Swi5 acts in meiotic DNA joint molecule formation in Schizosaccharomyces pombe

Positive and negative regulators of RAD51/DMC1 in homologous recombination and DNA replication

RAD51 recombinase plays a central role in homologous recombination (HR) by forming a nucleoprotein filament on single-stranded DNA (ssDNA) to catalyze homology search and strand exchange between the ssDNA and a homologous double-stranded DNA (dsDNA). The catalytic activity of RAD51 assembled on ssDNA is critical for the DNA-homology-mediated repair of DNA double-strand breaks in somatic and meiotic cells and restarting stalled replication forks during DNA replication. The RAD51-ssDNA complex also plays a structural role in protecting the regressed/reversed replication fork. Two types of regulators control RAD51 filament formation, stability, and dynamics, namely positive regulators, including mediators, and negative regulators, so-called remodelers. The appropriate balance of action by the two regulators assures genome stability. This review describes the roles of positive and negative RAD51 regulators in HR and DNA replication and its meiosis-specific homolog DMC1 in meiotic recombination. We also provide future study directions for a comprehensive understanding of RAD51/DMC1-mediated regulation in maintaining and inheriting genome integrity.

The Fission Yeast Mating-Type Switching Motto: "One-for-Two" and "Two-for-One"

Schizosaccharomyces pombe is an ascomycete fungus that divides by medial fission; it is thus commonly referred to as fission yeast, as opposed to the distantly related budding yeast Saccharomyces cerevisiae. The reproductive lifestyle of S. pombe relies on an efficient genetic sex determination system generating a 1:1 sex ratio and using alternating haploid/diploid phases in response to environmental conditions. In this review, we address how one haploid cell manages to generate two sister cells with opposite mating types, a prerequisite to conjugation and meiosis. This mating-type switching process depends on two highly efficient consecutive asymmetric cell divisions that rely on DNA replication, repair, and recombination as well as the structure and components of heterochromatin. We pay special attention to the intimate interplay between the genetic and epigenetic partners involved in this process to underscore the importance of basic research and its profound implication for a better understanding of chromatin biology.

Mapping and Analysis of Swi5 and Sfr1 Phosphorylation Sites

The evolutionarily conserved Swi5-Sfr1 complex plays an important role in homologous recombination, a process crucial for the maintenance of genomic integrity. Here, we purified Schizosaccharomyces pombe Swi5-Sfr1 complex from meiotic cells and analyzed it by mass spectrometry. Our analysis revealed new phosphorylation sites on Swi5 and Sfr1. We found that mutations that prevent phosphorylation of Swi5 and Sfr1 do not impair their function but swi5 and sfr1 mutants encoding phosphomimetic aspartate at the identified phosphorylation sites are only partially functional. We concluded that during meiosis, Swi5 associates with Sfr1 and both Swi5 and Sfr1 proteins are phosphorylated. However, the functional relevance of Swi5 and Sfr1 phosphorylation remains to be determined.

Repression of a large number of genes requires interplay between homologous recombination and HIRA

During homologous recombination, Dbl2 protein is required for localisation of Fbh1, an F-box helicase that efficiently dismantles Rad51-DNA filaments. RNA-seq analysis of dbl2Δ transcriptome showed that the dbl2 deletion results in upregulation of more than 500 loci in Schizosaccharomyces pombe. Compared with the loci with no change in expression, the misregulated loci in dbl2Δ are closer to long terminal and long tandem repeats. Furthermore, the misregulated loci overlap with antisense transcripts, retrotransposons, meiotic genes and genes located in subtelomeric regions. A comparison of the expression profiles revealed that Dbl2 represses the same type of genes as the HIRA histone chaperone complex. Although dbl2 deletion does not alleviate centromeric or telomeric silencing, it suppresses the silencing defect at the outer centromere caused by deletion of hip1 and slm9 genes encoding subunits of the HIRA complex. Moreover, our analyses revealed that cells lacking dbl2 show a slight increase of nucleosomes at transcription start sites and increased levels of methylated histone H3 (H3K9me2) at centromeres, subtelomeres, rDNA regions and long terminal repeats. Finally, we show that other proteins involved in homologous recombination, such as Fbh1, Rad51, Mus81 and Rad54, participate in the same gene repression pathway.

Two auxiliary factors promote Dmc1-driven DNA strand exchange via stepwise mechanisms

Homologous recombination (HR) is a universal mechanism operating in somatic and germ-line cells, where it contributes to the maintenance of genome stability and ensures the faithful distribution of genetic material, respectively. The ability to identify and exchange the strands of two homologous DNA molecules lies at the heart of HR and is mediated by RecA-family recombinases. Dmc1 is a meiosis-specific RecA homolog in eukaryotes, playing a predominant role in meiotic HR. However, Dmc1 cannot function without its two major auxiliary factor complexes, Swi5-Sfr1 and Hop2-Mnd1. Through biochemical reconstitutions, we demonstrate that Swi5-Sfr1 and Hop2-Mnd1 make unique contributions to stimulate Dmc1-driven strand exchange in a synergistic manner. Mechanistically, Swi5-Sfr1 promotes establishment of the Dmc1 nucleoprotein filament, whereas Hop2-Mnd1 defines a critical, rate-limiting step in initiating strand exchange. Following execution of this function, we propose that Swi5-Sfr1 then promotes strand exchange with Hop2-Mnd1. Thus, our findings elucidate distinct yet complementary roles of two auxiliary factors in Dmc1-driven strand exchange, providing mechanistic insights into some of the most critical steps in meiotic HR.

Intragenic meiotic recombination in Schizosaccharomyces pombe is sensitive to environmental temperature changes

Changes in environmental temperature influence cellular processes and their dynamics, and thus affect the life cycle of organisms that are unable to control their cell/body temperature. Meiotic recombination is the cellular process essential for producing healthy haploid gametes by providing physical links (chiasmata) between homologous chromosomes to guide their accurate segregation. Additionally, meiotic recombination—initiated by programmed DNA double-strand breaks (DSBs)—can generate genetic diversity and, therefore, is a driving force of evolution. Environmental temperature influencing meiotic recombination outcome thus may be a crucial determinant of reproductive success and genetic diversity. Indeed, meiotic recombination frequency in fungi, plants and invertebrates changes with temperature. In most organisms, these temperature-induced changes in meiotic recombination seem to be mediated through the meiosis-specific chromosome axis organization, the synaptonemal complex in particular. The fission yeast Schizosaccharomyces pombe does not possess a synaptonemal complex. Thus, we tested how environmental temperature modulates meiotic recombination frequency in the absence of a fully-fledged synaptonemal complex. We show that intragenic recombination (gene conversion) positively correlates with temperature within a certain range, especially at meiotic recombination hotspots. In contrast, crossover recombination, which manifests itself as chiasmata, is less affected. Based on our observations, we suggest that, in addition to changes in DSB frequency, DSB processing could be another temperature-sensitive step causing temperature-induced recombination rate alterations.

Immediate visualization of recombination events and chromosome segregation defects in fission yeast meiosis

Schizosaccharomyces pombe, also known as fission yeast, is an established model for studying chromosome biological processes. Over the years, research employing fission yeast has made important contributions to our knowledge about chromosome segregation during meiosis, as well as meiotic recombination and its regulation. Quantification of meiotic recombination frequency is not a straightforward undertaking, either requiring viable progeny for a genetic plating assay, or relying on laborious Southern blot analysis of recombination intermediates. Neither of these methods lends itself to high-throughput screens to identify novel meiotic factors. Here, we establish visual assays novel to Sz. pombe for characterizing chromosome segregation and meiotic recombination phenotypes. Genes expressing red, yellow, and/or cyan fluorophores from spore-autonomous promoters have been integrated into the fission yeast genomes, either close to the centromere of chromosome 1 to monitor chromosome segregation, or on the arm of chromosome 3 to form a genetic interval at which recombination frequency can be determined. The visual recombination assay allows straightforward and immediate assessment of the genetic outcome of a single meiosis by epi-fluorescence microscopy without requiring tetrad dissection. We also demonstrate that the recombination frequency analysis can be automatized by utilizing imaging flow cytometry to enable high-throughput screens. These assays have several advantages over traditional methods for analyzing meiotic phenotypes.

Genetic interactions between the chromosome axis-associated protein Hop1 and homologous recombination determinants in Schizosaccharomyces pombe

Hop1 is a component of the meiosis-specific chromosome axis and belongs to the evolutionarily conserved family of HORMA domain proteins. Hop1 and its orthologs in higher eukaryotes are a major factor in promoting double-strand DNA break formation and inter-homolog recombination. In budding yeast and mammals, they are also involved in a meiotic checkpoint kinase cascade monitoring the completion of double-strand DNA break repair. We used the fission yeast, Schizosaccharomyces pombe, which lacks a canonical synaptonemal complex to test whether Hop1 has a role beyond supporting the generation of double-strand DNA breaks and facilitating inter-homolog recombination events. We determined how mutants of homologous recombination factors genetically interact with hop1, studied the role(s) of the HORMA domain of Hop1, and characterized a bio-informatically predicted interactor of Hop1, Aho1 (SPAC688.03c). Our observations indicate that in fission yeast, Hop1 does require its HORMA domain to support wild-type levels of meiotic recombination and localization to meiotic chromatin. Furthermore, we show that hop1∆ only weakly interacts genetically with mutants of homologous recombination factors, and in fission yeast likely has no major role beyond break formation and promoting inter-homolog events. We speculate that after the evolutionary loss of the synaptonemal complex, Hop1 likely has become less important for modulating recombination outcome during meiosis in fission yeast, and that this led to a concurrent rewiring of genetic pathways controlling meiotic recombination.

The differentiated and conserved roles of Swi5-Sfr1 in homologous recombination

Homologous recombination (HR) is the process whereby two DNA molecules that share high sequence similarity are able to recombine to generate hybrid DNA molecules. Throughout evolution, the ability of HR to identify highly similar DNA sequences has been adopted for numerous biological phenomena including DNA repair, meiosis, telomere maintenance, ribosomal DNA amplification and immunological diversity. Although Rad51 and Dmc1 are the key proteins that promote HR in mitotic and meiotic cells, respectively, accessory proteins that allow Rad51 and Dmc1 to effectively fulfil their functions have been identified in all examined model systems. In this Review, we discuss the roles of the highly conserved Swi5‐Sfr1 accessory complex in yeast, mice and humans, and explore similarities and differences between these species.

Two separable functions of Ctp1 in the early steps of meiotic DNA double-strand break repair

Meiotic programmed DNA double-strand break (DSB) repair is essential for crossing-over and viable gamete formation and requires removal of Spo11-oligonucleotide complexes from 5′ ends (clipping) and their resection to generate invasive 3′-end single-stranded DNA (resection). Ctp1 (Com1, Sae2, CtIP homolog) acting with the Mre11-Rad50-Nbs1 (MRN) complex is required in both steps. We isolated multiple S. pombe ctp1 mutants deficient in clipping but proficient in resection during meiosis. Remarkably, all of the mutations clustered in or near the conserved CxxC or RHR motif in the C-terminal portion. The mutants tested, like ctp1Δ, were clipping-deficient by both genetic and physical assays­. But, unlike ctp1Δ, these mutants were recombination-proficient for Rec12 (Spo11 homolog)-independent break-repair and resection-proficient by physical assay. We conclude that the intracellular Ctp1 C-terminal portion is essential for clipping, while the N-terminal portion is sufficient for DSB end-resection. This conclusion agrees with purified human CtIP resection and endonuclease activities being independent. Our mutants provide intracellular evidence for separable functions of Ctp1. Some mutations truncate Ctp1 in the same region as one of the CtIP mutations linked to the Seckel and Jawad severe developmental syndromes, suggesting that these syndromes are caused by a lack of clipping at DSB ends that require repair.

Rad51/Dmc1 paralogs and mediators oppose DNA helicases to limit hybrid DNA formation and promote crossovers during meiotic recombination

During meiosis programmed DNA double-strand breaks (DSBs) are repaired by homologous recombination using the sister chromatid or the homologous chromosome (homolog) as a template. This repair results in crossover (CO) and non-crossover (NCO) recombinants. Only CO formation between homologs provides the physical linkages guiding correct chromosome segregation, which are essential to produce healthy gametes. The factors that determine the CO/NCO decision are still poorly understood. Using Schizosaccharomyces pombe as a model we show that the Rad51/Dmc1-paralog complexes Rad55-Rad57 and Rdl1-Rlp1-Sws1 together with Swi5-Sfr1 play a major role in antagonizing both the FANCM-family DNA helicase/translocase Fml1 and the RecQ-type DNA helicase Rqh1 to limit hybrid DNA formation and promote Mus81-Eme1-dependent COs. A common attribute of these protein complexes is an ability to stabilize the Rad51/Dmc1 nucleoprotein filament, and we propose that it is this property that imposes constraints on which enzymes gain access to the recombination intermediate, thereby controlling the manner in which it is processed and resolved.

Increased meiotic crossovers and reduced genome stability in absence of Schizosaccharomyces pombe Rad16 (XPF)

Schizosaccharomyces pombe Rad16 is the ortholog of the XPF structure-specific endonuclease, which is required for nucleotide excision repair and implicated in the single strand annealing mechanism of recombination. We show that Rad16 is important for proper completion of meiosis. In its absence, cells suffer reduced spore viability and abnormal chromosome segregation with evidence for fragmentation. Recombination between homologous chromosomes is increased, while recombination within sister chromatids is reduced, suggesting that Rad16 is not required for typical homolog crossovers but influences the balance of recombination between the homolog and the sister. In vegetative cells, rad16 mutants show evidence for genome instability. Similar phenotypes are associated with mutants affecting Rhp14(XPA) but are independent of other nucleotide excision repair proteins such as Rad13(XPG). Thus, the XPF/XPA module of the nucleotide excision repair pathway is incorporated into multiple aspects of genome maintenance even in the absence of external DNA damage.

Dual regulation of Dmc1-driven DNA strand exchange by Swi5-Sfr1 activation and Rad22 inhibition

Meiotic recombination requires two key recombinases: the ubiquitously expressed Rad51 and the meiosis-specific Dmc1. Rad52 and its fission yeast ortholog, Rad22, are mediators that help load Rad51 onto ssDNA coated with replication protein A (RPA). Here, Iwasaki and colleagues reveal how the Swi5–Sfr1 complex functions as both a mediator (loading DMC1 onto ssDNA) and an activator (stimulating Dmc1-driven strand exchange). In contrast, Rad22 inhibits Dmc1 by competing for binding to RPA-coated ssDNA. This study thus provides a novel regulatory mechanism for meiotic recombination.

Both ubiquitously expressed Rad51 and meiosis-specific Dmc1 are required for crossover production during meiotic recombination. The budding yeast Rad52 and its fission yeast ortholog, Rad22, are “mediators;” i.e., they help load Rad51 onto ssDNA coated with replication protein A (RPA). Here we show that the Swi5–Sfr1 complex from fission yeast is both a mediator that loads Dmc1 onto ssDNA and a direct “activator” of DNA strand exchange by Dmc1. In stark contrast, Rad22 inhibits Dmc1 action by competing for its binding to RPA-coated ssDNA. Thus, Rad22 plays dual roles in regulating meiotic recombination: activating Rad51 and inhibiting Dmc1.

Enhancement of ADP release from the RAD51 presynaptic filament by the SWI5-SFR1 complex

Homologous recombination catalyzed by the RAD51 recombinase eliminates deleterious DNA lesions from the genome. In the presence of ATP, RAD51 forms a nucleoprotein filament on single-stranded DNA, termed the presynaptic filament, to initiate homologous recombination-mediated DNA double-strand break repair. The SWI5-SFR1 complex stabilizes the presynaptic filament and enhances its ability to mediate the homologous DNA pairing reaction. Here we characterize the RAD51 presynaptic filament stabilization function of the SWI5-SFR1 complex using optical tweezers. Biochemical experiments reveal that SWI5-SFR1 enhances ATP hydrolysis by single-stranded DNA-bound RAD51. Importantly, we show that SWI5-SFR1 acts by facilitating the release of ADP from the presynaptic filament. Our results thus provide mechanistic understanding of the function of SWI5-SFR1 in RAD51-mediated DNA recombination.

The fission yeast FANCM ortholog directs non-crossover recombination during meiosis

The formation of healthy gametes depends on programmed DNA double-strand breaks (DSBs), which are each repaired as a crossover (CO) or non-crossover (NCO) from a homologous template. Although most of these DSBs are repaired without giving COs, little is known about the genetic requirements of NCO-specific recombination. We show that Fml1, the Fanconi anemia complementation group M (FANCM)-ortholog of Schizosaccharomyces pombe, directs the formation of NCOs during meiosis in competition with the Mus81-dependent pro-CO pathway. We also define the Rad51/Dmc1-mediator Swi5-Sfr1 as a major determinant in biasing the recombination process in favor of Mus81, to ensure the appropriate amount of COs to guide meiotic chromosome segregation. The conservation of these proteins from yeast to humans suggests that this interplay may be a general feature of meiotic recombination.

Rad51 presynaptic filament stabilization function of the mouse Swi5-Sfr1 heterodimeric complex

Homologous recombination (HR) represents a major error-free pathway to eliminate pre-carcinogenic chromosomal lesions. The DNA strand invasion reaction in HR is mediated by a helical filament of the Rad51 recombinase assembled on single-stranded DNA that is derived from the nucleolytic processing of the primary lesion. Recent studies have found that the human and mouse Swi5 and Sfr1 proteins form a complex that influences Rad51-mediated HR in cells. Here, we provide biophysical evidence that the mouse Swi5–Sfr1 complex has a 1:1 stoichiometry. Importantly, the Swi5–Sfr1 complex, but neither Swi5 nor Sfr1 alone, physically interacts with Rad51 and stimulates Rad51-mediated homologous DNA pairing. This stimulatory effect stems from the stabilization of the Rad51–ssDNA presynaptic filament. Moreover, we provide evidence that the RSfp (rodent Sfr1 proline rich) motif in Sfr1 serves as a negative regulatory element. These results thus reveal an evolutionarily conserved function in the Swi5–Sfr1 complex and furnish valuable information as to the regulatory role of the RSfp motif that isspecific to themammalianSfr1 orthologs.

Homologous recombination and its regulation

Homologous recombination (HR) is critical both for repairing DNA lesions in mitosis and for chromosomal pairing and exchange during meiosis. However, some forms of HR can also lead to undesirable DNA rearrangements. Multiple regulatory mechanisms have evolved to ensure that HR takes place at the right time, place and manner. Several of these impinge on the control of Rad51 nucleofilaments that play a central role in HR. Some factors promote the formation of these structures while others lead to their disassembly or the use of alternative repair pathways. In this article, we review these mechanisms in both mitotic and meiotic environments and in different eukaryotic taxa, with an emphasis on yeast and mammal systems. Since mutations in several proteins that regulate Rad51 nucleofilaments are associated with cancer and cancer-prone syndromes, we discuss how understanding their functions can lead to the development of better tools for cancer diagnosis and therapy.

Fission yeast Swi5-Sfr1 protein complex, an activator of Rad51 recombinase, forms an extremely elongated dogleg-shaped structure

In eukaryotes, DNA strand exchange is the central reaction of homologous recombination, which is promoted by Rad51 recombinases forming a right-handed nucleoprotein filament on single-stranded DNA, also known as a presynaptic filament. Accessory proteins known as recombination mediators are required for the formation of the active presynaptic filament. One such mediator in the fission yeast Schizosaccharomyces pombe is the Swi5-Sfr1 complex, which has been identified as an activator of Rad51 that assists in presynaptic filament formation and stimulates its strand exchange reaction. Here, we determined the 1:1 binding stoichiometry between the two subunits of the Swi5-Sfr1 complex using analytical ultracentrifugation and electrospray ionization mass spectrometry. Small-angle x-ray scattering experiments revealed that the Swi5-Sfr1 complex displays an extremely elongated dogleg-shaped structure in solution, which is consistent with its exceptionally high frictional ratio (f/f(0)) of 2.0 ± 0.2 obtained by analytical ultracentrifugation. Furthermore, we determined a rough topology of the complex by comparing the small-angle x-ray scattering-based structures of the Swi5-Sfr1 complex and four Swi5-Sfr1-Fab complexes, in which the Fab fragments of monoclonal antibodies were specifically bound to experimentally determined sites of Sfr1. We propose a model for how the Swi5-Sfr1 complex binds to the Rad51 filament, in which the Swi5-Sfr1 complex fits into the groove of the Rad51 filament, leading to an active and stable presynaptic filament.

A homolog of male sex-determining factor SRY cooperates with a transposon-derived CENP-B protein to control sex-specific directed recombination

Schizosaccharomyces pombe cells switch mating type by replacing genetic information at the expressed mat1 locus with sequences copied from mat2-P or mat3-M silent donor loci. The choice of donor locus is dictated by cell type, such that mat2 is the preferred donor in M cells and mat3 is the preferred donor in P cells. Donor choice involves a recombination-promoting complex (RPC) containing Swi2 and Swi5. In P cells, the RPC localizes to a specific DNA element located adjacent to mat3, but in M cells it spreads across the silent mating-type region, including mat2-P. This differential distribution of the RPC regulates nonrandom choice of donors. However, cell-type-specific differences in RPC localization are not understood. Here we show that the mat1-M-encoded factor Mc, which shares structural and functional similarities with the male sex-determining factor SRY, is highly enriched at the swi2 and swi5 loci and promotes elevated levels of RPC components. Loss of Mc reduces Swi2 and Swi5 to levels comparable to those in P cells and disrupts RPC spreading across the mat2/3 region. Mc also localizes to loci expressed preferentially in M cells and to retrotransposon LTRs. We demonstrate that Mc localization at LTRs and at swi2 requires Abp1, a homolog of transposon-derived CENP-B protein and that loss of Abp1 impairs Swi2 protein expression and the donor choice mechanism. These results suggest that Mc modulates levels of recombination factors, which is important for mating-type donor selection and for the biased gene conversion observed during meiosis, where M cells serve as preferential donors of genetic information.

The fission yeast meiosis-specific Dmc1 recombinase mediates formation and branch migration of Holliday junctions by preferentially promoting strand exchange in a direction opposite to that of Rad51

Homologous recombination proceeds via the formation of several intermediates including Holliday junctions (HJs), which are important for creating crossover products. DNA strand exchange is a core reaction that produces these intermediates that is directly catalyzed by RecA family recombinases, of which there are two types in eukaryotes: universal Rad51 and meiosis-specific Dmc1. We demonstrated previously that Rad51 promotes four-strand exchange, mimicking the formation and branch migration of HJs. Here we show that Dmc1 from fission yeast has a similar activity, which requires ATP hydrolysis and is independent of an absolute requirement for the Swi5-Sfr1 complex. These features are critically different from three-strand exchange mediated by Dmc1, but similar to those of four-strand exchange mediated by Rad51, suggesting that strand exchange reactions between duplex-duplex and single-duplex DNAs are mechanistically different. Interestingly, despite similarities in protein structure and in reaction features, the preferential polarities of Dmc1 and Rad51 strand exchange are different (Dmc1 promotes exchange in the 5'-to-3' direction and Rad51 promotes exchange in the 3'-to-5' direction relative to the ssDNA region of the DNA substrate). The significance of the Dmc1 polarity is discussed within the context of the necessity for crossover production.

The role of the human SWI5-MEI5 complex in homologous recombination repair

The Swi5-Mei5 complex and its homologues are involved in specialized recombination pathways in budding and fission yeasts. Although the fission yeast homologue Swi5-Sfr1 is critical for homologous recombination repair, the budding yeast counterpart Sae3-Mei5 is meiosis-specific, interacts with Dmc1, and promotes assembly of Dmc1 on meiotic chromosomes. Here, we identify and characterize the human SWI5-MEI5 (C9orf119-C10orf78) complex. We showed that SWI5 and MEI5 form a stable complex in vitro and in vivo. The C-terminal Swi5 domain of SWI5 and the middle coiled-coil region of MEI5 dictate this conserved interaction. In addition, SWI5-MEI5 directly interacts with RAD51 in vitro. Depletion of SWI5 or MEI5 in human cells causes defects in homologous recombination repair. Finally, SWI5- or MEI5-depleted cells display enhanced sensitivity to ionizing radiation, consistent with the role of this complex in HR repair. Our results suggest that human SWI5-MEI5 has an evolutionarily conserved function in homologous recombination repair.

Role for the mammalian Swi5-Sfr1 complex in DNA strand break repair through homologous recombination

In fission yeast, the Swi5-Sfr1 complex plays an important role in homologous recombination (HR), a pathway crucial for the maintenance of genomic integrity. Here we identify and characterize mammalian Swi5 and Sfr1 homologues. Mouse Swi5 and Sfr1 are nuclear proteins that form a complex in vivo and in vitro. Swi5 interacts in vitro with Rad51, the DNA strand-exchange protein which functions during HR. By generating Swi5^−/− and Sfr1^−/− embryonic stem cell lines, we found that both proteins are mutually interdependent for their stability. Importantly, the Swi5-Sfr1 complex plays a role in HR when Rad51 function is perturbed in vivo by expression of a BRC peptide from BRCA2. Swi5^−/− and Sfr1^−/− cells are selectively sensitive to agents that cause DNA strand breaks, in particular ionizing radiation, camptothecin, and the Parp inhibitor olaparib. Consistent with a role in HR, sister chromatid exchange induced by Parp inhibition is attenuated in Swi5^−/− and Sfr1^−/− cells, and chromosome aberrations are increased. Thus, Swi5-Sfr1 is a newly identified complex required for genomic integrity in mammalian cells with a specific role in the repair of DNA strand breaks.

Our genome constantly undergoes DNA damage as a result of agents in the environment, as well as from metabolic processes. One method of repairing DNA damage is homologous recombination (HR), in which genetic information from a duplicate sequence (the sister chromatid) is copied into the damaged site in DNA. In model organisms (the yeasts), a protein complex termed Swi5-Sfr1 functions in DNA damage repair by HR. In this study, we characterize mouse homologues of this complex. We find that mouse cells lacking this complex are sensitive to DNA damaging agents, in particular, those that cause breaks in DNA strands and that serve as cancer chemotherapeutics. These cells also have increased numbers of chromosome aberrations when exposed to DNA damaging agents. Moreover, HR is decreased in Swi5 and Sfr1 mutant cells under conditions where the cell is challenged. Together, these results demonstrate a requirement for the Swi5-Sfr1 protein complex in maintaining genomic integrity in mammalian cells.

Crossover invariance determined by partner choice for meiotic DNA break repair

Crossovers between meiotic homologs are crucial for their proper segregation, and crossover number and position are carefully controlled. Crossover homeostasis in budding yeast maintains crossovers at the expense of noncrossovers when double-strand DNA break (DSB) frequency is reduced. The mechanism of maintaining constant crossover levels in other species has been unknown. Here we investigate in fission yeast a different aspect of crossover control--the near invariance of crossover frequency per kb of DNA despite large variations in DSB intensity across the genome. Crossover invariance involves the choice of sister chromatid versus homolog for DSB repair. At strong DSB hotspots, intersister repair outnumbers interhomolog repair approximately 3:1, but our genetic and physical data indicate the converse in DSB-cold regions. This unanticipated mechanism of crossover control may operate in many species and explain, for example, the large excess of DSBs over crossovers and the repair of DSBs on unpaired chromosomes in diverse species.

An improved strategy for tandem affinity purification-tagging of Schizosaccharomyces pombe genes

Tandem affinity purification (TAP) is a method that allows rapid purification of native protein complexes. We developed an improved technique to fuse the fission yeast genes with a TAP tag. Our technique is based on tagging constructs that contain regions homologous to the target gene cloned into vectors carrying a TAP tag. We used this technique to design strategies for TAP-tagging of predicted Schizosaccharomyces pombe genes (http://mendel.imp.ac.at/Pombe_tagging/). To validate the approach, we purified the proteins, which associated with two evolutionarily conserved proteins Swi5 and Sfr1 as well as three protein kinases Ksg1, Orb6 and Sid1.

Meiotic DNA double-strand break repair requires two nucleases, MRN and Ctp1, to produce a single size class of Rec12 (Spo11)-oligonucleotide complexes

Programmed DNA double-strand breaks (DSBs) in meiosis are formed by Spo11 (Rec12 in fission yeast), a topoisomerase II-like protein, which becomes covalently attached to DNA 5' ends. For DSB repair through homologous recombination, the protein must be removed from these DNA ends. We show here that Rec12 is endonucleolytically removed from DSB ends attached to a short oligonucleotide (Rec12-oligonucleotide complex), as is Spo11 in budding yeast. Fission yeast, however, has only one size class of Rec12-oligonucleotide complexes, whereas budding yeast has two size classes, suggesting different endonucleolytic regulatory mechanisms. Rec12-oligonucleotide generation strictly requires Ctp1 (Sae2 nuclease homolog), the Rad32 (Mre11) nuclease domain, and Rad50 of the MRN complex. Surprisingly, Nbs1 is not strictly required, indicating separable roles for the MRN subunits. On the basis of these and other data, we propose that Rad32 nuclease has the catalytic site for Rec12-oligonucleotide generation and is activated by Ctp1, which plays an additional role in meiotic recombination.

The Mei5-Sae3 protein complex mediates Dmc1 activity in Saccharomyces cerevisiae

During homologous recombination, a number of proteins cooperate to catalyze the loading of recombinases onto single-stranded DNA. Single-stranded DNA-binding proteins stimulate recombination by coating single-stranded DNA and keeping it free of secondary structure; however, in order for recombinases to load on single-stranded-DNA-binding protein-coated DNA, the activity of a class of proteins known as recombination mediators is required. Mediator proteins coordinate the handoff of single-stranded DNA from single-stranded DNA-binding protein to recombinase. Here we show that a complex of Mei5 and Sae3 from Saccharomyces cerevisiae preferentially binds single-stranded DNA and relieves the inhibition of the strand assimilation and DNA binding abilities of the meiotic recombinase Dmc1 imposed by the single-stranded DNA-binding protein replication protein A. Additionally, we demonstrate the physical interaction of Mei5-Sae3 with replication protein A. Our results, together with previous in vivo studies, indicate that Mei5-Sae3 is a mediator of Dmc1 assembly during meiotic recombination in S. cerevisiae.

Improved tools for efficient mapping of fission yeast genes: identification of microtubule nucleation modifier mod22-1 as an allele of chromatin- remodelling factor gene swr1

Fission yeast genes identified in genetic screens are usually cloned by transformation of mutants with plasmid libraries. However, for some genes this can be difficult, and positional cloning approaches are required. The mutation swi5-39 reduces recombination frequency in homozygous crosses and has been used as a tool in mapping gene position (Schmidt, 1993). However, strain construction in swi5-39-based mapping is significantly more laborious than is desirable. Here we describe a set of strains designed to make swi5-based mapping more efficient and more powerful. The first improvement is the use of a swi5Δ strain marked with kanamycin (G418) resistance, which greatly facilitates identification of swi5 mutants. The second improvement, which follows directly from the first, is the introduction of a large number of auxotrophic markers into mapping strains, increasing the likelihood of finding close linkage between a marker and the mutation of interest. We combine these new mapping strains with a rec12Δ-based approach for initial mapping of a mutation to an individual chromosome. Together, the two methods allow an approximate determination of map position in only a small number of crosses. We used these to determine that mod22-1, a modifier of microtubule nucleation phenotypes, encodes a truncation allele of Swr1, a chromatin-remodelling factor involved in nucleosomal deposition of H2A.Z histone variant Pht1. Expression microarray analysis of mod22-1, swr1Δ and pht1Δ cells suggests that the modifier phenotype of mod22-1 mutants may be due to small changes in expression of one or more genes involved in tubulin function. Copyright © 2009 John Wiley & Sons, Ltd.

Indistinguishable landscapes of meiotic DNA breaks in rad50+ and rad50S strains of fission yeast revealed by a novel rad50+ recombination intermediate

The fission yeast Schizosaccharomyces pombe Rec12 protein, the homolog of Spo11 in other organisms, initiates meiotic recombination by creating DNA double-strand breaks (DSBs) and becoming covalently linked to the DNA ends of the break. This protein–DNA linkage has previously been detected only in mutants such as rad50S in which break repair is impeded and DSBs accumulate. In the budding yeast Saccharomyces cerevisiae, the DSB distribution in a rad50S mutant is markedly different from that in wild-type (RAD50) meiosis, and it was suggested that this might also be true for other organisms. Here, we show that we can detect Rec12-DNA linkages in Sc. pombe rad50⁺ cells, which are proficient for DSB repair. In contrast to the results from Sa. cerevisiae, genome-wide microarray analysis of Rec12-DNA reveals indistinguishable meiotic DSB distributions in rad50⁺ and rad50S strains of Sc. pombe. These results confirm our earlier findings describing the occurrence of widely spaced DSBs primarily in large intergenic regions of DNA and demonstrate the relevance and usefulness of fission yeast studies employing rad50S. We propose that the differential behavior of rad50S strains reflects a major difference in DSB regulation between the two species—specifically, the requirement for the Rad50-containing complex for DSB formation in budding yeast but not in fission yeast. Use of rad50S and related mutations may be a useful method for DSB analysis in other species.

During meiosis, which creates haploid gametes from diploid cells, recombination between two homologous chromosomes increases genetic diversity and, in most organisms, is crucial for proper segregation of chromosomes into haploid nuclei. To better understand where recombination occurs and why it occurs there, we investigated in fission yeast the initiating step in recombination—formation of DNA double-strand breaks (DSBs). A genome-wide DSB map is crucial to understand how DNA sequence and chromatin structure affect DSB formation and may help answer these questions in other organisms, including humans. Mutants in which DSBs accumulate are particularly useful for determining the DSB distribution. As recently reported, however, in budding yeast the DSB distribution in one such widely used mutant, rad50S, differs markedly from that in a dmc1 mutant, in which DSBs also accumulate and appear to have a more nearly wild-type distribution. We have detected in fission yeast a DNA–protein intermediate of recombination assumed to exist, but never before detected, in a recombination-proficient strain (rad50⁺). The distributions of this intermediate, and therefore those of DSBs, in rad50⁺ and rad50S strains are indistinguishable. rad50S-like mutations may also accurately reflect the wild-type DSB distribution in other species and may be particularly useful in species lacking Dmc1 orthologs.

The mating-type-related bias of gene conversion in Schizosaccharomyces pombe

The mating-type bias (mat-bias) of gene conversion was previously described as a phenomenon in which the number of prototrophic recombinants in an ura4A heteroallelic two-factor cross relates to the mating types of the parents. We show now that the mat-bias is restricted neither to ura4A nor to recombination hotspots, but occurs at other genomic loci, too. It is specific for gene conversion and absent in azygotic meiosis. Thus, the mat-bias must originate from mating-type-specific "imprinting" events before karyogamy takes place. Structural variations of the mating-type locus, such as h(+N), h(+S), h(-S), h(+smtDelta), or h(-smtDelta), showed mat-bias manifestation. Mutations in genes coding for histone acetylase (gcn5, ada2) and histone deacetylase (hos2, clr6) activities smooth or abolish the mat-bias. In addition, the mat-bias depends on the presence of Swi5. We propose a new role for Swi5 and the histone acetylation status in mat-bias establishment through directionality of repair from the intact chromatid to the broken chromatid.

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 fission yeast BLM homolog Rqh1 promotes meiotic recombination

RecQ helicases are found in organisms as diverse as bacteria, fungi, and mammals. These proteins promote genome stability, and mutations affecting human RecQ proteins underlie premature aging and cancer predisposition syndromes, including Bloom syndrome, caused by mutations affecting the BLM protein. In this study we show that mutants lacking the Rqh1 protein of the fission yeast Schizosaccharomyces pombe, a RecQ and BLM homolog, have substantially reduced meiotic recombination, both gene conversions and crossovers. The relative proportion of gene conversions having associated crossovers is unchanged from that in wild type. In rqh1 mutants, meiotic DNA double-strand breaks are formed and disappear with wild-type frequency and kinetics, and spore viability is only moderately reduced. Genetic analyses and the wild-type frequency of both intersister and interhomolog joint molecules argue against these phenotypes being explained by an increase in intersister recombination at the expense of interhomolog recombination. We suggest that Rqh1 extends hybrid DNA and biases the recombination outcome toward crossing over. Our results contrast dramatically with those from the budding yeast ortholog, Sgs1, which has a meiotic antirecombination function that suppresses recombination events involving more than two DNA duplexes. These observations underscore the multiple recombination functions of RecQ homologs and emphasize that even conserved proteins can be adapted to play different roles in different organisms.

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.

Meiotic Recombination in Schizosaccharomyces pombe: A Paradigm for Genetic and Molecular Analysis

The fission yeast Schizosaccharomyces pombe is especially well-suited for both genetic and biochemical analysis of meiotic recombination. Recent studies have revealed ~50 gene products and two DNA intermediates central to recombination, which we place into a pathway from parental to recombinant DNA. We divide recombination into three stages - chromosome alignment accompanying nuclear "horsetail" movement, formation of DNA breaks, and repair of those breaks - and we discuss the roles of the identified gene products and DNA intermediates in these stages. Although some aspects of recombination are similar to those in the distantly related budding yeast Saccharomyces cerevisiae, other aspects are distinctly different. In particular, many proteins required for recombination in one species have no clear ortholog in the other, and the roles of identified orthologs in regulating recombination often differ. Furthermore, in S. pombe the dominant joint DNA molecule intermediates contain single Holliday junctions, and intersister joint molecules are more frequent than interhomolog types, whereas in S. cerevisiae interhomolog double Holliday junctions predominate. We speculate that meiotic recombination in other organisms shares features of each of these yeasts.

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.

Mus81-Eme1-dependent and -independent crossovers form in mitotic cells during double-strand break repair in Schizosaccharomyces pombe

During meiosis, double-strand breaks (DSBs) lead to crossovers, thought to arise from the resolution of double Holliday junctions (HJs) by an HJ resolvase. In Schizosaccharomyces pombe, meiotic crossovers are produced primarily through a mechanism requiring the Mus81-Eme1 endonuclease complex. Less is known about the processes that produces crossovers during the repair of DSBs in mitotic cells. We employed an inducible DSB system to determine the role of Rqh1-Top3 and Mus81-Eme1 in mitotic DSB repair and crossover formation in S. pombe. In agreement with the meiotic data, crossovers are suppressed in cells lacking Mus81-Eme1. And relative to the wild type, rqh1Delta cells show a fourfold increase in crossover frequency. This suppression of crossover formation by Rqh1 is dependent on its helicase activity. We found that the synthetic lethality of cells lacking both Rqh1 and Eme1 is suppressed by loss of swi5(+), which allowed us to show that the excess crossovers formed in an rqh1Delta background are independent of Mus81-Eme1. This result suggests that a second process for crossover formation exists in S. pombe and is consistent with our finding that deletion of swi5(+) restored meiotic crossovers in eme1Delta cells. Evidence suggesting that Rqh1 also acts downstream of Swi5 in crossover formation was uncovered in these studies. Our results suggest that during Rhp51-dependent repair of DSBs, Rqh1-Top3 suppresses crossovers in the Rhp57-dependent pathway while Mus81-Eme1 and possibly Rqh1 promote crossovers in the Swi5-dependent pathway.

Fission yeast Swi5/Sfr1 and Rhp55/Rhp57 differentially regulate Rhp51-dependent recombination outcomes

Several accessory proteins referred to as mediators are required for the full activity of the Rad51 (Rhp51 in fission yeast) recombinase. In this study, we analyzed in vivo functions of the recently discovered Swi5/Sfr1 complex from fission yeast. In normally growing cells, the Swi5-GFP protein localizes to the nucleus, where it forms a diffuse nuclear staining pattern with a few distinct foci. These spontaneous foci do not form in swi2Delta mutants. Upon UV irradiation, Swi5 focus formation is induced in swi2Delta mutants, a response that depends on Sfr1 function, and Sfr1 also forms foci that colocalize with damage-induced Rhp51 foci. The number of UV-induced Rhp51 foci is partially reduced in swi5Delta and rhp57Delta mutants and completely abolished in an swi5Delta rhp57Delta double mutant. An assay for products generated by HO endonuclease-induced DNA double-strand breaks (DSBs) reveals that Rhp51 and Rhp57, but not Swi5/Sfr1, are essential for crossover production. These results suggest that Swi5/Sfr1 functions as an Rhp51 mediator but processes DSBs in a manner different from that of the Rhp55/57 mediator.

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.

Cohesins are required for meiotic DNA breakage and recombination in Schizosaccharomyces pombe

In preparation for the unique segregation of homologs at the first meiotic division, chromosomes undergo dramatic changes. The meiosis-specific sister chromatid cohesins Rec8 and Rec11 of Schizosaccharomyces pombe are recruited around the time of premeiotic replication, and Rec10, a component of meiosis-specific linear elements, is subsequently added. Here we report that Rec10 is essential for meiosis-specific DNA breakage by Rec12 (Spo11 homolog) and for meiotic recombination. DNA breakage and recombination also depend on the Rec8 and Rec11 cohesins, strictly in some genomic intervals but less so in others. Thus, in addition to their previously recognized role in meiotic chromosome segregation, cohesins have a direct role, as do linear element components, in meiotic recombination by enabling double-strand DNA break formation by Rec12. Our results reveal a pathway, whose regulation is significantly different from that in the distantly related yeast Saccharomyces cerevisiae, for meiosis-specific chromosome differentiation and high-frequency recombination.

Homologous recombination: needing to have my say

Recent studies in budding and fission yeasts have revealed Mei5 and Sae3 as factors necessary for the proper function of the recombinases Dmc1 and Rad51 in DNA repair and meiotic recombination, providing new insights into how strand exchange proteins are directed along specific recombination pathways.

Fission yeast mating-type switching: programmed damage and repair

Mating-type switching in fission yeast follows similar rules as in budding yeast, but the underlying mechanisms are entirely different. Whilst the initiating double-strand cut in Saccharomyces cerevisiae requires recombinational repair for survival, the initial damage in Schizosaccharomyces pombe only affects a single strand, which can be sealed by gap repair in situ, whether or not it serves as an imprint for subsequent switching of mating type from an appropriate donor cassette. Recent papers have linked the transient stalling of a replication fork to the generation of a site-specific nick. This discontinuity then remains protected for a full cell cycle, until it interferes with replication in the next S-phase. It, thereby, represents a valuable model system to study the molecular safeguards to protect a replication fork at a predetermined hindrance to leading-strand extension. The versatility of this experimental system has increased further yet by the recent development of a conditional setup, where imprinting and switching can be repressed or derepressed in response to external stimuli.

A natural meiotic DNA break site in Schizosaccharomyces pombe is a hotspot of gene conversion, highly associated with crossing over

In Schizosaccharomyces pombe, meiosis-specific DNA breaks that initiate recombination are observed at prominent but widely separated sites. We investigated the relationship between breakage and recombination at one of these sites, the mbs1 locus on chromosome I. Breaks corresponding to 10% of chromatids were mapped to four clusters spread over a 2.1-kb region. Gene conversion of markers within the clusters occurred in 11% of tetrads (3% of meiotic chromatids), making mbs1 a conversion hotspot when compared to other fission yeast markers. Approximately 80% of these conversions were associated with crossing over of flanking markers, suggesting a strong bias in meiotic break repair toward the generation of crossovers. This bias was observed in conversion events at three other loci, ade6, ade7, and ura1. A total of 50-80% of all crossovers seen in a 90-kb region flanking mbs1 occurred in a 4.8-kb interval containing the break sites. Thus, mbs1 is also a hotspot of crossing over, with breakage at mbs1 generating most of the crossovers in the 90-kb interval. Neither Rec12 (Spo11 ortholog) nor I-SceI-induced breakage at mbs1 was significantly associated with crossing over in an apparently break-free interval >25 kb away. Possible mechanisms for generating crossovers in such break-free intervals are discussed.