be mediated by different molecular mechanisms because both target DNAs and DNA modification products are quite distinct. However, involvement of activation-induced cytidine deaminase (AID) in both CSR and SHM has revealed that the two genetic alteration mechanisms are surprisingly similar. Accumulating data led us to propose the following scenario: AID is likely to be an RNA editing enzyme that modifies an unknown pre-mRNA to generate mRNA encoding a nicking endonuclease specific to the stem-loop structure. Transcription of the S and V regions, which contain palindromic sequences, leads to transient denaturation, forming the stem-loop structure that is cleaved by the AID-regulated endonuclease. Cleaved single-strand tails will be processed by error-prone DNA polymerase-mediated gap-filling or exonuclease-mediated resection. Mismatched bases will be corrected or fixed by mismatch repair enzymes. CSR ends are then ligated by the NHEJ system while SHM nicks are repaired by another ligation system.

AID is required to initiate Nbs1/gamma-H2AX focus formation and mutations at sites of class switching

Class switch recombination (CSR) is a region-specific DNA recombination reaction that replaces one immunoglobulin heavy-chain constant region (Ch) gene with another. This enables a single variable (V) region gene to be used in conjunction with different downstream Ch genes, each having a unique biological activity. The molecular mechanisms that mediate CSR have not been defined, but activation-induced cytidine deaminase (AID), a putative RNA-editing enzyme, is required for this reaction. Here we report that the Nijmegen breakage syndrome protein (Nbs1) and phosphorylated H2A histone family member X (gamma-H2AX, also known as gamma-H2afx), which facilitate DNA double-strand break (DSB) repair, form nuclear foci at the Ch region in the G1 phase of the cell cycle in cells undergoing CSR, and that switching is impaired in H2AX-/- mice. Localization of Nbs1 and gamma-H2AX to the Igh locus during CSR is dependent on AID. In addition, AID is required for induction of switch region (S mu)-specific DNA lesions that precede CSR. These results place AID function upstream of the DNA modifications that initiate CSR.

RAD51 supports spontaneous non-homologous recombination in mammalian cells, but not the corresponding process induced by topoisomerase inhibitors

The RAD51 protein has been shown to participate in homologous recombination by promoting ATP-dependent homologous pairing and strand transfer reactions. In the present study, we have investigated the possible involvement of RAD51 in non-homologous recombination. We demonstrate that overexpression of CgRAD51 enhances the frequency of spontaneous non-homologous recombination in the hprt gene of Chinese hamster cells. However, the rate of non-homologous recombination induced by the topoisomerase inhibitors campothecin and etoposide was not altered by overexpression of RAD51. These results indicate that the RAD51 protein may perform a function in connection with spontaneous non-homologous recombination that is not essential to or not rate-limiting for non-homologous recombination induced by camptothecin or etoposide. We discuss the possibility that the role played by RAD51 in non-homologous recombination observed here may not be linked to non-homologous end-joining.

Coordinated response of mammalian Rad51 and Rad52 to DNA damage

Biochemical analysis has shown that mammalian Rad51 and Rad52 interact and synergize in DNA recombination reactions in vitro, but these proteins have not been shown to function together in response to DNA damage in vivo. By analysis of murine cells expressing murine Rad52 tagged with green fluorescent protein (GFP)-Rad52, we now show that DNA damage causes Rad51 and GFP-Rad52 to colocalize in distinct nuclear foci. Cells expressing GFP-Rad52 show both increased survival and an increased number of Rad51 foci, raising the possibility that Rad52 is limiting for repair. These observations provide evidence of coordinated function of Rad51 and Rad52 in vivo and support the hypothesis that Rad52 plays an important role in the DNA damage response in mammalian cells.

Cell-free recombination of immunoglobulin switch-region DNA with nuclear extracts

We have developed an in vitro recombination system employing cell-free nuclear extracts from human B lymphocytes capable of detecting the recombination between human mu switch (Smu) and Sepsilon sequences in a model plasmid. Nuclear extracts from CD40-stimulated B lymphocytes gave a higher frequency of recombination in the assay than the unstimulated B cells. Recombination between Smu and Sepsilon was mediated by the nuclear extracts as the recombinational products could be amplified prior to bacterial transformation. Characterization of the recombination products demonstrated that the recombination process had the characteristics of immunoglobulin (Ig) isotype switching, as it was (i) switch-region-sequence specific, (ii) nonhomologous recombination, and (iii) enhanced by CD40 stimulation. Transcription through the S region DNA was not required for recombination in the system. These results demonstrate that Ig switch-region DNA recombination can be accomplished in vitro by cell-free nuclear extracts. This in vitro system for Ig switch-region DNA recombination using cell-free nuclear extracts will permit the dissection of the events involved in IgE class switch recombination, a critical event in the development of allergic diseases.

Human 75-kDa DNA-pairing protein is identical to the pro-oncoprotein TLS/FUS and is able to promote D-loop formation

Homologous recombination plays a fundamental role in DNA double-strand break repair. Previously, we detected two mammalian nuclear proteins of 100 and 75 kDa (POMp100 and POMp75, respectively) that are able to promote homologous DNA pairing, a key step in homologous recombination. Here we describe the identification of human (h) POMp75 as the pro-oncoprotein TLS/FUS. hPOMp75/TLS binds both single- and double-stranded DNAs and mediates annealing of complementary DNA strands. More important, it promotes the uptake of a single-stranded oligonucleotide into a homologous superhelical DNA to form a D-loop. The formation of a D-loop is an essential step in DNA double-strand break repair through recombination. DNA annealing and D-loop formation catalyzed by hPOMp75/TLS require Mg(2+) and are ATP-independent. Interestingly, the oncogenic fusion form TLS-CHOP is not able to promote DNA pairing. These data suggest a possible role for hPOMp75/TLS in maintenance of genomic integrity.

Sensitive analysis of recombination activity using integrated cell surface reporter substrates

BACKGROUND

Recombination processes play a crucial role in the functioning of the immune system and are also involved in mutation events that result in various malignancies. So far the study of recombination activity has frequently relied on the use of reporter substrates that are limited by low sensitivity as well as tedious and distorting readout procedures.

METHODS

Immunoglobulin class switch recombination substrates were generated which, upon recombination, resulted in the surface expression of human CD4 or murine MHC class I H-2K(k) and thus allowed for cytometric evaluation.

RESULTS

Recombining cells harboring integrated reporter substrates were analyzed by immunofluorescence and flow cytometry and could easily be isolated by high-gradient magnetic cell sorting (MACS). The analysis was not influenced by cloning efficiencies, as would be the case after drug selection, or prokaryotic recombination that might occur after analysis of recovered substrates in bacteria. In addition, cytometric readout is much faster, as it can be performed immediately after recombination. The substrate exhibited properties compatible with the detection of immunoglobulin class switch recombination and permitted the detection of recombination events down to 10(-5) per cell and generation.

CONCLUSIONS

The high sensitivity of this system allows precise detection of very rare recombination events and thus permits the study of cell types with extremely low recombination activities.

Localization and dynamic relocalization of mammalian Rad52 during the cell cycle and in response to DNA damage

The importance of RAD52 in establishment and maintenance of genomic structure has been established by genetic experiments in the yeast Saccharomyces cerevisiae, where mutation of RAD52 has been shown to diminish DNA repair and recombination of a variety of markers, including the rDNA [1] [2] [3]. Biochemical analysis has shown that yeast and mammalian Rad52 proteins have some identical functions in vitro [4] [5] [6], but targeted deletion of Rad52 in vertebrates has little effect on repair and recombination [7] [8]. These results raise the question of whether mammalian Rad52 does indeed function in recombination and/or repair. Here we show that Rad52 is distributed throughout the nucleoplasm in actively cycling mammalian cells and is localized specifically to the nucleoli in S phase. In response to ionizing radiation, Rad52 relocalizes to form distinctive foci which are distributed throughout the nucleus and which colocalize with Rad50 foci in the DNA damage response. These data suggest that rDNA recombination and DNA repair are functions shared by mammalian Rad52 and its S. cerevisiae homolog, and provide evidence for the coordinated action of Rad50 and Rad52 in DNA repair.

Somatic hypermutation and the three R's: repair, replication and recombination

Somatic hypermutation introduces single base changes into the rearranged variable (V) regions of antigen activated B cells at a rate of approximately 1 mutation per kilobase per generation. This is nearly a million-fold higher than the typical mutation rate in a mammalian somatic cell. Rampant mutation at this level could have a devastating effect, but somatic hypermutation is accurately targeted and tightly regulated. Here, we provide an overview of immunoglobulin gene somatic hypermutation; discuss mechanisms of mutation in model organisms that may be relevant to the hypermutation mechanism; and review recent advances toward understanding the possible role(s) of DNA repair, replication, and recombination in this fascinating process.

Human Rad51 protein can form homologous joints in the absence of net strand exchange

The eukaryotic homologs of RecA protein are central enzymes of recombination and repair, and notwithstanding a high degree of conservation they differ sufficiently from RecA to offer insights into mechanisms and biological roles. The yield of DNA strand exchange reactions driven by both Escherichia coli RecA protein and its human homolog HsRad51 protein was inversely related to the GC content of oligonucleotide substrates, but at any given GC composition, HsRad51 promoted less exchange than RecA. When 40% of bases were GC pairs, the rate constant for strand exchange by HsRad51 was unmeasurable, whereas the rate constants for homologous pairing were unaltered relative to more AT-rich DNA. The ability of HsRad51 to form joints in the absence of net strand exchange was confirmed by experiments in which heterologous blocks at both ends of linear duplex oligonucleotides produced joints that instantly dissociated upon deproteinization. These findings suggest that HsRad51 acting alone on human DNA in vivo is a pairing protein that cannot form extensive heteroduplex DNA.

A Role for RAD51 in the Generation of Immunoglobulin Gene Diversity in Rabbits

Ig VDJ genes in rabbit somatically diversify by both hyperpointmutation and gene conversion. To elucidate the mechanism of gene conversion of IgH genes, we cloned a rabbit homologue of RAD51, a gene involved in gene conversion in Saccharomyces cerevisiae (yeast), and tested whether it could complement a yeast rad51 mutant deficient in recombination repair. We found that rabbit RAD51 partially complemented the defect in switching mating types by gene conversion as well as in DNA double-strand break repair after γ-irradiation. Further, by Western blot analysis, we found that levels of Rad51 were higher in appendix-derived B lymphocytes of 6-wk-old rabbits, a time at which IgH genes diversify by somatic gene conversion. We suggest that Rad51 is involved in somatic gene conversion of rabbit Ig genes.

The contribution of homologous recombination in preserving genome integrity in mammalian cells

Although it is clear that mammalian somatic cells possess the enzymatic machinery to perform homologous recombination of DNA molecules, the importance of this process in mitigating DNA damage has been uncertain. An initial genetic framework for studying homologous recombinational repair (HRR) has come from identifying relevant genes by homology or by their ability to correct mutants whose phenotypes are suggestive of recombinational defects. While yeast has been an invaluable guide, higher eukaryotes diverge in the details and complexity of HRR. For eliminating DSBs, HRR and end-joining pathways share the burden, with HRR contributing critically during S and G2 phases. It is likely that the removal of interstrand cross-links is absolutely dependent on efficient HRR, as suggested by the extraordinary sensitivity of the ercc1, xpf/ercc4, xrcc2, and xrcc3 mutants to cross-linking chemicals. Similarly, chromosome stability in untreated cells requires intact HRR, which may eliminate DSBs arising during DNA replication and thereby prevent chromosome aberrations. Complex regulation of HRR by cell cycle checkpoint and surveillance functions is suggested not only by direct interactions between human Rad51 and p53, c-Abl, and BRCA2, but also by very high recombination rates in p53-deficient cells.

Interaction of human rad51 recombination protein with single-stranded DNA binding protein, RPA

Replication protein A (RPA), a heterotrimeric single-stranded DNA binding protein, is required for recombination, and stimulates homologous pairing and DNA strand exchange promoted in vitro by human recombination protein HsRad51. Co-immunoprecipitation revealed that purified RPA interacts physically with HsRad51, as well as with HsDmc1, the homolog that is expressed specifically in meiosis. The interaction with HsRad51 was mediated by the 70 kDa subunit of RPA, and according to experiments with deletion mutants, this interaction required amino acid residues 169-326. In exponentially growing mammalian cells, 22% of nuclei showed foci of RPA protein and 1-2% showed foci of Rad51. After gamma-irradiation, the percentage of cells with RPA foci increased to approximately 50%, and those with Rad51 foci to 30%. All of the cells with foci of Rad51 had foci of RPA, and in those cells the two proteins co-localized in a high fraction of foci. The interactions of human RPA with Rad51, replication proteins and DNA are suited to the linking of recombination to replication.

Localization of Splenic B Cells Activated for Switch Recombination by In Situ Hybridization with Iγ1 Switch Transcript and Rad51 Probes

B cells are activated for switch recombination by signals from Th cells, but the site at which this first occurs in vivo has yet to be identified. By in situ hybridization of splenic sections using riboprobes specific for the Iγ1 switch transcript and Rad51 mRNA, we have visualized B cells that are newly activated for switch recombination and characterized the spatial and temporal patterns of Iγ1 and Rad51 mRNA expression. Within 2 days after immunization with (4-hydroxy-3-nitrophenyl)acetyl-chicken gamma-globulin, expression of Iγ1 switch transcripts and Rad51 mRNA was evident and was localized to B220+ B cells clustered within the T cell-rich periarteriolar lymphoid sheath (PALS) and surrounding follicles. By Ab staining, we have shown previously that cells switching from IgM to IgG expression can be visualized at 3 to 5 days postimmunization and colocalize to clusters of Rad51+ cells. Hybridization of adjacent sections with probes for Cμ and Cγ1 mRNA now shows that switching from μ to γ expression occurs within Rad51+Iγ1+ regions of the PALS and peaks between days 3 and 5. Colocalized expression of Iγ1 and Rad51 transcripts was observed from days 2 through 12 of the immune response. Iγ1 and Rad51 transcripts were down-regulated but still detectable at 12 days postimmunization, when they were evident in peanut agglutinin-positive germinal center B cells. Taken together, these observations show that B cells are first activated for switch recombination in the T cell-rich PALS.

c-Fos Induces Apoptosis in Germinal Center B Cells

We examined the role of c-Fos in the differentiation of mature B cells into IgG-producing cells using transgenic mice carrying the c-fos gene under the control of the IFN-α/β-inducible Mx promoter (Mx-c-fos) or the constitutive H-2Kb promoter (H2-c-fos). Splenic B cells from Mx-c-fos mice were cultured with LPS and rIL-4, and IgG1+ B cells were developed in the culture after day 3. When IFN-α/β was added to the culture from day 2, development of IgG1+ B cells was perturbed, and the number of apoptotic cells increased within 24 h, suggesting that c-Fos induces apoptosis in Ig class-switching B cells. To confirm the effect of c-Fos on B cell differentiation in vivo, H2-c-fos mice were immunized with DNP-OVA. The mice produced primary IgM, but not IgG, anti-DNP Ab in serum and failed to generate germinal centers in spleen. The perturbation of germinal center formation in H2-c-fos mice was rescued by mating them with transgenic mice carrying the bcl-2 gene with the Ig promoter. However, primary IgG1 anti-DNP Ab production was still suppressed in doubly transgenic mice, suggesting that Bcl-2 can delay the time of c-Fos-induced apoptosis in Ig class-switching B cells but cannot rescue the death. Since c-Fos is induced in mature B cells reacted with Ags, and clonal deletion of self-reactive B cells in germinal centers is insensitive to Bcl-2, these results suggest that c-Fos plays a causal role in clonal deletion of germinal center B cells.

Characterization of Human γ4 Switch Region Polymorphisms Suggests a Meiotic Recombinational Hot Spot Within the Ig Locus: Influence of S Region Length on IgG4 Production

Human γ4 gene RFLPs, revealed after BamHI digestion, show IGHG4 alleles of 9.0 (9.2), 9.4, and 9.6 kb at various frequencies in different ethnic populations. Studies in immunodeficient individuals have previously suggested that the 9.4 BamHI allele is associated with a higher serum level of IgG4 than the 9.0 (9.2) BamHI allele, but it is not clear whether this is associated with the S region itself or other control elements. In addition, a duplication of the 9.4-kb γ4 allele has recently been observed in a high proportion of normal donors. We therefore undertook a study of the structural basis for the difference in Ab levels in the various γ4 alleles. We demonstrate that the Sγ4 alleles differ in length due to deletions and insertions of a varying number of 79-bp Sγ4 repeat units. Two novel RFLPs, 8.8 and 9.1 kb, were also observed. The alleles are likely to be generated by unequal crossing over, and the breakpoints cluster in Sγ4 repeat units that contain chi-like motifs, implicating chi-like sequences in the meiotic recombination. Our data support the idea that the 9.4-kb BamHI allele is more productive than the 9.0 (9.2)-kb allele in normal healthy donors, possibly due to the extended switch regions, whereas duplication of the γ4 gene has no effect on switching and IgG4 serum levels.

Xrcc3 is required for assembly of Rad51 complexes in vivo

Rad51 is a member of a family of eukaryotic proteins related to the bacterial recombinational repair protein RecA. Rad51 protein localizes to multiple subnuclear foci in Chinese hamster ovary cells. Subnuclear Rad51 foci are induced by ionizing radiation or the DNA cross-linking agent cisplatin. Formation of these foci is likely to reflect assembly of a multimeric form of Rad51 that promotes DNA repair. Formation of damage-induced Rad51 foci does not occur in the Chinese hamster ovary cell line irs1SF, which is sensitive to DNA damaging agents. The Rad51 focus formation defect of irs1SF cells is corrected by a construct that encodes the repair protein Xrcc3. Xrcc3 is a human homolog of Rad51 previously isolated by virtue of its ability to correct the radiation sensitivity of irs1SF cells. Changes in the steady state level of Rad51 protein do not account for the irs1SF defect nor do they account for the appearance of foci following DNA damage. These results suggest that Xrcc3 is required for the assembly or stabilization of a multimeric form of Rad51 during DNA repair. Cell lines defective in two different components of DNA protein kinase formed Rad51 foci in response to damage, indicating DNA protein kinase is not required for damaged-induced mobilization of Rad51.

Polarity of DNA strand exchange promoted by recombination proteins of the RecA family

Homologs of Escherichia coli RecA recombination protein, which have been found throughout the living kingdom, promote homologous pairing and strand exchange. The nucleoprotein filament, within which strand exchange occurs, has been conserved through evolution, but conservation of the polarity of exchange and the significance of that directionality has not been settled. Using oligonucleotides as substrates, and assays based on fluorescence resonance energy transfer (FRET), we distinguished the biased formation of homologous joints at either end of duplex DNA from the subsequent directionality of strand exchange. As with E. coli RecA protein, the homologous Rad51 proteins from both Homo sapiens (HsRad51) and Saccharomyces cerevisiae (ScRad51) propagated DNA strand exchange preferentially in the 5' to 3' direction. The data suggest that 5' to 3' polarity is a conserved intrinsic property of recombination filaments.

Sequence analysis and expression of a novel mouse homolog of Escherichia coli recA gene

Escherichia coli recA and its yeast homologs RAD51 and DMC1 play crucial roles in mitotic and/or meiotic recombination and in repair of double-strand DNA breaks. We have identified a murine novel recA-like gene (MmTRAD). The predicted 329 amino acid protein showed significant homology to mouse Rec2, Rad51, Dmc1 (or Lim15) and E. coli RecA. Northern blot analysis revealed that MmTRAD was ubiquitously transcribed in various tissues.

Identification and characterization of the RAD51 gene from the ciliate Tetrahymena thermophila

The RAD51 gene is a eukaryotic homolog of rec A, a critical component in homologous recombination and DNA repair pathways in Escherichia coli . We have cloned the RAD51 homolog from Tetrahymena thermophila , a ciliated protozoan. Tetrahymena thermophila RAD51 encodes a 36.3 kDa protein whose amino acid sequence is highly similar to representative Rad51 homologs from other eukaryotic taxa. Recombinant Rad51 protein was purified to near homogeneity following overproduction in a bacterial expression system. The purified protein binds to both single- and double-stranded DNA, possesses a DNA-dependent ATPase activity and promotes intermolecular ligation of linearized plasmid DNA. While steady-state levels of Rad51 mRNA are low in normally growing cells, treatment with UV light resulted in a >100-fold increase in mRNA levels. This increase in mRNA was time dependent, but relatively independent of UV dose over a range of 1400-5200 J/m2. Western blot analysis confirmed that Rad51 protein levels increase upon UV irradiation. Exposure to the alkylating agent methyl methane sulfonate also resulted in substantially elevated Rad51 protein levels in treated cells, with pronounced localization in the macronucleus. These data are consistent with the hypothesis that ciliates such as T.thermophila utilize a Rad51-dependent pathway to repair damaged DNA.

Ku70 is required for late B cell development and immunoglobulin heavy chain class switching

Immunoglobulin (Ig) heavy chain (HC) class switch recombination (CSR) is a late B cell process that involves intrachromosomal DNA rearrangement. Ku70 and Ku80 form a DNA end-binding complex required for DNA double strand break repair and V(D)J recombination. Ku70(-/-) (K70T) mice, like recombination activating gene (RAG)-1- or RAG-2-deficient (R1T or R2T) mice, have impaired B and T cell development at an early progenitor stage, which is thought to result at least in part from defective V(D)J recombination (Gu, Y., K.J. Seidl, G.A. Rathbun, C. Zhu, J.P. Manis, N. van der Stoep, L. Davidson, H.L. Cheng, J.M. Sekiguchi, K. Frank, et al. 1997. Immunity. 7:653-665; Ouyang, H., A. Nussenzweig, A. Kurimasa, V.C. Soares, X. Li, C. Cordon-Cardo, W. Li, N. Cheong, M. Nussenzweig, G. Iliakis, et al. 1997. J. Exp. Med. 186:921-929). Therefore, to examine the potential role of Ku70 in CSR, we generated K70T mice that carry a germline Ig HC locus in which the JH region was replaced with a functionally rearranged VH(D)JH and Ig lambda light chain transgene (referred to as K70T/HL mice). Previously, we have shown that B cells from R1T or R2T mice carrying these rearranged Ig genes (R1T/HL or R2T/HL mice) can undergo CSR to IgG isotypes (Lansford, R., J. Manis, E. Sonoda, K. Rajewsky, and F. Alt. 1998. Int. Immunol. 10:325-332). K70T/HL mice had significant numbers of peripheral surface IgM+ B cells, which generated serum IgM levels similar to those of R2T/HL mice. However, in contrast to R2T/HL mice, K70T/HL mice had no detectable serum IgG isotypes. In vitro culture of K70T/HL B cells with agents that induce CSR in normal or R2T/HL B cells did lead to the induction of germline CH transcripts, indicating that initial signaling pathways for CSR were intact in K70T/HL cells. However, treatment with such agents did not lead to detectable CSR by K70T/HL B cells, and instead, led to cell death within 72 h. We conclude that Ku70 is required for the generation of B cells that have undergone Ig HC class switching. Potential roles for Ku70 in the CSR process are discussed.

The human HNRPD locus maps to 4q21 and encodes a highly conserved protein

The hnRNP D protein interacts with nucleic acids both in vivo and in vitro. Like many other proteins that interact with RNA, it contains RBD (or "RRM") domains and arg-gly-gly (RGG) motifs. We have examined the organization and localization of the human and murine genes that encode the hnRNP D protein. Comparison of the predicted sequences of the hnRNP D proteins in human and mouse shows that they are 96.9% identical (98.9% similar). This very high level of conservation suggests a critical function for hnRNP D. Sequence analysis of the human HNRPD gene shows that the protein is encoded by eight exons and that two additional exons specify sequences in the 3' UTR. Use of two of the coding exons is determined by alternative splicing of the HNRPD mRNA. The human HNRPD gene maps to 4q21. The mouse Hnrpd gene maps to the F region of chromosome 3, which is syntenic with the human 4q21 region.

Recombination-based mechanisms for somatic hypermutation

We review some experiments designed to test recombination-based mechanisms for somatic hypermutation in mice, particularly mechanisms involving templated mutation or gene conversion. As recombination and repair functions are highly conserved among prokaryotes and eukaryotes, pathways of mutation in microorganisms may prove relevant to the mechanism of somatic hypermutation. Escherichia coli initiates a recombination-based pathway of mutation in response to environmental stimuli, and this "adaptive" pathway of mutation has striking similarities with somatic hypermutation, as does a process of mutagenic repair that occurs at double-strand breaks in Saccharomyces cerevisiae. We present a model for recombination-based hypermutation of the immunoglobulin loci which could result in either templated or non-templated mutation.

The preference for GT-rich DNA by the yeast Rad51 protein defines a set of universal pairing sequences

The Rad51 protein of Saccharomyces cerevisiae is a eukaryotic homolog of the RecA protein, the prototypic DNA strand-exchange protein of Escherichia coli. RAD51 gene function is required for efficient genetic recombination and for DNA double-strand break repair. Recently, we demonstrated that RecA protein has a preferential affinity for GT-rich DNA sequences-several of which exhibit enhanced RecA protein-promoted homologous pairing activity. The fundamental similarity between the RecA and Rad51 proteins suggests that Rad51 might display an analogous bias. Using in vitro selection, here we show that the yeast Rad51 protein shares the same preference for GT-rich sequences as its prokaryotic counterpart. This bias is also manifest as an increased ability of Rad51 protein to promote the invasion of supercoiled DNA by homologous GT-rich single-stranded DNA, an activity not previously described for the eukaryotic pairing protein. We propose that the preferred utilization of GT-rich sequences is a conserved feature among all homologs of RecA protein, and that GT-rich regions are loci for increased genetic exchange in both prokaryotes and eukaryotes.

A novel nucleic acid-binding protein that interacts with human rad51 recombinase

Using the yeast two-hybrid system, we isolated a cDNA encoding a novel human protein, named Pir51, that strongly interacts with human Rad51 recombinase. Analysis in vitro confirmed the interaction between Rad51 and Pir51. Pir51 mRNA is expressed in a number of human organs, most notably in testis, thymus, colon and small intestine. The Pir51 gene locus was mapped to chromosome 12p13.1-13. 2 by fluorescence in situ hybridization. The Pir51 protein was expressed in Escherichia coli and purified to near homogeneity. Biochemical analysis shows that the Pir51 protein binds both single- and double-stranded DNA, and is capable of aggregating DNA. The protein also binds RNA. The Pir51 protein may represent a new member of the multiprotein complexes postulated to carry out homologous recombination and DNA repair in mammalian cells.

Nuclear Rad51 foci induced by DNA damage are distinct from Rad51 foci associated with B cell activation and recombination

Lipopolysaccharide (LPS) is a B cell mitogen which can stimulate murine primary B cells to proliferate and carry out immunoglobulin heavy chain class switch recombination. LPS can also function as an endotoxin, which may cause DNA damage and apoptosis in certain types of cells. We have previously reported that LPS-activated primary murine B cells contain nuclear foci that stain brightly with anti-Rad51 antibodies (Li et al. (1996) Proc. Natl. Acad. Sci. USA 93, 10222-10227). We have now analyzed Rad51 nuclear foci induced in both primary and immortalized B cells by treatment with the DNA damaging agent, methyl methanesulfonate (MMS). We have found that, in LPS-cultured primary B cells, MMS treatment increases the fraction of cells containing Rad51 foci and induces formation of a very high number of foci per cell. The foci induced by MMS treatment are small, punctate, and numerous; in contrast, the foci induced by LPS activation are large, brightly staining, and relatively few in number. In LPS-cultured primary B cells, Rad51 relocalizes during the cell cycle, and large, brightly staining nuclear foci are present in only restricted stages of the cell cycle. Rad51 foci similar to those present in LPS-activated primary B cells are also observed in immortalized B cells lines cultured in the absence of LPS. These foci are unaltered in number or appearance by culture with LPS, but treatment of immortalized B cell lines with MMS induces foci which are small and punctate in staining, like those induced by MMS in primary B cells. These data show that distinctive Rad51 foci are induced by DNA damaging agents and cell activation and that the response to DNA damage may involve pathways distinct from those associated with B cell activation and switch recombination.

Augmented expression of a human gene for 8-oxoguanine DNA glycosylase (MutM) in B lymphocytes of the dark zone in lymph node germinal centers

B cells that mediate normal, T cell-dependent, humoral immune responses must first pass through germinal centers (GCs) within the cortex of antigenically stimulated lymph nodes. As they move through the dark zone and then the light zone in the GC, B cells are subjected to somatic hypermutation and switch recombination within their rearranged immunoglobulin genes and also participate in a number of other processes that control development into memory cells or cells specialized for antibody secretion. To investigate the molecular mechanisms that contribute to B cell development within GCs, we constructed a recombinant DNA library enriched for cDNAs derived from human genes expressed in B cells at this site. This library was found to contain a cDNA structurally and functionally related to genes in bacteria and yeast for the DNA repair enzyme 8-oxoguanine DNA glycosylase. Northern blot analysis indicated that the human gene is expressed as two alternatively spliced messenger RNAs within GC B cells at levels greatly exceeding that found in other tissues. In situ hybridization studies revealed that expression of this gene is most abundant within the dark zones of GCs. Both the function and localized expression of this gene suggest that it may play a role in somatic hypermutation of immunoglobulin genes.

Recombination activities of HsDmc1 protein, the meiotic human homolog of RecA protein

Meiosis-specific homologs of RecA protein have been identified in Saccharomyces cerevisiae and higher eukaryotes including mammals, but their enzymatic activities have not been described. We have purified the human protein HsDmc1 produced in Escherichia coli from a cloned copy of the cDNA. The recombinant enzyme had DNA-dependent ATPase activity with an estimated kcat of 1.5 min-1. DNase protection experiments with oligonucleotides as substrates indicated that HsDmc1 protein binds preferentially to single-stranded DNA with a stoichiometry of approximately one molecule of protein per three nucleotide residues. HsDmc1 protein catalyzed the formation of D-loops in superhelical DNA, as well as strand exchange between single-stranded and double-stranded oligonucleotides. The requirements for strand exchange catalyzed by HsDmc1 were similar to those of RecA protein, but exchange caused by HsDmc1 was not supported by ATPgammaS.

Interaction of p53 with the human Rad51 protein

p53 is thought to function in the maintenance of genomic stability by modulating transcription and interacting with cellular proteins to influence the cell cycle, DNA repair and apoptosis. p53 mutations occur in >50% of human cancers, and cells which lack wild type p53 accumulate karyotypic abnormalities such as amplifications, deletions, inversions and translocations. We propose that p53 hinders these promiscuous recombinational events by interacting with cellular recombination and repair machinery. We recently reported that p53 can directly bind in vivo to human Rad51 (hRad51) protein and in vitro to its bacterial homologue RecA. We used GST-fusion and his-tagged protein systems to further investigate the physical interaction between p53 and hRad51, homologue of the yeast Rad51 protein that is involved in recombination and DNA double strand repair. The hRad51 binds to wild-type p53 and to a lesser extent, point mutants 135Y, 249S and 273H. This binding is not mediated by a DNA or RNA intermediate. Mapping studies using a panel of p53 deletion mutants indicate that hRad51 could bind to two regions of p53; one between amino acids 94 and 160 and a second between 264 and 315. Addition of anti-p53 antibody PAb421 (epitope 372-381 amino acids) inhibited the interaction with hRad51. In contrast, p53 interacts with the region between aa 125 and 220 of hRad51, which is highly conserved among Rad51 related proteins from bacteria to human. In Escherichia coli ecA protein, this region is required for homo-oligomerization, suggesting that p53 might disrupt the interaction between RecA and Rad51 subunits, thus inhibiting biochemical functions of Rad51 like proteins. These data are consistent with the hypothesis that p53 interaction with hRAD51 may influence DNA recombination and repair and that additional modifications of p53 by mutation and protein binding may affect this interaction.

hMre11 and hRad50 nuclear foci are induced during the normal cellular response to DNA double-strand breaks

We previously identified a conserved multiprotein complex that includes hMre11 and hRad50. In this study, we used immunofluorescence to investigate the role of this complex in DNA double-strand break (DSB) repair. hMre11 and hRad50 form discrete nuclear foci in response to treatment with DSB-inducing agents but not in response to UV irradiation. hMre11 and hRad50 foci colocalize after treatment with ionizing radiation and are distinct from those of the DSB repair protein, hRad51. Our data indicate that an irradiated cell is competent to form either hMre11-hRad50 foci or hRad51 foci, but not both. The multiplicity of hMre11 and hRad50 foci is much higher in the DSB repair-deficient cell line 180BR than in repair-proficient cells. hMre11-hRad50 focus formation is markedly reduced in cells derived from ataxia-telangiectasia patients, whereas hRad51 focus formation is markedly increased. These experiments support genetic evidence from Saccharomyces cerevisiae indicating that Mre11-Rad50 have roles distinct from that of Rad51 in DSB repair. Further, these data indicate that hMre11-hRad50 foci form in response to DNA DSBs and are dependent upon a DNA damage-induced signaling pathway.

The human Rad51 protein: polarity of strand transfer and stimulation by hRP-A

The human Rad51 protein is homologous to the RecA protein and catalyses homologous pairing and strand transfer reactions in vitro. Using single-stranded circular and homologous linear duplex DNA, we show that hRad51 forms stable joint molecules by transfer of the 5' end of the complementary strand of the linear duplex to the ssDNA. The polarity of strand transfer is therefore 3' to 5', defined relative to the ssDNA on which hRad51 initiates filament formation. This polarity is opposite to that observed with RecA. Homologous pairing and strand transfer require stoichiometric amounts of hRad51, corresponding to one hRad51 monomer per three nucleotides of ssDNA. Joint molecules are not observed when the protein is present in limiting or excessive amounts. The human ssDNA binding-protein, hRP-A, stimulates hRad51-mediated reactions. Its effect is consistent with a role in the removal of secondary structures from ssDNA, thereby facilitating the formation of continuous Rad51 filaments.

Purification of human Rad51 protein by selective spermidine precipitation

The human Rad51 protein is a structural homolog of Escherichia coli RecA. The exact role of human Rad51 within the cell is poorly understood but, like its bacterial and yeast homologs, hRad51 is believed to play a central role in homologous recombination. However, recent reports that transgenic mice lacking the RAD51 gene die early in development suggest an additional and essential function for mammalian Rad51 in cell proliferation or genome maintenance. In this paper we describe a simple and quick method for the purification of human Rad51 overproduced in E. coli. Dialysis of cell-free extracts against buffer containing low concentrations of spermidine result in the formation of hRad51 microcrystals as observed by light and electron microscopy. The crystals were easily redissolved in phosphate buffer and hRad51 was further purified to homogeneity using hydroxylapatite, affi-gel heparin and Q-sepharose chromatography. When purified by this method hRad51 is free of endo- and exonuclease activities and suitable for enzymological studies. Spermidine precipitation also provides a rapid method for the large scale purification of hRad51 suitable for physical analysis.

RAB22 and RAB163/mouse BRCA2: proteins that specifically interact with the RAD51 protein

The human RAD51 protein is a homologue of the bacteria RecA and yeast RAD51 proteins that are involved in homologous recombination and DNA repair. RAD51 interacts with proteins involved in recombination and also with tumor suppressor proteins p53 and breast cancer susceptibility gene 1 (BRCA1). We have used the yeast two-hybrid system to clone murine cDNA sequences that encode two RAD51-associated molecules, RAB22 and RAB163. RAB163 encodes the C-terminal portion of mouse BRCA2, the homologue of the second breast cancer susceptibility gene protein in humans, demonstrating an in vitro association between RAD51 and BRCA2. RAB22 is a novel gene product that also interacts with RAD51 in vitro. To detect RAD51 interactions in vivo, we developed a transient nuclear focus assay that was used to demonstrate a complete colocalization of RAB22 with RAD51 in large nuclear foci.

Disruption of mouse RAD54 reduces ionizing radiation resistance and homologous recombination

Double-strand DNA break (DSB) repair by homologous recombination occurs through the RAD52 pathway in Saccharomyces cerevisiae. Its biological importance is underscored by the conservation of many RAD52 pathway genes, including RAD54, from fungi to humans. We have analyzed the phenotype of mouse RAD54-/- (mRAD54-/-) cells. Consistent with a DSB repair defect, these cells are sensitive to ionizing radiation, mitomycin C, and methyl methanesulfonate, but not to ultraviolet light. Gene targeting experiments demonstrate that homologous recombination in mRAD54-/- cells is reduced compared to wild-type cells. These results imply that, besides DNA end-joining mediated by DNA-dependent protein kinase, homologous recombination contributes to the repair of DSBs in mammalian cells. Furthermore, we show that mRAD54-/- mice are viable and exhibit apparently normal V(D)J and immunoglobulin class-switch recombination. Thus, mRAD54 is not required for the recombination processes that generate functional immunoglobulin and T cell receptor genes.

Nucleolin is one component of the B cell-specific transcription factor and switch region binding protein, LR1

LR1 is a B cell-specific, sequence-specific DNA binding activity that regulates transcription in activated B cells. LR1 also binds Ig heavy chain switch region sequences and may function in class switch recombination. LR1 contains two polypeptides, of 106 kDa and 45 kDa, and here we report that the 106-kDa component of LR1 is nucleolin. This identification, initially made by microsequence analysis, was verified by showing that (i) LR1-DNA binding activity increased in B cells transfected with a nucleolin cDNA expression construct; (ii) LR1-DNA binding activity was recognized by antibodies raised against recombinant human nucleolin; and (iii) in B cells transfected with epitope-tagged nucleolin expression constructs, the LR1-DNA complex was recognized by the anti-tag antibody. Nucleolin is an abundant nucleolar protein which is believed to play a role in rDNA transcription or organization, or rRNA processing. Homology between nucleolin and histone H1 suggests that nucleolin may alter DNA organization in response to cell cycle controls, and the nucleolin component of LR1 may therefore function to organize switch regions before, during, or after switch recombination. The demonstration that nucleolin is a component of a B cell-specific complex that binds switch region sequences suggests that the G-rich switch regions may have evolved from rDNA.

Activities of human recombination protein Rad51

Homologous pairing and strand exchange, which are catalyzed by Escherichia coli RecA protein, are central to homologous recombination. Homologs of this protein are found in eukaryotes; however, little has been reported on the recombinase activities of the mammalian homologs, including the human protein, denoted HsRad51. For the studies described here, we purified HsRad51 form E. coli. Although the activities of HsRad51 and RecA were qualitatively similar in the presence of ATP, there were also striking differences. The stoichiometry of binding to DNA and the rate of renaturation of complementary strands were similar for the two proteins, but rates of ATP hydrolysis, homologous pairing, and subsequent strand exchange promoted by HsRad51 were less than 1/10 those of RecA. In addition, HsRad51 bound gamma-thio-ATP and formed stable presynaptic complexes that promoted renaturation as rapidly as RecA, but the recombinant human protein catalyzed neither strand exchange nor homologous pairing of a single strand with duplex DNA in the presence of the ATP analog. By contrast, RecA promoted both of the latter reactions in control experiments. These observations suggest that among RecA-like proteins, HsRad51 may be a variant in which homologous pairing and strand exchange are more closely linked to the hydrolysis of ATP.