The yeast Red1 protein localizes to the cores of meiotic chromosomes

Physical and functional interactions among basic chromosome organizational features govern early steps of meiotic chiasma formation

Analysis of meiotic recombination by functional genomic approaches reveals prominent spatial and functional interactions among diverse organizational determinants. Recombination occurs between chromatin loop sequences; however, these sequences are spatially tethered to underlying chromosome axes via their recombinosomes. Meiotic chromosomal protein, Red1, localizes to chromosome axes; however, Red1 loading is modulated by R/G-bands isochores and thus by bulk chromatin state. Recombination is also modulated by isochore determinants: R-bands differentially favor double-strand break (DSB) formation but disfavor subsequent loading of meiotic RecA homolog, Dmc1. Red1 promotes DSB formation in both R- and G-bands and then promotes Dmc1 loading, specifically counteracting disfavoring R-band effects. These complexities are discussed in the context of chiasma formation as a series of coordinated local changes at the DNA and chromosome-axis levels.

Asy1, a protein required for meiotic chromosome synapsis, localizes to axis-associated chromatin in Arabidopsis and Brassica

The Arabidopsis thaliana ASY1 gene is essential for homologous chromosome synapsis. Antibodies specific to Asy1 protein and its homologue BoAsy1 from the related crop species Brassica oleracea have been used to investigate the temporal expression and localization of the protein in both species. Asy1 is initially detected in pollen mother cells during meiotic interphase as numerous punctate foci distributed over the chromatin. As leptotene progresses the signal appears to be increasingly continuous and is closely associated with the axial elements but not to the extended chromatin loops associated with them. By the end of zygotene the signal extends almost the entire length of the synapsed homologues, although not to the telomeres. The protein begins to disappear as the homologues desynapse, until by late diplotene it is no longer associated with the chromosomes. Immunogold labelling in conjunction with electron microscopy established that Asy1 localizes to regions of chromatin that associate with the axial/lateral elements of meiotic chromosomes rather than being a component of the synaptonemal complex itself. These data together with the previously observed asynaptic phenotype of the asy1 mutant suggest that Asy1 is required for morphogenesis of the synaptonemal complex, possibly by defining regions of chromatin that associate with the developing synaptonemal complex structure.

Mechanism and control of meiotic recombination initiation

Homologous recombination is essential during meiosis in most sexually reproducing organisms. In budding yeast, and most likely in other organisms as well, meiotic recombination proceeds via the formation and repair of DNA double-strand breaks (DSBs). These breaks appear to be formed by the Spo11 protein, with assistance from a large number of other gene products, by a topoisomerase-like transesterase mechanism. Recent studies in fission yeast, multicellular fungi, flies, worms, plants, and mammals indicate that the role of Spo11 in meiotic recombination initiation is highly conserved. This chapter reviews the properties of Spo11 and the other gene products required for meiotic DSB formation in a number of organisms and discusses ways in which recombination initiation is coordinated with other events occurring in the meiotic cell.

A role for Ddc1 in signaling meiotic double-strand breaks at the pachytene checkpoint

The pachytene checkpoint prevents meiotic cell cycle progression in response to unrepaired recombination intermediates. We show that Ddc1 is required for the pachytene checkpoint in Saccharomyces cerevisiae. During meiotic prophase, Ddc1 localizes to chromosomes and becomes phosphorylated; these events depend on the formation and processing of double-strand breaks (DSBs). Ddc1 colocalizes with Rad51, a DSB-repair protein, indicating that Ddc1 associates with sites of DSB repair. The Rad24 checkpoint protein interacts with Ddc1 and with recombination proteins (Sae1, Sae2, Rad57, and Msh5) in the two-hybrid protein system, suggesting that Rad24 also functions at DSB sites. Ddc1 phosphorylation and localization depend on Rad24 and Mec3, consistent with the hypothesis that Rad24 loads the Ddc1/Mec3/Rad17 complex onto chromosomes. Phosphorylation of Ddc1 depends on the meiosis-specific kinase Mek1. In turn, Ddc1 promotes the stable association of Mek1 with chromosomes and is required for Mek1-dependent phosphorylation of the meiotic chromosomal protein Red1. Ddc1 therefore appears to operate in a positive feedback loop that promotes Mek1 function.

The molecular basis of sister-chromatid cohesion

The replicated copies of each chromosome, the sister chromatids, are attached prior to their segregation in mitosis and meiosis. This association or cohesion is critical for each sister chromatid to bind to microtubules from opposite spindle poles and thus segregate away from each other at anaphase of mitosis or meiosis II. The cohesin protein complex is essential for cohesion in both mitosis and meiosis, and cleavage of one of the subunits is sufficient for loss of cohesion at anaphase. The localization of the cohesin complex and other cohesion proteins permits evaluation of the positions of sister-chromatid associations within the chromosome structure, as well as the relationship between cohesion and condensation. Recently, two key riddles in the mechanism of meiotic chromosome segregation have yielded to molecular answers. First, analysis of the cohesin complex in meiosis provides molecular support for the long-standing hypothesis that sister-chromatid cohesion links homologs in meiosis I by stabilizing chiasmata. Second, the isolation of the monopolin protein that controls kinetochore behavior in meiosis I defines a functional basis by which sister kinetochores are directed toward the same pole in meiosis I.

A model for chromosome structure during the mitotic and meiotic cell cycles

The chromosome scaffold model in which loops of chromatin are attached to a central, coiled chromosome core (scaffold) is the current paradigm for chromosome structure. Here we present a modified version of the chromosome scaffold model to describe chromosome structure and behavior through the mitotic and meiotic cell cycles. We suggest that a salient feature of chromosome structure is established during DNA replication when sister loops of DNA extend in opposite directions from replication sites on nuclear matrix strands. This orientation is maintained into prophase when the nuclear matrix strand is converted into two closely associated sister chromatid cores with sister DNA loops extending in opposite directions. We propose that chromatid cores are contractile and show, using a physical model, that contraction of cores during late prophase can result in coiled chromatids. Coiling accounts for the majority of chromosome shortening that is needed to separate sister chromatids within the confines of a cell. In early prophase I of meiosis, the orientation of sister DNA loops in opposite directions from axial elements assures that DNA loops interact preferentially with homologous DNA loops rather than with sister DNA loops. In this context, we propose a bar code model for homologous presynaptic chromosome alignment that involves weak paranemic interactions of homologous DNA loops. Opposite orientation of sister loops also suppresses crossing over between sister chromatids in favor of crossing over between homologous non-sister chromatids. After crossing over is completed in pachytene and the synaptonemal complex breaks down in early diplotene (= diffuse stage), new contractile cores are laid down along each chromatid. These chromatid cores are comparable to the chromatid cores in mitotic prophase chromosomes. As an aside, we propose that leptotene through early diplotene represent the 'missing' G2 period of the premeiotic interphase. The new chromosome cores, along with sister chromatid cohesion, stabilize chiasmata. Contraction of cores in late diplotene causes chromosomes to coil in a configuration that encourages subsequent syntelic orientation of sister kinetochores and amphitelic orientation of homologous kinetochore pairs on the spindle at metaphase I.

A meiotic chromosomal core consisting of cohesin complex proteins recruits DNA recombination proteins and promotes synapsis in the absence of an axial element in mammalian meiotic cells

The behavior of meiotic chromosomes differs in several respects from that of their mitotic counterparts, resulting in the generation of genetically distinct haploid cells. This has been attributed in part to a meiosis-specific chromatin-associated protein structure, the synaptonemal complex. This complex consist of two parallel axial elements, each one associated with a pair of sister chromatids, and a transverse filament located between the synapsed homologous chromosomes. Recently, a different protein structure, the cohesin complex, was shown to be associated with meiotic chromosomes and to be required for chromosome segregation. To explore the functions of the two different protein structures, the synaptonemal complex and the cohesin complex, in mammalian male meiotic cells, we have analyzed how absence of the axial element affects early meiotic chromosome behavior. We find that the synaptonemal complex protein 3 (SCP3) is a main determinant of axial-element assembly and is required for attachment of this structure to meiotic chromosomes, whereas SCP2 helps shape the in vivo structure of the axial element. We also show that formation of a cohesin-containing chromosomal core in meiotic nuclei does not require SCP3 or SCP2. Our results also suggest that the cohesin core recruits recombination proteins and promotes synapsis between homologous chromosomes in the absence of an axial element. A model for early meiotic chromosome pairing and synapsis is proposed.

The budding yeast Msh4 protein functions in chromosome synapsis and the regulation of crossover distribution

The budding yeast MSH4 gene encodes a MutS homolog produced specifically in meiotic cells. Msh4 is not required for meiotic mismatch repair or gene conversion, but it is required for wild-type levels of crossing over. Here, we show that a msh4 null mutation substantially decreases crossover interference. With respect to the defect in interference and the level of crossing over, msh4 is similar to the zip1 mutant, which lacks a structural component of the synaptonemal complex (SC). Furthermore, epistasis tests indicate that msh4 and zip1 affect the same subset of meiotic crossovers. In the msh4 mutant, SC formation is delayed compared to wild type, and full synapsis is achieved in only about half of all nuclei. The simultaneous defects in synapsis and interference observed in msh4 (and also zip1 and ndj1/tam1) suggest a role for the SC in mediating interference. The Msh4 protein localizes to discrete foci on meiotic chromosomes and colocalizes with Zip2, a protein involved in the initiation of chromosome synapsis. Both Zip2 and Zip1 are required for the normal localization of Msh4 to chromosomes, raising the possibility that the zip1 and zip2 defects in crossing over are indirect, resulting from the failure to localize Msh4 properly.

Coordinating the segregation of sister chromatids during the first meiotic division: evidence for sexual dimorphism

Errors during the first meiotic division are common in our species, but virtually all occur during female meiosis. The reason why oogenesis is more error prone than spermatogenesis remains unknown. Normal segregation of homologous chromosomes at the first meiotic division (MI) requires coordinated behavior of the sister chromatids of each homolog. Failure of sister kinetochores to act cooperatively at MI, or precocious sister chromatid segregation (PSCS), has been postulated to be a major contributor to human nondisjunction. To investigate the factors that influence PSCS we utilized the XO mouse, since the chromatids of the single X chromosome frequently segregate at MI, and the propensity for PSCS is influenced by genetic background. Our studies demonstrate that the strain-specific differences in PSCS are due to the actions of an autosomal trans-acting factor or factors. Since components of the synaptonemal complex are thought to play a role in centromere cohesion and kinetochore orientation, we evaluated the behavior of the X chromosome at prophase to determine if this factor influenced the propensity of the chromosome for self-synapsis. We were unable to directly correlate synaptic differences with subsequent segregation behavior. However, unexpectedly, we uncovered a sexual dimorphism that may partially explain sex-specific differences in the fidelity of meiotic chromosome segregation. Specifically, in the male remnants of the synaptonemal complex remain associated with the centromeres until anaphase of the second meiotic division (MII), whereas in the female, all traces of synaptonemal complex (SC) protein components are lost from the chromosomes before the onset of the first meiotic division. This finding suggests a sex-specific difference in the components used to correctly segregate chromosomes during meiosis, and may provide a reason for the high error frequency during female meiosis.

Checkpoint and DNA-repair proteins are associated with the cores of mammalian meiotic chromosomes

Meiotic checkpoints are manifested through protein complexes capable of detecting an abnormality in chromosome metabolism and signaling it to effector molecules that subsequently delay or arrest the progression of meiosis. Some checkpoints act during the first meiotic prophase to monitor the repair of chromosomal DSBs, predominantly by meiotic recombination, or to ensure the correct establishment of synapsis and its well-timed dissolution. In mammals, a number of checkpoint and repair proteins localize to the meiotic chromosomal cores, sometimes in the context of the synaptonemal complex (SC). Here we discuss possible functions of these proteins in the accomplishment of meiotic recombination and normal progression of the meiotic pathway. Also, we present arguments for a structural role of cores and SCs in the assembly of the repair and checkpoint protein complexes on the chromosomes.

The Saccharomyces cerevisiae MUM2 gene interacts with the DNA replication machinery and is required for meiotic levels of double strand breaks

The Saccharomyces cerevisiae MUM2 gene is essential for meiotic, but not mitotic, DNA replication and thus sporulation. Genetic interactions between MUM2 and a component of the origin recognition complex and polymerase alpha-primase suggest that MUM2 influences the function of the DNA replication machinery. Early meiotic gene expression is induced to a much greater extent in mum2 cells than in meiotic cells treated with the DNA synthesis inhibitor hydroxyurea. This result indicates that the mum2 meiotic arrest is downstream of the arrest induced by hydroxyurea and suggests that DNA synthesis is initiated in the mutant. Genetic analyses indicate that the recombination that occurs in mum2 mutants is dependent on the normal recombination machinery and on synaptonemal complex components and therefore is not a consequence of lesions created by incompletely replicated DNA. Both meiotic ectopic and allelic recombination are similarly reduced in the mum2 mutant, and the levels are consistent with the levels of meiosis-specific DSBs that are generated. Cytological analyses of mum2 mutants show that chromosome pairing and synapsis occur, although at reduced levels compared to wild type. Given the near-wild-type levels of meiotic gene expression, pairing, and synapsis, we suggest that the reduction in DNA replication is directly responsible for the reduced level of DSBs and meiotic recombination.

The pachytene checkpoint prevents accumulation and phosphorylation of the meiosis-specific transcription factor Ndt80

In budding yeast, many mutants defective in meiotic recombination and chromosome synapsis undergo checkpoint-mediated arrest at the pachytene stage of meiotic prophase. We recovered the NDT80 gene in a screen for genes whose overexpression bypasses the pachytene checkpoint. Ndt80 is a meiosis-specific transcription factor that promotes expression of genes required for exit from pachytene and entry into meiosis I. Herein, we show that the Ndt80 protein accumulates and is extensively phosphorylated during meiosis in wild type but not in cells arrested at the pachytene checkpoint. Our results indicate that inhibition of Ndt80 activity is one mechanism used to achieve pachytene arrest.

Role for the silencing protein Dot1 in meiotic checkpoint control

During the meiotic cell cycle, a surveillance mechanism called the "pachytene checkpoint" ensures proper chromosome segregation by preventing meiotic progression when recombination and chromosome synapsis are defective. The silencing protein Dot1 (also known as Pch1) is required for checkpoint-mediated pachytene arrest of the zip1 and dmc1 mutants of Saccharomyces cerevisiae. In the absence of DOT1, the zip1 and dmc1 mutants inappropriately progress through meiosis, generating inviable meiotic products. Other components of the pachytene checkpoint include the nucleolar protein Pch2 and the heterochromatin component Sir2. In dot1, disruption of the checkpoint correlates with the loss of concentration of Pch2 and Sir2 in the nucleolus. In addition to its checkpoint function, Dot1 blocks the repair of meiotic double-strand breaks by a Rad54-dependent pathway of recombination between sister chromatids. In vegetative cells, mutation of DOT1 results in delocalization of Sir3 from telomeres, accounting for the impaired telomeric silencing in dot1.

Tid1/Rdh54 promotes colocalization of rad51 and dmc1 during meiotic recombination

Two RecA homologs, Rad51 and Dmc1, assemble as cytologically visible complexes (foci) at the same sites on meiotic chromosomes. Time course analysis confirms that co-foci appear and disappear as the single predominant form. A large fraction of co-foci are eliminated in a red1 mutant, which is expected as a characteristic of the interhomolog-specific recombination pathway. Previous studies suggested that normal Dmc1 loading depends on Rad51. We show here that a mutation in TID1/RDH54, encoding a RAD54 homolog, reduces Rad51-Dmc1 colocalization relative to WT. A rad54 mutation, in contrast, has relatively little effect on RecA homolog foci except when strains also contain a tid1/rdh54 mutation. The role of Tid1/Rdh54 in coordinating RecA homolog assembly may be very direct, because Tid1/Rdh54 is known to physically bind both Dmc1 and Rad51. Also, Dmc1 foci appear early in a tid1/rdh54 mutant. Thus, Tid1 may normally act with Rad51 to promote ordered RecA homolog assembly by blocking Dmc1 until Rad51 is present. Finally, whereas double-staining foci predominate in WT nuclei, a subset of nuclei with expanded chromatin exhibit individual Rad51 and Dmc1 foci side-by-side, suggesting that a Rad51 homo-oligomer and a Dmc1 homo-oligomer assemble next to one another at the site of a single double-strand break (DSB) recombination intermediate.

Meiotic segregation, synapsis, and recombination checkpoint functions require physical interaction between the chromosomal proteins Red1p and Hop1p

In yeast, HOP1 and RED1 are required during meiosis for proper chromosome segregation and the consequent formation of viable spores. Mutations in either HOP1 or RED1 create unique as well as overlapping phenotypes, indicating that the two proteins act alone as well as in concert with each other. To understand which meiotic processes specifically require Red1p-Hop1p hetero-oligomers, a novel genetic screen was used to identify a single-point mutation of RED1, red1-K348E, that separates Hop1p binding from Red1p homo-oligomerization. The Red1-K348E protein is stable, phosphorylated in a manner equivalent to Red1p, and undergoes efficient homo-oligomerization; however, its ability to interact with Hop1p both by two-hybrid and coimmunoprecipitation assays is greatly reduced. Overexpression of HOP1 specifically suppresses red1-K348E, supporting the idea that the only defect in the protein is a reduced affinity for Hop1p. red1-K348E mutants exhibit reduced levels of crossing over and spore viability and fail to undergo chromosome synapsis, thereby implicating a role for Red1p-Hop1p hetero-oligomers in these processes. Furthermore, red1-K348E suppresses the sae2/com1 defects in meiotic progression and sporulation, indicating a previously unknown role for HOP1 in the meiotic recombination checkpoint.

DNA replication and damage checkpoints and meiotic cell cycle controls in the fission and budding yeasts

The cell cycle checkpoint mechanisms ensure the order of cell cycle events to preserve genomic integrity. Among these, the DNA-replication and DNA-damage checkpoints prevent chromosome segregation when DNA replication is inhibited or DNA is damaged. Recent studies have identified an outline of the regulatory networks for both of these controls, which apparently operate in all eukaryotes. In addition, it appears that these checkpoints have two arrest points, one is just before entry into mitosis and the other is prior to chromosome separation. The former point requires the central cell-cycle regulator Cdc2 kinase, whereas the latter involves several key regulators and substrates of the ubiquitin ligase called the anaphase promoting complex. Linkages between these cell-cycle regulators and several key checkpoint proteins are beginning to emerge. Recent findings on post-translational modifications and protein-protein interactions of the checkpoint proteins provide new insights into the checkpoint responses, although the functional significance of these biochemical properties often remains unclear. We have reviewed the molecular mechanisms acting at the DNA-replication and DNA-damage checkpoints in the fission yeast Schizosaccharomyces pombe, and the modifications of these controls during the meiotic cell cycle. We have made comparisons with the controls in fission yeast and other organisms, mainly the distantly related budding yeast.

Bypass of a meiotic checkpoint by overproduction of meiotic chromosomal proteins

The Saccharomyces cerevisiae zip1 mutant, which exhibits defects in synaptonemal complex formation and meiotic recombination, triggers a checkpoint that causes cells to arrest at the pachytene stage of meiotic prophase. Overproduction of either the meiotic chromosomal protein Red1 or the meiotic kinase Mek1 bypasses this checkpoint, allowing zip1 cells to sporulate. Red1 or Mek1 overproduction also promotes sporulation of other mutants (zip2, dmc1, hop2) that undergo checkpoint-mediated arrest at pachytene. In addition, Red1 overproduction antagonizes interhomolog interactions in the zip1 mutant, substantially decreasing double-strand break formation, meiotic recombination, and homologous chromosome pairing. Mek1 overproduction, in contrast, suppresses checkpoint-induced arrest without significantly decreasing meiotic recombination. Cooverproduction of Red1 and Mek1 fails to bypass the checkpoint; moreover, overproduction of the meiotic chromosomal protein Hop1 blocks the Red1 and Mek1 overproduction phenotypes. These results suggest that meiotic chromosomal proteins function in the signaling of meiotic prophase defects and that the correct stoichiometry of Red1, Mek1, and Hop1 is needed to achieve checkpoint-mediated cell cycle arrest at pachytene.

Multiple roles of Spo11 in meiotic chromosome behavior

Spo11, a type II topoisomerase, is likely to be required universally for initiation of meiotic recombination. However, a dichotomy exists between budding yeast and the animals Caenorhabditis elegans and Drosophila melanogaster with respect to additional roles of Spo11 in meiosis. In Saccharomyces cerevisiae, Spo11 is required for homolog pairing, as well as axial element (AE) and synaptonemal complex (SC) formation. All of these functions are Spo11 independent in C.elegans and D.melanogaster. We examined Spo11 function in a multicellular fungus, Coprinus cinereus. The C.cinereus spo11-1 mutant shows high levels of homolog pairing and occasionally forms full-length AEs, but no SC. In C.cinereus, Spo11 is also required for maintenance of meiotic chromosome condensation and proper spindle formation. Meiotic progression in spo11-1 is aberrant; late in meiosis basidia undergo programmed cell death (PCD). To our knowledge, this is the first example of meiotic PCD outside the animal kingdom. Ionizing radiation can partially rescue spo11-1 for both AE and SC formation and viable spore production, suggesting that the double-strand break function of Spo11 is conserved and is required for these functions.

Pachytene exit controlled by reversal of Mek1-dependent phosphorylation

During yeast meiosis, a checkpoint prevents exit from pachytene in response to defects in meiotic recombination and chromosome synapsis. This pachytene checkpoint requires two meiotic chromosomal proteins, Red1 and Mek1; Mek1 is a kinase that phosphorylates Red1. In mutants that undergo checkpoint-mediated pachytene arrest, Mek1 is active and Red1 remains phosphorylated. Activation of Mek1 requires the initiation of meiotic recombination and certain DNA damage checkpoint proteins. Mek1 kinase activity and checkpoint-induced pachytene arrest are counteracted by protein phosphatase type 1 (Glc7). Glc7 coimmunoprecipitates with Red1, colocalizes with Red1 on chromosomes, and dephosphorylates Red1 in vitro. We speculate that phosphorylated Red1 prevents exit from pachytene and that completion of meiotic recombination triggers Glc7-dependent dephosphorylation of Red1.

Meiotic chromosomes: integrating structure and function

Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, a feature which greatly facilitates the study of chromosome behavior. The complex events involved in homolog juxtaposition necessitate prolongation of prophase, thus permitting resolution of events that are temporally compressed in the mitotic cycle. Furthermore, once homologs are paired, the chromosomes are connected by a specific structure: the synaptonemal complex. Finally, interaction of homologs includes recombination at the DNA level, which is intimately linked to structural features of the chromosomes. In consequence, recombination-related events report on diverse aspects of chromosome morphogenesis, notably relationships between sisters, development of axial structure, and variations in chromatin status. The current article reviews recent information on these topics in an historical context. This juxtaposition has suggested new relationships between structure and function. Additional issues were addressed in a previous chapter (551).

Progression of meiotic DNA replication is modulated by interchromosomal interaction proteins, negatively by Spo11p and positively by Rec8p

Spo11p is a key mediator of interhomolog interactions during meiosis. Deletion of the SPO11 gene decreases the length of S phase by ∼25%. Rec8p is a key coordinator of meiotic interhomolog and intersister interactions. Deletion of the REC8 gene increases S-phase length, by ∼10% in wild-type and ∼30% in aspo11Δ background. Thus, the progression of DNA replication is modulated by interchromosomal interaction proteins. Thespo11–Y135F DSB (double strand break) catalysis-defective mutant is normal for S-phase modulation and DSB-independent homolog pairing but is defective for later events, formation of DSBs, and synaptonemal complexes. Thus, earlier and later functions of Spo11 are defined. We propose that meiotic S-phase progression is linked directly to development of specific chromosomal features required for meiotic interhomolog interactions and that this feedback process is built upon a more fundamental mechanism, common to all cell types, by which S-phase progression is coupled to development of nascent intersister connections and/or related aspects of chromosome morphogenesis. Roles for Rec8 and/or Spo11 in progression through other stages of meiosis are also revealed.

Progression of meiotic DNA replication is modulated by interchromosomal interaction proteins, negatively by Spo11p and positively by Rec8p

Spo11p is a key mediator of interhomolog interactions during meiosis. Deletion of the SPO11 gene decreases the length of S phase by approximately 25%. Rec8p is a key coordinator of meiotic interhomolog and intersister interactions. Deletion of the REC8 gene increases S-phase length, by approximately 10% in wild-type and approximately 30% in a spo11Delta background. Thus, the progression of DNA replication is modulated by interchromosomal interaction proteins. The spo11-Y135F DSB (double strand break) catalysis-defective mutant is normal for S-phase modulation and DSB-independent homolog pairing but is defective for later events, formation of DSBs, and synaptonemal complexes. Thus, earlier and later functions of Spo11 are defined. We propose that meiotic S-phase progression is linked directly to development of specific chromosomal features required for meiotic interhomolog interactions and that this feedback process is built upon a more fundamental mechanism, common to all cell types, by which S-phase progression is coupled to development of nascent intersister connections and/or related aspects of chromosome morphogenesis. Roles for Rec8 and/or Spo11 in progression through other stages of meiosis are also revealed.

Organization of the yeast Zip1 protein within the central region of the synaptonemal complex

The yeast Zip1 protein is a component of the central region of the synaptonemal complex (SC). Zip1 is predicted to form an alpha-helical coiled coil, flanked by globular domains at the NH(2) and COOH termini. Immunogold labeling with domain-specific anti-Zip1 antibodies demonstrates that the NH(2)-terminal domain of Zip1 is located in the middle of the central region of the SC, whereas the COOH-terminal domain is embedded in the lateral elements of the complex. Previous studies have shown that overproduction of Zip1 results in the assembly of two types of aggregates, polycomplexes and networks, that are unassociated with chromatin. Our epitope mapping data indicate that the organization of Zip1 within polycomplexes is similar to that of the SC, whereas the organization of Zip1 within networks is fundamentally different. Zip1 protein purified from bacteria assembles into dimers in vitro, and electron microscopic analysis demonstrates that the two monomers within a dimer are arranged in parallel and in register. Together, these results suggest that two Zip1 dimers, lying head-to-head, span the width of the SC.

Yeast meiosis-specific protein Hop1 binds to G4 DNA and promotes its formation

DNA molecules containing stretches of contiguous guanine residues can assume a stable configuration in which planar quartets of guanine residues joined by Hoogsteen pairing appear in a stacked array. This conformation, called G4 DNA, has been implicated in several aspects of chromosome behavior including immunoglobulin gene rearrangements, promoter activation, and telomere maintenance. Moreover, the ability of the yeast SEP1 gene product to cleave DNA in a G4-DNA-dependent fashion, as well as that of the SGS1 gene product to unwind G4 DNA, has suggested a crucial role for this structure in meiotic synapsis and recombination. Here, we demonstrate that the HOP1 gene product, which plays a crucial role in the formation of synaptonemal complex in Saccharomyces cerevisiae, binds robustly to G4 DNA. The apparent dissociation constant for interaction with G4 DNA is 2 x 10(-10), indicative of binding that is about 1,000-fold stronger than to normal duplex DNA. Oligonucleotides of appropriate sequence bound Hop1 protein maximally if the DNA was first subjected to conditions favoring the formation of G4 DNA. Furthermore, incubation of unfolded oligonucleotides with Hop1 led to their transformation into G4 DNA. Methylation interference experiments confirmed that modifications blocking G4 DNA formation inhibit Hop1 binding. In contrast, neither bacterial RecA proteins that preferentially interact with GT-rich DNA nor histone H1 bound strongly to G4 DNA or induced its formation. These findings implicate specific interactions of Hop1 protein with G4 DNA in the pathway to chromosomal synapsis and recombination in meiosis.

An integration of old and new perspectives of mammalian meiotic sterility

The many events of meiotic prophase can now be viewed as a series of specialized incidents that are monitored by meiotic checkpoints, some of which are similar to their mitotic counterparts, and some of which are probably unique to meiosis. This shift in perspective means that meiotic sterility in mammals must be reexamined and viewed as the result of errors subject to meiotic checkpoint controls. Like their mitotic counterparts, the meiotic checkpoints detect defects and halt normal progression until these mistakes can be repaired. Some of these checkpoints utilize mitotic checkpoint proteins, others may involve meiotic-specific proteins, or splice forms. If repair is impossible, the checkpoints then either trigger immediate apoptosis or cause an arrest of meiotic progression followed by eventual cell death. If a sufficient number of spermatocytes are involved, either alternative results in sterility. Identification of these meiotic checkpoints and delineation of the signal transduction cascades involved has only just begun. While yeast, or other model organisms, may provide clues to some of these pathways, others appears to have arisen during vertebrate evolution. The study of mammalian meiosis has entered a new era and the foundations are being laid for a growing understanding of the many problems that may contribute to sterility.

The murine SCP3 gene is required for synaptonemal complex assembly, chromosome synapsis, and male fertility

During meiosis, the homologous chromosomes pair and recombine. An evolutionarily conserved protein structure, the synaptonemal complex (SC), is located along the paired meiotic chromosomes. We have studied the function of a structural component in the axial/lateral element of the SC, the synaptonemal complex protein 3 (SCP3). A null mutation in the SCP3 gene was generated, and we noted that homozygous mutant males were sterile due to massive apoptotic cell death during meiotic prophase. The SCP3-deficient male mice failed to form axial/lateral elements and SCs, and the chromosomes in the mutant spermatocytes did not synapse. While the absence of SCP3 affected the nuclear distribution of DNA repair and recombination proteins (Rad51 and RPA), as well as synaptonemal complex protein 1 (SCP1), a residual chromatin organization remained in the mutant meiotic cells.

The pachytene checkpoint in S. cerevisiae depends on Swe1-mediated phosphorylation of the cyclin-dependent kinase Cdc28

Mutants defective in meiotic recombination and synaptonemal complex formation undergo checkpoint-mediated arrest in mid-meiotic prophase. In S. cerevisiae, this checkpoint requires Swe1, which phosphorylates and inactivates the cyclin-dependent kinase Cdc28. A swe1 deletion allows mutants that normally arrest in meiotic prophase to sporulate at wild-type levels, though sporulation is delayed. This delay is eliminated by overproducing Clb1, the major cyclin required for meiosis I. The Swe1 protein accumulates and is hyperphosphorylated in checkpoint-arrested cells. Our results suggest that meiotic arrest is mediated both by increasing Swe1 activity and limiting cyclin production, with Swe1 being the primary downstream target of checkpoint control. The requirement for Swe1 distinguishes the pachytene checkpoint from the DNA damage checkpoints operating in vegetative cells.

RAD51 and DMC1 form mixed complexes associated with mouse meiotic chromosome cores and synaptonemal complexes

The eukaryotic RecA homologues RAD51 and DMC1 function in homology recognition and formation of joint-molecule recombination intermediates during yeast meiosis. The precise immunolocalization of these two proteins on the meiotic chromosomes of plants and animals has been complicated by their high degree of identity at the amino acid level. With antibodies that have been immunodepleted of cross-reactive epitopes, we demonstrate that RAD51 and DMC1 have identical distribution patterns in extracts of mouse spermatocytes in successive prophase I stages, suggesting coordinate functionality. Immunofluorescence and immunoelectron microscopy with these antibodies demonstrate colocalization of the two proteins on the meiotic chromosome cores at early prophase I. We also show that mouse RAD51 and DMC1 establish protein-protein interactions with each other and with the chromosome core component COR1(SCP3) in a two-hybrid system and in vitro binding analyses. These results suggest that the formation of a multiprotein recombination complex associated with the meiotic chromosome cores is essential for the development and fulfillment of the meiotic recombination process.

Genetic control of recombination partner preference in yeast meiosis. Isolation and characterization of mutants elevated for meiotic unequal sister-chromatid recombination

Meiotic exchange occurs preferentially between homologous chromatids, in contrast to mitotic recombination, which occurs primarily between sister chromatids. To identify functions that direct meiotic recombination events to homologues, we screened for mutants exhibiting an increase in meiotic unequal sister-chromatid recombination (SCR). The msc (meiotic sister-chromatid recombination) mutants were quantified in spo13 meiosis with respect to meiotic unequal SCR frequency, disome segregation pattern, sporulation frequency, and spore viability. Analysis of the msc mutants according to these criteria defines three classes. Mutants with a class I phenotype identified new alleles of the meiosis-specific genes RED1 and MEK1, the DNA damage checkpoint genes RAD24 and MEC3, and a previously unknown gene, MSC6. The genes RED1, MEK1, RAD24, RAD17, and MEC1 are required for meiotic prophase arrest induced by a dmc1 mutation, which defines a meiotic recombination checkpoint. Meiotic unequal SCR was also elevated in a rad17 mutant. Our observation that meiotic unequal SCR is elevated in meiotic recombination checkpoint mutants suggests that, in addition to their proposed monitoring function, these checkpoint genes function to direct meiotic recombination events to homologues. The mutants in class II, including a dmc1 mutant, confer a dominant meiotic lethal phenotype in diploid SPO13 meiosis in our strain background, and they identify alleles of UBR1, INP52, BUD3, PET122, ELA1, and MSC1-MSC3. These results suggest that DMC1 functions to bias the repair of meiosis-specific double-strand breaks to homologues. We hypothesize that the genes identified by the class II mutants function in or are regulators of the DMC1-promoted interhomologue recombination pathway. Class III mutants may be elevated for rates of both SCR and homologue exchange.

Saccharomyces cerevisiae checkpoint genes MEC1, RAD17 and RAD24 are required for normal meiotic recombination partner choice

Checkpoint gene function prevents meiotic progression when recombination is blocked by mutations in the recA homologue DMC1. Bypass of dmc1 arrest by mutation of the DNA damage checkpoint genes MEC1, RAD17, or RAD24 results in a dramatic loss of spore viability, suggesting that these genes play an important role in monitoring the progression of recombination. We show here that the role of mitotic checkpoint genes in meiosis is not limited to maintaining arrest in abnormal meioses; mec1-1, rad24, and rad17 single mutants have additional meiotic defects. All three mutants display Zip1 polycomplexes in two- to threefold more nuclei than observed in wild-type controls, suggesting that synapsis may be aberrant. Additionally, all three mutants exhibit elevated levels of ectopic recombination in a novel physical assay. rad17 mutants also alter the fraction of recombination events that are accompanied by an exchange of flanking markers. Crossovers are associated with up to 90% of recombination events for one pair of alleles in rad17, as compared with 65% in wild type. Meiotic progression is not required to allow ectopic recombination in rad17 mutants, as it still occurs at elevated levels in ndt80 mutants that arrest in prophase regardless of checkpoint signaling. These observations support the suggestion that MEC1, RAD17, and RAD24, in addition to their proposed monitoring function, act to promote normal meiotic recombination.

Phosphorylated proteins are involved in sister-chromatid arm cohesion during meiosis I

Sister-chromatid arm cohesion is lost during the metaphase I/anaphase I transition to allow homologue separation. To obtain needed information on this process we have analysed in grasshopper bivalents the sequential release of arm cohesion in relation to the behaviour of chromatid axes. Results show that sister axes are associated during early metaphase I but separate during late metaphase I leading to a concomitant change of chromosome structure that implies the loss of sister-kinetochore cohesion. Afterwards, homologues initiate their separation asynchronously depending on their size, and number and position of chiasmata. In all bivalents thin chromatin strands at the telomeres appeared as the last point of contact between sister chromatids. Additionally, we have analysed the participation of phosphoproteins recognised by the MPM-2 monoclonal antibody against mitotic phosphoproteins in arm cohesion in bivalents and two different kinds of univalents. Results show the absence of MPM-2 phosphoproteins at the interchromatid domain in mitotic chromosomes and meiotic univalents, but their presence in metaphase I bivalents. These phosphoproteins are lost at the onset of anaphase I. Taken together, these data have prompted us to propose a ‘working’ model for the release of arm cohesion during meiosis I. The model suggests that MPM-2 phosphoproteins may act as cohesive proteins associating sister axes. Their modification, once all bivalents are correctly aligned at the metaphase plate, would trigger a change of chromosome structure and the sequential release of sister-kinetochore, arm, and telomere cohesions.

Synapsis and chiasma formation in Caenorhabditis elegans require HIM-3, a meiotic chromosome core component that functions in chromosome segregation

Meiotic chromosomes are organized about a proteinaceous core that forms between replicated sister chromatids. We have isolated a Caenorhabditis elegans gene, him-3, which encodes a meiosis-specific component of chromosome cores with some similarity to the yeast lateral element protein Hop1p. Antibodies raised against HIM-3 localize the protein to condensing chromosomes in early prophase I and to the cores of both synapsed and desynapsed chromosomes. In RNA interference experiments, chromosomes appear to condense normally in the absence of detectable protein but fail to synapse and form chiasmata, indicating that HIM-3 is essential for these processes. Hypomorphs of him-3, although being synapsis proficient, show severe reductions in the frequency of crossing-over, demonstrating that HIM-3 has a role in establishing normal levels of interhomolog exchange. Him-3 mutants also show defects in meiotic chromosome segregation and the persistence of the protein at the chromosome core until the metaphase I-anaphase I transition suggests that HIM-3 may play a role in sister chromatid cohesion. The analysis of him-3 provides the first functional description of a chromosome core component in a multicellular organism and suggests that a mechanistic link exists between the early meiotic events of synapsis and recombination, and later events such as segregation.

Spo76p is a conserved chromosome morphogenesis protein that links the mitotic and meiotic programs

Spo76p is conserved and related to the fungal proteins Pds5p and BIMD and the human AS3 prostate proliferative shutoff-associated protein. Spo76p localizes to mitotic and meiotic chromosomes, except at metaphase(s) and anaphase(s). During meiotic prophase, Spo76p assembles into strong lines in correlation with axial element formation. As inferred from spo76-1 mutant phenotypes, Spo76p is required for sister chromatid cohesiveness, chromosome axis morphogenesis, and chromatin condensation during critical transitions at mitotic prometaphase and meiotic midprophase. Spo76p is also required for meiotic interhomolog recombination, likely at postinitiation stage(s). We propose that a disruptive force coordinately promotes chromosomal axial compaction and destabilization of sister connections and that Spo76p restrains and channels the effects of this force into appropriate morphogenetic mitotic and meiotic outcomes.

A central role for cohesins in sister chromatid cohesion, formation of axial elements, and recombination during yeast meiosis

A multisubunit complex, called cohesin, containing Smc1p, Smc3p, Scc1p, and Scc3p, is required for sister chromatid cohesion in mitotic cells. We show here that Smc3p and a meiotic version of Scc1p called Rec8p are required for cohesion between sister chromatids, for formation of axial elements, for reciprocal recombination, and for preventing hyperresection of double-strand breaks during meiosis. Both Rec8p and Smc3p colocalize with chromosome cores independently of synapsis during prophase I and largely disappear from chromosome arms after pachytene but persist in the neighborhood of centromeres until the onset of anaphase II. The eukaryotic cell's cohesion apparatus is required both for the repair of recombinogenic lesions and for chromosome segregation and therefore appears to lie at the heart of the meiotic process.

Heterologously expressed polypeptide from the yeast meiotic gene HOP1 binds preferentially to yeast DNA

HOP1 protein, present in sporulating cells of Saccharomyces cerevisiae and believed to be a component of the synaptonemal complex, has been expressed in Escherichia coli fused to a biotinylated tag protein. Once solubilized from bacterial inclusion bodies, the HOP1 fusion protein was purified by using a combination of avidin-affinity chromatography and gel filtration FPLC and refolded. Sequence comparisons indicate that the HOP1 gene product contains a zinc finger motif, which may confer DNA binding properties, and the recombinant polypeptide was used to assess the putative DNA binding properties of the product of native HOP1 protein using a gel-shift assay. Protein and protein-DNA complexes were detected by exploiting the affinity of streptavidin-alkaline phosphatase for the biotinylated tag protein after Western blotting. The HOP1 fusion protein bound unambiguously to digested genomic yeast DNA. This binding possessed some degree of specificity, was maintained under a wide range of salt concentrations, and was unaffected by the presence of high concentrations of competitor DNA (synthetic poly[dI-dC].poly[dI-dC]). In contrast, no shift was detected when the fusion protein was incubated with digested genomic DNA from Arabidopsis, or with lambda/HindIII DNA. Incubation with digested genomic DNA from Lilium produced a small change in the mobility of the protein. The biotinylated tag protein failed to show any DNA binding activity. Scatchard analysis indicated an apparent yeast genomic DNA:HOP1 fusion protein dissociation constant of K(d) = 5 x 10(-7) M.

Multiple pathways of recombination induced by double-strand breaks in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been the principal organism used in experiments to examine genetic recombination in eukaryotes. Studies over the past decade have shown that meiotic recombination and probably most mitotic recombination arise from the repair of double-strand breaks (DSBs). There are multiple pathways by which such DSBs can be repaired, including several homologous recombination pathways and still other nonhomologous mechanisms. Our understanding has also been greatly enriched by the characterization of many proteins involved in recombination and by insights that link aspects of DNA repair to chromosome replication. New molecular models of DSB-induced gene conversion are presented. This review encompasses these different aspects of DSB-induced recombination in Saccharomyces and attempts to relate genetic, molecular biological, and biochemical studies of the processes of DNA repair and recombination.

Multiple pathways of recombination induced by double-strand breaks in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been the principal organism used in experiments to examine genetic recombination in eukaryotes. Studies over the past decade have shown that meiotic recombination and probably most mitotic recombination arise from the repair of double-strand breaks (DSBs). There are multiple pathways by which such DSBs can be repaired, including several homologous recombination pathways and still other nonhomologous mechanisms. Our understanding has also been greatly enriched by the characterization of many proteins involved in recombination and by insights that link aspects of DNA repair to chromosome replication. New molecular models of DSB-induced gene conversion are presented. This review encompasses these different aspects of DSB-induced recombination in Saccharomyces and attempts to relate genetic, molecular biological, and biochemical studies of the processes of DNA repair and recombination.

Rec8p, a meiotic recombination and sister chromatid cohesion phosphoprotein of the Rad21p family conserved from fission yeast to humans

Our work and that of others defined mitosis-specific (Rad21 subfamily) and meiosis-specific (Rec8 subfamily) proteins involved in sister chromatid cohesion in several eukaryotes, including humans. Mutation of the fission yeast Schizosaccharomyces pombe rec8 gene was previously shown to confer a number of meiotic phenotypes, including strong reduction of recombination frequencies in the central region of chromosome III, absence of linear element polymerization, reduced pairing of homologous chromosomes, reduced sister chromatid cohesion, aberrant chromosome segregation, defects in spore formation, and reduced spore viability. Here we extend the description of recombination reduction to the central regions of chromosomes I and II. We show at the protein level that expression of rec8 is meiosis specific and that Rec8p localizes to approximately 100 foci per prophase nucleus. Rec8p was present in an unphosphorylated form early in meiotic prophase but was phosphorylated prior to meiosis I, as demonstrated by analysis of the mei4 mutant blocked before meiosis I. Evidence for the persistence of Rec8p beyond meiosis I was obtained by analysis of the mutant mes1 blocked before meiosis II. A human gene, which we designate hrec8, showed significant primary sequence similarity to rec8 and was mapped to chromosome 14. High mRNA expression of mouse and human rec8 genes was found only in germ line cells, specifically in testes and, interestingly, in spermatids. hrec8 was also expressed at a low level in the thymus. Sequence similarity and testis-specific expression indicate evolutionarily conserved functions of Rec8p in meiosis. Possible roles of Rec8p in the integration of different meiotic events are discussed.

Pch2 links chromatin silencing to meiotic checkpoint control

The PCH2 gene of Saccharomyces cerevisiae is required for the meiotic checkpoint that prevents chromosome segregation when recombination and chromosome synapsis are defective. Mutation of PCH2 relieves the checkpoint-induced pachytene arrest of the zip1, zip2, and dmc1 mutants, resulting in chromosome missegregation and low spore viability. Most of the Pch2 protein localizes to the nucleolus, where it represses meiotic interhomolog recombination in the ribosomal DNA, apparently by excluding the meiosis-specific Hop1 protein. Nucleolar localization of Pch2 depends on the silencing factor Sir2, and mutation of SIR2 also bypasses the zip1 pachytene arrest. Under certain circumstances, Sir3-dependent localization of Pch2 to telomeres also provides checkpoint function. These unexpected findings link the nucleolus, chromatin silencing, and the pachytene checkpoint.

The leptotene-zygotene transition of meiosis

The leptotene/zygotene transition of meiosis, as defined by classical cytological studies, is the period when homologous chromosomes, already being discernible individualized entities, begin to be close together or touching over portions of their lengths. This period also includes the bouquet stage: Chromosome ends, which have already become integral components of the inner nuclear membrane, move into a polarized configuration, along with other nuclear envelope components. Chromosome movements, active or passive, also occur. The detailed nature of interhomologue interactions during this period, with special emphasis on the involvement of chromosome ends, and the overall role for meiosis and recombination of chromosome movement and, especially, the bouquet stage are discussed.

Red1p, a MEK1-dependent phosphoprotein that physically interacts with Hop1p during meiosis in yeast

The synaptonemal complex (SC) is a proteinaceous structure formed between pairs of homologous chromosomes during prophase I of meiosis. The proper assembly of axial elements (AEs), lateral components of the SC, during meiosis in the yeast, Saccharomyces cerevisiae, is essential for wild-type levels of recombination and for the accurate segregation of chromosomes at the first meiotic division. Genetic experiments have indicated that the stoichiometry between two meiosis-specific components of AEs in S. cerevisiae, HOP1 and RED1, is critical for proper assembly and function of the SC. A third meiosis-specific gene, MEK1, which encodes a putative serine/threonine protein kinase, is also important for proper AE function, suggesting that AE formation is regulated by phosphorylation. In this paper, we demonstrate that Mek1p is a functional kinase in vitro and that catalytic activity is an essential part of the meiotic function of Mek1 in vivo. Immunoblot analysis revealed that Red1p is a MEK1-dependent phosphoprotein. Co-immunoprecipitation experiments demonstrated that the interaction between Hop1p and Red1p is enhanced by the presence of MEK1. Thus, MEK1-dependent phosphorylation of Red1p facilitates the formation of Hop1p/Red1p hetero-oligomers, thereby enabling the formation of functional AEs.

Synaptonemal complex morphogenesis and sister-chromatid cohesion require Mek1-dependent phosphorylation of a meiotic chromosomal protein

Development of yeast meiotic chromosome cores into full-length synaptonemal complexes requires the MEK1 gene product, a meiosis-specific protein kinase homolog. The Mek1 protein associates with meiotic chromosomes and colocalizes with the Red1 protein, which is a component of meiotic chromosome cores. Mek1 and Red1 interact physically in meiotic cells, as demonstrated by coimmunoprecipitation and the two-hybrid protein system. Hop1, another protein associated with meiotic chromosome cores, also interacts with Mek1 but only in the presence of Red1. Red1 displays Mek1-dependent phosphorylation, both in vitro and in vivo, and Mek1 kinase activity is necessary for Mek1 function in vivo. Fluorescent in situ hybridization analysis indicates that Mek1-mediated phosphorylation of Red1 is required for meiotic sister-chromatid cohesion, raising the possibility that cohesion is regulated by protein phosphorylation.

Telomere-mediated chromosome pairing during meiosis in budding yeast

Certain haploid strains of Saccharomyces cerevisiae can undergo meiosis, but meiotic prophase progression and subsequent nuclear division are delayed if these haploids carry an extra chromosome (i. e., are disomic). Observations indicate that interactions between homologous chromosomes cause a delay in meiotic prophase, perhaps to allow time for interhomolog interactions to be completed. Analysis of meiotic mutants demonstrates that the relevant aspect of homolog recognition is independent of meiotic recombination and synaptonemal complex formation. A disome in which the extra chromosome is circular sporulates without a delay, indicating that telomeres are important for homolog recognition. Consistent with this hypothesis, fluorescent in situ hybridization demonstrates that a circular chromosome has a reduced capacity to pair with its homolog, and a telomere-associated meiotic protein (Ndj1) is required to delay sporulation in disomes. A circular dimer containing two copies of the same chromosome delays meiosis to the same extent as two linear homologs, implying that physical proximity bypasses the requirement for telomeres in homolog pairing. Analysis of a disome carrying two linear permuted chromosomes suggests that even nonhomologous chromosome ends can promote homolog pairing to a limited extent. We speculate that telomere-mediated chromosome movement and/or telomere clustering promote homolog pairing.

The synaptonemal complex protein SCP3 can form multistranded, cross-striated fibers in vivo

The synaptonemal complex protein SCP3 is part of the lateral element of the synaptonemal complex, a meiosis-specific protein structure essential for synapsis of homologous chromosomes. We have investigated the fiber-forming properties of SCP3 to elucidate its role in the synaptonemal complex. By synthesis of SCP3 in cultured somatic cells, it has been shown that SCP3 can self-assemble into thick fibers and that this process requires the COOH-terminal coiled coil domain of SCP3, as well as the NH2-terminal nonhelical domain. We have further analyzed the thick SCP3 fibers by transmission electron microscopy and immunoelectron microscopy. We found that the fibers display a transversal striation with a periodicity of approximately 20 nm and consist of a large number of closely associated, thin fibers, 5-10 nm in diameter. These features suggest that the SCP3 fibers are structurally related to intermediate filaments. It is known that in some species the lateral elements of the synaptonemal complex show a highly ordered striated structure resembling that of the SCP3 fibers. We propose that SCP3 fibers constitute the core of the lateral elements of the synaptonemal complex and function as a molecular framework to which other proteins attach, regulating DNA binding to the chromatid axis, sister chromatid cohesion, synapsis, and recombination.

Meiotic chromosome morphology and behavior in zip1 mutants of Saccharomyces cerevisiae

The yeast Zip1 protein (Zip1p) is a component of the central region of the synaptonemal complex (SC). Zip1p is predicted to form a dimer consisting of a coiled-coil domain flanked by globular domains. To analyze the organization of Zip1p within the SC, in-frame deletions of ZIP1 were constructed and analyzed. The results demonstrate that the C terminus but not the N terminus of Zip1p is required for its localization to chromosomes. Deletions in the carboxy half of the predicted coiled-coil region cause decreases in the width of the SC. Based on these results, a model for the organization of Zip1p within the SC is proposed. zip1 deletion mutations were also examined for their effects on sporulation, spore viability, crossing over, and crossover interference. The results demonstrate that the extent of synapsis is positively correlated with the levels of spore viability, crossing over, and crossover interference. In contrast, the role of Zip1p in synapsis is separable from its role in meiotic cell cycle progression. zip1 mutants display interval-specific effects on crossing over.

SCP2: a major protein component of the axial elements of synaptonemal complexes of the rat

In the axial elements of synaptonemal complexes (SCs) of the rat, major protein components have been identified, with relative electrophoretic mobilities (M rs) of 30 000-33 000 and 190 000. Using monoclonal anti-SC antibodies, we isolated cDNA fragments which encode the 190 000 M r component of rat SCs. The translation product predicted from the nucleotide sequence of the cDNA, called SCP2 (for synaptonemal complex protein 2), is a basic protein (pI = 8.0) with a molecular mass of 173 kDa. At the C-terminus, a stretch of approximately 50 amino acid residues is predicted to be capable of forming coiled-coil structures. SCP2 contains two clusters of S/T-P motifs, which are common in DNA-binding proteins. These clusters flank the central, most basic part of the protein (pI = 9.5). Three of the S/T-P motifs are potential target sites for p34(cdc2) protein kinase. In addition, SCP2 has eight potential cAMP/cGMP-dependent protein kinase target sites. The gene encoding SCP2 is transcribed specifically in the testis, in meiotic prophase cells. At the amino acid sequence and secondary structural level, SCP2 shows some similarity to the Red1 protein, which is involved in meiotic recombination and the assembly of axial elements of SCs in yeast. We speculate that SCP2 is a DNA-binding protein involved in the structural organization of meiotic prophase chromosomes.

Regulation of yeast glycogen metabolism and sporulation by Glc7p protein phosphatase

Glc7p is an essential serine/threonine type 1 protein phosphatase (PP1) from the yeast Saccharomyces cerevisiae, which has a role in many processes including cell cycle progression, sporulation, glycogen accumulation, translation initiation, and glucose repression. Two hallmarks of PP1 enzymes are very high amino acid sequence conservation and association of the catalytic subunit with a variety of noncatalytic, regulatory subunits. We tested the hypothesis that PP1 sequence conservation was the result of each PP1 residue playing a role in multiple intermolecular interactions. Analysis of 24 glc7 mutants, isolated primarily by their glycogen accumulation traits, revealed that every mutated Glc7p residue altered many noncatalytic subunit affinities and conferred unselected sporulation traits to various degrees. Furthermore, quantitative analysis showed that Glc7p affinity for the glycogen-binding noncatalytic subunit Gac1p was not the only parameter that determines the glycogen accumulation by a glc7 mutant. Sds22p is one Glc7p noncatalytic subunit that is essential for mitotic growth. Surprisingly, several mutant Glc7p proteins had undetectable affinity for Sds22p, yet grew apparently normally. The characterization of glc7 diploid sporulation revealed that Glc7p has at least two meiotic roles. Premeiotic DNA synthesis was undetectable in glc7 mutants with the poorest sporulation. In the glc7 diploids examined, expression of the meiotic inducer IME1 was proportional to the glc7 diploid sporulation frequency. Moreover, IME1 hyperexpression could not suppress glc7 sporulation traits. The Glc7p/Gip1p holoenzyme may participate in completion of meiotic divisions or spore packaging because meiotic dyads predominate when some glc7 diploids sporulate.

Zip2, a meiosis-specific protein required for the initiation of chromosome synapsis

We describe the identification and characterization of the Saccharomyces cerevisiae ZIP2 gene, which encodes a novel meiosis-specific protein essential for synaptonemal complex formation. In the zip2 mutant, chromosomes are homologously paired but not synapsed. The Zip2 protein localizes to discrete foci on meiotic chromosomes; these foci correspond to sites of convergence between paired homologs that are believed to be sites of synapsis initiation. Localization of Zip2p requires the initiation of meiotic recombination. In a mutant defective in double-strand break repair, Zip2p colocalizes with proteins involved in double-strand break formation and processing. We propose that Zip2p promotes the initiation of chromosome synapsis and that localization of Zip2p to sites of interhomolog recombination ensures synapsis between homologous chromosomes.

Recombination at work for meiosis

In sexually reproducing organisms, homologous recombination increases genetic diversity in gametes and ensures proper chromosome segregation. Recent publications have provided details of the molecular intermediates and proteins involved, the control of the distribution of recombination events at the chromosomal level, and the surveillance mechanisms that coordinate recombination with the meiotic cell cycle.