Fission yeast rad51 and dmc1, two efficient DNA recombinases forming helical nucleoprotein filaments

Dysfunction of Gpl1-Gih35-Wdr83 Complex in S. pombe Affects the Splicing of DNA Damage Repair Factors Resulting in Increased Sensitivity to DNA Damage

Pre-mRNA splicing plays a key role in the regulation of gene expression. Recent discoveries suggest that defects in pre-mRNA splicing, resulting from the dysfunction of certain splicing factors, can impact the expression of genes crucial for genome surveillance mechanisms, including those involved in cellular response to DNA damage. In this study, we analyzed how cells with a non-functional spliceosome-associated Gpl1-Gih35-Wdr83 complex respond to DNA damage. Additionally, we investigated the role of this complex in regulating the splicing of factors involved in DNA damage repair. Our findings reveal that the deletion of any component within the Gpl1-Gih35-Wdr83 complex leads to a significant accumulation of unspliced pre-mRNAs of DNA repair factors. Consequently, mutant cells lacking this complex exhibit increased sensitivity to DNA-damaging agents. These results highlight the importance of the Gpl1-Gih35-Wdr83 complex in regulating the expression of DNA repair factors, thereby protecting the stability of the genome following DNA damage.

Hop2-Mnd1 and Swi5-Sfr1 stimulate Dmc1 filament assembly using distinct mechanisms

In meiosis, Dmc1 recombinase and the general recombinase Rad51 are responsible for pairing homologous chromosomes and exchanging strands. Fission yeast (Schizosaccharomyces pombe) Swi5-Sfr1 and Hop2-Mnd1 stimulate Dmc1-driven recombination, but the stimulation mechanism is unclear. Using single-molecule fluorescence resonance energy transfer (smFRET) and tethered particle motion (TPM) experiments, we showed that Hop2-Mnd1 and Swi5-Sfr1 individually enhance Dmc1 filament assembly on single-stranded DNA (ssDNA) and adding both proteins together allows further stimulation. FRET analysis showed that Hop2-Mnd1 enhances the binding rate of Dmc1 while Swi5-Sfr1 specifically reduces the dissociation rate during the nucleation, about 2-fold. In the presence of Hop2-Mnd1, the nucleation time of Dmc1 filaments shortens, and doubling the ss/double-stranded DNA (ss/dsDNA) junctions of DNA substrates reduces the nucleation times in half. Order of addition experiments confirmed that Hop2-Mnd1 binds on DNA to recruit and stimulate Dmc1 nucleation at the ss/dsDNA junction. Our studies directly support the molecular basis of how Hop2-Mnd1 and Swi5-Sfr1 act on different steps during the Dmc1 filament assembly. DNA binding of these accessory proteins and nucleation preferences of recombinases thus dictate how their regulation can take place.

Can PCNA and LIM15 gene expression levels predict sperm retrieval success in men with non-obstructive azoospermia?

INTRODUCTION AND OBJECTIVES

It is necessary to be able to predict sperm retrieval before microdissection testicular sperm extraction (mTESE) in azoospermic men. This study established the importance of proliferating cell nuclear antigen (PCNA) and LIM15 gene expression levels in predicting the success of sperm retrieval by mTESE.

MATERIALS AND METHODS

One hundred and forty-three men who were diagnosed with non-obstructive azoospermia (NOA) were included in the study. Patients' age, total testosterone and follicle stimulating hormone values, testicular volume and testicular histology were recorded by prospectively. PCNA and LIM15 gene expression levels were determined by real-time PCR in the materials from both ejaculate and testicular specimens.

RESULTS

Testis volume and histology were the most important factors in predicting the sperm retrieval rate (SRR). The PCNA and LIM15 gene expression levels measured in testicular tissues and the LIM15 gene expression levels measured in ejaculate significantly correlated with the SRR in mTESE (p=0.038, p=0.022, and p=0.004, respectively). Although the PCNA gene expression level measured in ejaculate was higher in men with successful sperm retrieval, the difference was not statistically significant (p=0.061). According to the multivariate logistic regression analysis, testicular volume and LIM15 gene expression level in ejaculate were independent predictive parameters for sperm retrieval.

CONCLUSION

The data showed that LIM15 gene expression level in ejaculate is a useful molecular marker to predict the SRR before mTESE.

FIGNL1 Inhibits Non-homologous Chromosome Association and Crossover Formation

Meiotic crossovers (COs) not only generate genetic diversity but also ensure the accuracy of homologous chromosome segregation. Here, we identified FIGNL1 as a new inhibitor for extra crossover formation in rice. The fignl1 mutant displays abnormal interactions between non-homologous chromosomes at diakinesis, and chromosome bridges and fragmentation at subsequent stages of meiosis, but shows normal homologous chromosome pairing and synapsis during early prophase I. FIGNL1 participates in homologous chromosome recombination and functions downstream of DMC1. Mutation of FIGNL1 increases the number of bivalents in zip4 mutants, but does not change the number of HEI10 foci, indicating that FIGNL1 functions in limiting class II CO formation. FIGNL1 interacts with MEICA1, and colocalizes with MEICA1 in a dynamic pattern as punctate foci located between two linear homologous chromosomes. The localization of FIGNL1 depends on ZEP1-mediated assembly of the synaptonemal complex. Based on these results, we propose that FIGNL1 inhibits non-homologous chromosome interaction and CO formation during rice meiosis.

DNA Repair in Haploid Context

DNA repair is a well-covered topic as alteration of genetic integrity underlies many pathological conditions and important transgenerational consequences. Surprisingly, the ploidy status is rarely considered although the presence of homologous chromosomes dramatically impacts the repair capacities of cells. This is especially important for the haploid gametes as they must transfer genetic information to the offspring. An understanding of the different mechanisms monitoring genetic integrity in this context is, therefore, essential as differences in repair pathways exist that differentiate the gamete’s role in transgenerational inheritance. Hence, the oocyte must have the most reliable repair capacity while sperm, produced in large numbers and from many differentiation steps, are expected to carry de novo variations. This review describes the main DNA repair pathways with a special emphasis on ploidy. Differences between Saccharomyces cerevisiae and Schizosaccharomyces pombe are especially useful to this aim as they can maintain a diploid and haploid life cycle respectively.

Biochemical properties of fission yeast homologous recombination enzymes

Homologous recombination (HR) is a universal phenomenon conserved from viruses to humans. The mechanisms of HR are essentially the same in humans and simple unicellular eukaryotes like yeast. Two highly diverged yeast species, Saccharomyces cerevisiae and Schizosaccharomyces pombe, have proven exceptionally useful in understanding the fundamental mechanisms of eukaryotic HR by serving as a source for unique biological insights and also complementing each other. Here, we will review the features of S. pombe HR mechanisms in comparison to S. cerevisiae and other model organisms. Particular emphasis will be put on the biochemical characterization of HR mechanisms uncovered using S. pombe proteins.

Real-time tracking reveals catalytic roles for the two DNA binding sites of Rad51

During homologous recombination, Rad51 forms a nucleoprotein filament on single-stranded DNA to promote DNA strand exchange. This filament binds to double-stranded DNA (dsDNA), searches for homology, and promotes transfer of the complementary strand, producing a new heteroduplex. Strand exchange proceeds via two distinct three-strand intermediates, C1 and C2. C1 contains the intact donor dsDNA whereas C2 contains newly formed heteroduplex DNA. Here, we show that the conserved DNA binding motifs, loop 1 (L1) and loop 2 (L2) in site I of Rad51, play distinct roles in this process. L1 is involved in formation of the C1 complex whereas L2 mediates the C1–C2 transition, producing the heteroduplex. Another DNA binding motif, site II, serves as the DNA entry position for initial Rad51 filament formation, as well as for donor dsDNA incorporation. Our study provides a comprehensive molecular model for the catalytic process of strand exchange mediated by eukaryotic RecA-family recombinases.

Rad51 drives DNA strand exchange, the central reaction in recombinational DNA repair. Two sites of Rad51 are responsible for DNA binding, but the function of these sites has proven elusive. Here, the authors employ real-time assays to reveal catalytic roles for the two DNA binding sites of Rad51.

Rad51 facilitates filament assembly of meiosis-specific Dmc1 recombinase

Dmc1 recombinases are essential to homologous recombination in meiosis. Here, we studied the kinetics of the nucleoprotein filament assembly of Saccharomyces cerevisiae Dmc1 using single-molecule tethered particle motion experiments and in vitro biochemical assay. ScDmc1 nucleoprotein filaments are less stable than the ScRad51 ones because of the kinetically much reduced nucleation step. The lower nucleation rate of ScDmc1 results from its lower single-stranded DNA (ssDNA) affinity, compared to that of ScRad51. Surprisingly, ScDmc1 nucleates mostly on the DNA structure containing the single-stranded and duplex DNA junction with the allowed extension in the 5'-to-3' polarity, while ScRad51 nucleation depends strongly on ssDNA lengths. This nucleation preference is also conserved for mammalian RAD51 and DMC1. In addition, ScDmc1 nucleation can be stimulated by short ScRad51 patches, but not by EcRecA ones. Pull-down experiments also confirm the physical interactions of ScDmc1 with ScRad51 in solution, but not with EcRecA. Our results are consistent with a model that Dmc1 nucleation can be facilitated by a structural component (such as DNA junction and protein-protein interaction) and DNA polarity. They provide direct evidence of how Rad51 is required for meiotic recombination and highlight a regulation strategy in Dmc1 nucleoprotein filament assembly.

Characterization of the recombination activities of the Entamoeba histolytica Rad51 recombinase

The protozoan parasite responsible for human amoebiasis is Entamoeba histolytica. An important facet of the life cycle of E. histolytica involves the conversion of the mature trophozoite to a cyst. This transition is thought to involve homologous recombination (HR), which is dependent upon the Rad51 recombinase. Here, a biochemical characterization of highly purified ehRad51 protein is presented. The ehRad51 protein preferentially binds ssDNA, forms a presynaptic filament and possesses ATP hydrolysis activity that is stimulated by the presence of DNA. Evidence is provided that ehRad51 catalyzes robust DNA strand exchange over at least 5.4 kilobase pairs. Although the homologous DNA pairing activity of ehRad51 is weak, it is strongly enhanced by the presence of two HR accessory cofactors, calcium and Hop2-Mnd1. The biochemical system described herein was used to demonstrate the potential for targeting ehRad51 with two small molecule inhibitors of human RAD51. We show that 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) inhibited ehRad51 by interfering with DNA binding and attenuated encystation in Entamoeba invadens, while B02 had no effect on ehRad51 strand exchange activity. These results provide insight into the underlying mechanism of homology-directed DNA repair in E. histolytica.

DNA strand exchange and RecA homologs in meiosis

Homology search and DNA strand-exchange reactions are central to homologous recombination in meiosis. During meiosis, these processes are regulated such that the probability of choosing a homolog chromatid as recombination partner is enhanced relative to that of choosing a sister chromatid. This regulatory process occurs as homologous chromosomes pair in preparation for assembly of the synaptonemal complex. Two strand-exchange proteins, Rad51 and Dmc1, cooperate in regulated homology search and strand exchange in most organisms. Here, we summarize studies on the properties of these two proteins and their accessory factors. In addition, we review current models for the assembly of meiotic strand-exchange complexes and the possible mechanisms through which the interhomolog bias of recombination partner choice is achieved.

A role for nuclear envelope-bridging complexes in homology-directed repair

Persistent double-strand DNA breaks (DSBs) are recruited to the nuclear periphery, where they induce formation of associated nuclear envelope–spanning LINC complexes made up of the SUN protein Sad1 and the KASH protein Kms1. The LINC complex couples DSBs within the nucleus to cytoplasmic microtubules, which alters DSB repair pathway choice.

Unless efficiently and faithfully repaired, DNA double-strand breaks (DSBs) cause genome instability. We implicate a Schizosaccharomyces pombe nuclear envelope–spanning linker of nucleoskeleton and cytoskeleton (LINC) complex, composed of the Sad1/Unc84 protein Sad1 and Klarsicht/Anc1/SYNE1 homology protein Kms1, in the repair of DSBs. An induced DSB associates with Sad1 and Kms1 in S/G2 phases of the cell cycle, connecting the DSB to cytoplasmic microtubules. DSB resection to generate single-stranded DNA and the ATR kinase drive the formation of Sad1 foci in response to DNA damage. Depolymerization of microtubules or loss of Kms1 leads to an increase in the number and size of DSB-induced Sad1 foci. Further, Kms1 and the cytoplasmic microtubule regulator Mto1 promote the repair of an induced DSB by gene conversion, a type of homology-directed repair. kms1 genetically interacts with a number of genes involved in homology-directed repair; these same gene products appear to attenuate the formation or promote resolution of DSB-induced Sad1 foci. We suggest that the connection of DSBs with the cytoskeleton through the LINC complex may serve as an input to repair mechanism choice and efficiency.

Functional attributes of the Saccharomyces cerevisiae meiotic recombinase Dmc1

The role of Dmc1 as a meiosis-specific general recombinase was first demonstrated in Saccharomyces cerevisiae. Progress in understanding the biochemical mechanism of ScDmc1 has been hampered by its tendency to form inactive aggregates. We have found that the inclusion of ATP during protein purification prevents Dmc1 aggregation. ScDmc1 so prepared is capable of forming D-loops and responsive to its accessory factors Rad54 and Rdh54. Negative staining electron microscopy and iterative helical real-space reconstruction revealed that the ScDmc1-ssDNA nucleoprotein filament harbors 6.5 protomers per turn with a pitch of ∼106Å. The ScDmc1 purification procedure and companion molecular analyses should facilitate future studies on this recombinase.

Interactions between BRCA2 and RAD51 for promoting homologous recombination in Leishmania infantum

In most organisms, the primary function of homologous recombination (HR) is to allow genome protection by the faithful repair of DNA double-strand breaks. The vital step of HR is the search for sequence homology, mediated by the RAD51 recombinase, which is stimulated further by proteins mediators such as the tumor suppressor BRCA2. The biochemical interplay between RAD51 and BRCA2 is unknown in Leishmania or Trypanosoma. Here we show that the Leishmania infantum BRCA2 protein possesses several critical features important for the regulation of DNA recombination at the genetic and biochemical level. A BRCA2 null mutant, generated by gene disruption, displayed genomic instability and gene-targeting defects. Furthermore, cytological studies show that LiRAD51 can no longer localize to the nucleus in this mutant. The Leishmania RAD51 and BRCA2 interact together and the purified proteins bind single-strand DNA. Remarkably, LiBRCA2 is a recombination mediator that stimulates the invasion of a resected DNA double-strand break in an undamaged template by LiRAD51 to form a D-loop structure. Collectively, our data show that LiBRCA2 and LiRAD51 promote HR at the genetic and biochemical level in L. infantum, the causative agent of visceral leishmaniasis.

From meiosis to postmeiotic events: uncovering the molecular roles of the meiosis-specific recombinase Dmc1

In meiosis, the accurate segregation of maternal and paternal chromosomes is accomplished by homologous recombination. A central player in meiotic recombination is the Dmc1 recombinase, a member of the RecA/Rad51 recombinase superfamily, which is widely conserved from viruses to humans. Dmc1 is a meiosis-specific protein that functions with the ubiquitously expressed homolog, the Rad51 recombinase, which is essential for both mitotic and meiotic recombination. Since its discovery, it has been speculated that Dmc1 is important for unique aspects of meiotic recombination. Understanding the distinctive properties of Dmc1, namely, the features that distinguish it from Rad51, will further clarify the mechanisms of meiotic recombination. Recent structural, biochemical, and genetic findings are now revealing the molecular mechanisms of Dmc1-mediated homologous recombination and its regulation by various recombination mediators.

SSO1450--a CAS1 protein from Sulfolobus solfataricus P2 with high affinity for RNA and DNA

Clustered regularly interspaced short palindromic repeats (CRISPR) and their associated protein genes (cas genes) are ubiquitous in archaea and eubacteria. It has been suggested that CRISPR and CAS proteins act as an immune system preventing the invasion of foreign genomic elements at the DNA level. The protein SSO1450 from Sulfolobus solfataricus (Sso) P2 belongs to the CAS1 cluster which is one of the core protein clusters most frequently associated with CRISPR sequences. In this study we show that SSO1450 is a high-affinity nucleic acid binding protein. It binds DNA, RNA and DNA-RNA hybrid apparently sequence non-specific in a multi-site binding mode. Furthermore, SSO1450 promotes the hybridization of complementary nucleic acid strands.

Functional interactions of meiotic recombination factors Rdh54 and Dmc1

Genetic studies in budding and fission yeasts have provided evidence that Rdh54, a Swi2/Snf2-like factor, synergizes with the Dmc1 recombinase to mediate inter-homologue recombination during meiosis. Rdh54 associates with Dmc1 in the yeast two-hybrid assay, but whether the Rdh54-Dmc1 interaction is direct and the manner in which these two recombination factors may functionally co-operate to accomplish their biological task have not yet been defined. Here, using purified Schizosaccharomyces pombe proteins, we demonstrate complex formation between Rdh54 and Dmc1 and enhancement of the recombinase activity of Dmc1 by Rdh54. Consistent with published cytological and chromatin immunoprecipitation data that implicate Rdh54 in preventing the non-specific association of Dmc1 with chromatin, we show here that Rdh54 mediates the efficient removal of Dmc1 from dsDNA. These functional attributes of Rdh54 are reliant on its ATPase function. The results presented herein provide valuable information concerning the Rdh54-Dmc1 protein pair that is germane for understanding their role in meiotic recombination. The biochemical systems established in this study should be useful for the continuing dissection of the action mechanism of Rdh54 and Dmc1.

Rad51 polymerization reveals a new chromatin remodeling mechanism

Rad51 protein is a well known protagonist of homologous recombination in eukaryotic cells. Rad51 polymerization on single-stranded DNA and its role in presynaptic filament formation have been extensively documented. Rad51 polymerizes also on double-stranded DNA but the significance of this filament formation remains unclear. We explored the behavior of Saccharomyces cerevisiae Rad51 on dsDNA and the influence of nucleosomes on Rad51 polymerization mechanism to investigate its putative role in chromatin accessibility to recombination machinery. We combined biochemical approaches, transmission electron microscopy (TEM) and atomic force microscopy (AFM) for analysis of the effects of the Rad51 filament on chromatinized templates. Quantitative analyses clearly demonstrated the occurrence of chromatin remodeling during nucleoprotein filament formation. During Rad51 polymerization, recombinase proteins moved all the nucleosomal arrays in front of the progressing filament. This polymerization process had a powerful remodeling effect, as Rad51 destabilized the nucleosomes along considerable stretches of DNA. Similar behavior was observed with RecA. Thus, recombinase polymerization is a powerful mechanism of chromatin remodeling. These remarkable features open up new possibilities for understanding DNA recombination and reveal new types of ATP-dependent chromatin dynamics.

Cohesin and recombination proteins influence the G1-to-S transition in azygotic meiosis in Schizosaccharomyces pombe

To determine whether recombination and/or sister-chromatid cohesion affect the timing of meiotic prophase events, the horsetail stage and S phase were analyzed in Schizosaccharomyces pombe strains carrying mutations in the cohesin genes rec8 or rec11, the linear element gene rec10, the pairing gene meu13, the double-strand-break formation genes rec6, rec7, rec12, rec14, rec15, and mde2, and the recombination gene dmc1. The double-mutant strains rec8 rec11 and rec8 rec12 were also assayed. Most of the single and both double mutants showed advancement of bulk DNA synthesis, start of nuclear movement (horsetail stage), and meiotic divisions by up to 2 hr. Only mde2 and dmc1 deletion strains showed wild-type timing. Contrasting behavior was observed for rec8 deletions (delayed by 1 hr) compared to a rec8 point mutation (advanced by 1 hr). An hypothesis for the role of cohesin and recombination proteins in the control of the G(1)-to-S transition is proposed. Finally, differences between azygotic meiosis and two other types of fission yeast meiosis (zygotic and pat1-114 meiosis) are discussed with respect to possible control steps in meiotic G(1).

Mechanism of eukaryotic homologous recombination

Homologous recombination (HR) serves to eliminate deleterious lesions, such as double-stranded breaks and interstrand crosslinks, from chromosomes. HR is also critical for the preservation of replication forks, for telomere maintenance, and chromosome segregation in meiosis I. As such, HR is indispensable for the maintenance of genome integrity and the avoidance of cancers in humans. The HR reaction is mediated by a conserved class of enzymes termed recombinases. Two recombinases, Rad51 and Dmc1, catalyze the pairing and shuffling of homologous DNA sequences in eukaryotic cells via a filamentous intermediate on ssDNA called the presynaptic filament. The assembly of the presynaptic filament is a rate-limiting process that is enhanced by recombination mediators, such as the breast tumor suppressor BRCA2. HR accessory factors that facilitate other stages of the Rad51- and Dmc1-catalyzed homologous DNA pairing and strand exchange reaction have also been identified. Recent progress on elucidating the mechanisms of action of Rad51 and Dmc1 and their cohorts of ancillary factors is reviewed here.

Functional and structural basis for a bacteriophage homolog of human RAD52

In eukaryotes, homologous recombination proteins such as RAD51 and RAD52 play crucial roles in DNA repair and genome stability. Human RAD52 is a member of a large single-strand annealing protein (SSAP) family [1] and stimulates Rad51-dependent recombination [2, 3]. In prokaryotes and phages, it has been difficult to establish the presence of RAD52 homologs with conserved sequences. Putative SSAPs were recently found in several phages that infect strains of Lactococcus lactis[4]. One of these SSAPs was identified as Sak and was found in the virulent L. lactis phage ul36, which belongs to the Siphoviridae family [4, 5]. In this study, we show that Sak is homologous to the N terminus of human RAD52. Purified Sak binds single-stranded DNA (ssDNA) preferentially over double-stranded DNA (dsDNA) and promotes the renaturation of long complementary ssDNAs. Sak also binds RecA and stimulates homologous recombination reactions. Mutations shown to modulate RAD52 DNA binding [6] affect Sak similarly. Remarkably, electron-microscopic reconstruction of Sak reveals an undecameric (11) subunit ring, similar to the crystal structure of the N-terminal fragment of human RAD52 [7, 8]. For the first time, we propose a viral homolog of RAD52 at the amino acid, phylogenic, functional, and structural levels.

Reconstitution of DNA strand exchange mediated by Rhp51 recombinase and two mediators

In the fission yeast Schizosaccharomyces pombe, genetic evidence suggests that two mediators, Rad22 (the S. pombe Rad52 homolog) and the Swi5-Sfr1 complex, participate in a common pathway of Rhp51 (the S. pombe Rad51 homolog)-mediated homologous recombination (HR) and HR repair. Here, we have demonstrated an in vitro reconstitution of the central step of DNA strand exchange during HR. Our system consists entirely of homogeneously purified proteins, including Rhp51, the two mediators, and replication protein A (RPA), which reflects genetic requirements in vivo. Using this system, we present the first robust biochemical evidence that concerted action of the two mediators directs the loading of Rhp51 onto single-stranded DNA (ssDNA) precoated with RPA. Dissection of the reaction reveals that Rad22 overcomes the inhibitory effect of RPA on Rhp51-Swi5-Sfr1-mediated strand exchange. In addition, Rad22 negates the requirement for a strict order of protein addition to the in vitro system. However, despite the presence of Rad22, Swi5-Sfr1 is still essential for strand exchange. Importantly, Rhp51, but neither Rad22 nor the Swi5-Sfr1 mediator, is the factor that displaces RPA from ssDNA. Swi5-Sfr1 stabilizes Rhp51-ssDNA filaments in an ATP-dependent manner, and this stabilization is correlated with activation of Rhp51 for the strand exchange reaction. Rad22 alone cannot activate the Rhp51 presynaptic filament. AMP-PNP, a nonhydrolyzable ATP analog, induces a similar stabilization of Rhp51, but this stabilization is independent of Swi5-Sfr1. However, hydrolysis of ATP is required for processive strand transfer, which results in the formation of a long heteroduplex. Our in vitro reconstitution system has revealed that the two mediators have indispensable, but distinct, roles for mediating Rhp51 loading onto RPA-precoated ssDNA.

The Rad52 homologs Rad22 and Rti1 of Schizosaccharomyces pombe are not essential for meiotic interhomolog recombination, but are required for meiotic intrachromosomal recombination and mating-type-related DNA repair

Proteins of the RAD52 epistasis group play an essential role in repair of some types of DNA damage and genetic recombination. In Schizosaccharomyces pombe, Rad22 (a Rad52 ortholog) has been shown to be as necessary for repair and recombination events during vegetative growth as its Saccharomyces cerevisiae counterpart. This finding contrasts with previous reports where, due to suppressor mutations in the fbh1 gene, rad22 mutants did not display a severe defect. We have analyzed the roles of Rad22 and Rti1, another Rad52 homolog, during meiotic recombination and meiosis in general. Both proteins play an important role in spore viability. During meiotic prophase I, they partially colocalize and partially localize to Rad51 foci and linear elements. Genetic analysis showed that meiotic interchromosomal crossover and conversion events were unexpectedly not much affected by deletion of either or both genes. A strong decrease of intrachromosomal recombination assayed by a gene duplication construct was observed. Therefore, we propose that the most important function of Rad22 and Rti1 in S. pombe meiosis is repair of double-strand breaks with involvement of the sister chromatids. In addition, a novel mating-type-related repair function of Rad22 specific to meiosis and spore germination is described.

Structural and functional analyses of the DMC1-M200V polymorphism found in the human population

The M200V polymorphism of the human DMC1 protein, which is an essential, meiosis-specific DNA recombinase, was found in an infertile patient, raising the question of whether this homozygous human DMC1-M200V polymorphism may cause infertility by affecting the function of the human DMC1 protein. In the present study, we determined the crystal structure of the human DMC1-M200V variant in the octameric-ring form. Biochemical analyses revealed that the human DMC1-M200V variant had reduced stability, and was moderately defective in catalyzing in vitro recombination reactions. The corresponding M194V mutation introduced in the Schizosaccharomyces pombe dmc1 gene caused a significant decrease in the meiotic homologous recombination frequency. Together, these structural, biochemical and genetic results provide extensive evidence that the human DMC1-M200V mutation impairs its function, supporting the previous interpretation that this single-nucleotide polymorphism is a source of human infertility.

A comparative analysis of Dmc1 and Rad51 nucleoprotein filaments

The eukaryotic RecA homologs Rad51 and Dmc1 are essential for strand exchange between homologous chromosomes during meiosis. All members of the RecA family of recombinases polymerize on DNA to form helical nucleoprotein filaments, which is the active form of the protein. Here we compare the filament structures of the Rad51 and Dmc1 proteins from both human and budding yeast. Previous studies of Dmc1 filaments suggested that they might be structurally distinct from filaments of other members of the RecA family, including Rad51. The data presented here indicate that Rad51 and Dmc1 filaments are essentially identical with respect to several structural parameters, including persistence length, helical pitch, filament diameter, DNA base pairs per helical turn and helical handedness. These data, together with previous studies demonstrating similar in vitro recombinase activity for Dmc1 and Rad51, support the view that differences in the meiotic function of Rad51 and Dmc1 are more likely to result from the influence of distinct sets of accessory proteins than from intrinsic differences in filament structure.

Biochemistry of Meiotic Recombination: Formation, Processing, and Resolution of Recombination Intermediates

Meiotic recombination ensures accurate chromosome segregation during the first meiotic division and provides a mechanism to increase genetic heterogeneity among the meiotic products. Unlike homologous recombination in somatic (vegetative) cells, where sister chromatid interactions prevail and crossover formation is avoided, meiotic recombination is targeted to involve homologs, resulting in crossovers to connect the homologs before anaphase of the first meiotic division. The mechanisms responsible for homolog choice and crossover control are poorly understood, but likely involve meiosis-specific recombination proteins, as well as meiosis-specific chromosome organization and architecture. Much progress has been made to identify and biochemically characterize many of the proteins acting during meiotic recombination. This review will focus on the proteins that generate and process heteroduplex DNA, as well as those that process DNA junctions during meiotic recombination, with particular attention to how recombination activities promote crossover resolution between homologs.

Fission yeast Swi5 protein, a novel DNA recombination mediator

The Schizosaccharomyces pombe Swi5 protein forms two distinct protein complexes, Swi5-Sfr1 and Swi5-Swi2, each of which plays an important role in the related but functionally distinct processes of homologous recombination and mating-type switching, respectively. The Swi5-Sfr1 mediator complex has been shown to associate with the two RecA-like recombinases, Rhp51 (spRad51) and Dmc1, and to stimulate in vitro DNA strand exchange reactions mediated by these proteins. Genetic analysis indicates that Swi5-Sfr1 works independently of another mediator complex, Rhp55-Rhp57, during Rhp51-dependent recombinational repair. In addition, mutations affecting the two mediators generate distinct repair spectra of HO endonuclease-induced DNA double strand breaks, suggesting that these recombination mediators differently regulate recombination outcomes in an independent manner.

Stimulation of fission yeast and mouse Hop2-Mnd1 of the Dmc1 and Rad51 recombinases

Genetic analysis of fission yeast suggests a role for the spHop2-Mnd1 proteins in the Rad51 and Dmc1-dependent meiotic recombination pathways. In order to gain biochemical insights into this process, we purified Schizosaccharomyces pombe Hop2-Mnd1 to homogeneity. spHop2 and spMnd1 interact by co-immunoprecipitation and two-hybrid analysis. Electron microscopy reveals that S. pombe Hop2-Mnd1 binds single-strand DNA ends of 3'-tailed DNA. Interestingly, spHop2-Mnd1 promotes the renaturation of complementary single-strand DNA and catalyses strand exchange reactions with short oligonucleotides. Importantly, we show that spHop2-Mnd1 stimulates spDmc1-dependent strand exchange and strand invasion. Ca(2+) alleviate the requirement for the order of addition of the proteins on DNA. We also demonstrate that while spHop2-Mnd1 affects spDmc1 specifically, mHop2 or mHop2-Mnd1 stimulates both the hRad51 and hDmc1 recombinases in strand exchange assays. Thus, our results suggest a crucial role for S. pombe and mouse Hop2-Mnd1 in homologous pairing and strand exchange and reveal evolutionary divergence in their specificity for the Dmc1 and Rad51 recombinases.

Double-strand break repair and homologous recombination in Schizosaccharomyces pombe

The study of double-strand break repair and homologous recombination in Saccharomyces cerevisiae meiosis has provided important information about the mechanisms involved. However, it has become clear that the resulting recombination models are only partially applicable to repair in mitotic cells, where crossover formation is suppressed. In recent years our understanding of double-strand break repair and homologous recombination in Schizosaccharomyces pombe has increased significantly, and the identification of novel pathways and genes with homologues in higher eukaryotes has increased its value as a model organism for double-strand break repair. In this review we will focus on the involvement of homologous recombination and repair in different aspects of genome stability in Sz. pombe meiosis, replication and telomere maintenance. We will also discuss anti-recombination pathways (that suppress crossover formation), non-homologous end-joining, single-strand annealing and factors that influence the choice and prevalence of the different repair pathways in Sz. pombe.

Stimulation of Dmc1-mediated DNA strand exchange by the human Rad54B protein

The process of homologous recombination is indispensable for both meiotic and mitotic cell division, and is one of the major pathways for double-strand break (DSB) repair. The human Rad54B protein, which belongs to the SWI2/SNF2 protein family, plays a role in homologous recombination, and may function with the Dmc1 recombinase, a meiosis-specific Rad51 homolog. In the present study, we found that Rad54B enhanced the DNA strand-exchange activity of Dmc1 by stabilizing the Dmc1–single-stranded DNA (ssDNA) complex. Therefore, Rad54B may stimulate the Dmc1-mediated DNA strand exchange by stabilizing the nucleoprotein filament, which is formed on the ssDNA tails produced at DSB sites during homologous recombination.

Mechanism of homologous recombination: mediators and helicases take on regulatory functions

Homologous recombination (HR) 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 maintenance. As such, HR is indispensable for genome integrity, but it must be regulated to avoid deleterious events. Mutations in the tumour-suppressor protein BRCA2, which has a mediator function in HR, lead to cancer formation. DNA helicases, such as Bloom's syndrome protein (BLM), regulate HR at several levels, in attenuating unwanted HR events and in determining the outcome of HR. Defects in BLM are also associated with the cancer phenotype. The past several years have witnessed dramatic advances in our understanding of the mechanism and regulation of HR.

Double-stranded DNA breaks and gene functions in recombination and meiosis

Meiotic prophase I is a long and complex phase. Homologous recombination is an important process that occurs between homologous chromosomes during meiotic prophase I. Formation of chiasmata, which hold homologous chromosomes together until the metaphase I to anaphase I transition, is critical for proper chromosome segregation. Recent studies have suggested that the SPO11 proteins have conserved functions in a number of organisms in generating sites of double-stranded DNA breaks (DSBs) that are thought to be the starting points of homologous recombination. Processing of these sites of DSBs requires the function of RecA homologs, such as RAD51, DMC1, and others, as suggested by mutant studies; thus the failure to repair these meiotic DSBs results in abnormal chromosomal alternations, leading to disrupted meiosis. Recent discoveries on the functions of these RecA homologs have improved the understanding of the mechanisms underlying meiotic homologous recombination.

The Swi5-Sfr1 complex stimulates Rhp51/Rad51- and Dmc1-mediated DNA strand exchange in vitro

Nucleoprotein filaments made up of Rad51 or Dmc1 recombinases, the core structures of recombination, engage in ATP-dependent DNA-strand exchange. The ability of recombinases to form filaments is enhanced by recombination factors termed 'mediators'. Here, we show that the Schizosaccharomyces pombe Swi5-Sfr1 complex, a conserved eukaryotic protein complex, at substoichiometric concentrations stimulates strand exchange mediated by Rhp51 (the S. pombe Rad51 homolog) and Dmc1 on long DNA substrates. Reactions mediated by both recombinases are completely dependent on Swi5-Sfr1, replication protein A (RPA) and ATP, although RPA inhibits the reaction when it is incubated with single-stranded DNA (ssDNA) before the recombinase. The Swi5-Sfr1 complex overcomes, at least partly, the inhibitory effect of RPA, representing a novel class of mediator. Notably, the Swi5-Sfr1 complex preferentially stimulates the ssDNA-dependent ATPase activity of Rhp51, and it increases the amounts of Dmc1 bound to ssDNA.

Clarifying the mechanics of DNA strand exchange in meiotic recombination

During meiosis, accurate separation of maternal and paternal chromosomes requires that they first be connected to one another through homologous recombination. Meiotic recombination has many intriguing but poorly understood features that distinguish it from recombination in mitotically dividing cells, and several of these features depend on the meiosis-specific DNA strand exchange protein Dmc1 (disrupted meiotic cDNA1). Many questions about this protein have arisen since its discovery more than a decade ago, but recent genetic and biochemical breakthroughs promise to shed light on the unique behaviours and functions of this central player in the remarkable chromosome dynamics of meiosis.

Meiotic recombination proteins localize to linear elements in Schizosaccharomyces pombe

In fission yeast, meiotic prophase nuclei develop structures known as linear elements (LinEs), instead of a canonical synaptonemal complex. LinEs contain Rec10 protein. While Rec10 is essential for meiotic recombination, the precise role of LinEs in this process is unknown. Using in situ immunostaining, we show that Rec7 (which is required for meiosis-specific DNA double-strand break (DSB) formation) aggregates in foci on LinEs. The strand exchange protein Rad51, which is known to mark the sites of DSBs, also localizes to LinEs, although to a lesser degree. The number of Rec7 foci corresponds well with the average number of genetic recombination events per meiosis suggesting that Rec7 marks the sites of recombination. Rec7 and Rad51 foci do not co-localize, presumably because they act sequentially on recombination sites. The localization of Rec7 is dependent on Rec10 but independent of the DSB-inducing protein Rec12/Spo11. Neither Rec7 nor Rad51 localization depends on the LinE-associated proteins Hop1 and Mek1, but the formation of Rad51 foci depends on Rec10, Rec7, and, as expected, Rec12/Spo11. We propose that LinEs form around designated recombination sites before the induction of DSBs and that most, if not all, meiotic recombination initiates within the setting provided by LinEs.