Zip1-induced changes in synaptonemal complex structure and polycomplex assembly

Zip3 provides a link between recombination enzymes and synaptonemal complex proteins

In budding yeast, absence of the meiosis-specific Zip3 protein (also known as Cst9) causes synaptonemal complex formation to be delayed and incomplete. The Zip3 protein colocalizes with Zip2 at discrete foci on meiotic chromosomes, corresponding to the sites where synapsis initiates. Observations suggest that Zip3 promotes synapsis by recruiting the Zip2 protein to chromosomes and/or stabilizing the association of Zip2 with chromosomes. Zip3 interacts with a number of gene products involved in meiotic recombination, including proteins that act at both early (Mre11, Rad51, and Rad57) and late (Msh4 and Msh5) steps in the exchange process. We speculate that Zip3 is a component of recombination nodules and serves to link the initiation of synapsis to meiotic recombination.

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.

Filamentous polymers induced by overexpression of a novel centrosomal protein, Cep135

A novel 135 kDa centrosomal component (Cep135) was identified by immunoscreening of a mammalian expression library with monoclonal antibodies raised against clam centrosomes. It is predicted to be a highly coiled-coil protein with an extensive alpha-helix, suggesting that Cep135 is a structural component of the centrosome. To evaluate how the protein is arranged in the centrosomal structure, we overexpressed Cep135 polypeptides in CHO cells by transient transfection. HA- or GFP-tagged full (amino acids 1-1144) as well as truncated (#10, 29-1144; Delta3, 29-812) polypeptides become localized at the centrosome and induce cytoplasmic dots of various size and number in CHO cells. Centrosomes are associated with massive approximately 7 nm filaments and dense particles organized in a whorl-like arrangement in which parallel-oriented dense lines appear with a regular approximately 7 nm periodicity. The same filamentous aggregates are also detected in cytoplasmic dots, indicating that overexpressed Cep135 can assemble into elaborate higher-ordered structures in and outside the centrosome. Sf9 cells infected with baculovirus containing Cep135 sequences induce filamentous polymers which are distinctive from the whorl seen in CHO cells; #10 forms highly packed spheroids, but the Delta3-containing structure looks loose. Both structures show an internal repeating unit of dense and less dense stripes. Although the distance between the outer end of two adjacent dense lines is similar between two types of polymers ( approximately 120 nm), the dense stripe of Delta3 polymers ( approximately 40 nm) is wider than #10 ( approximately 30 nm). The light band of Delta3 ( approximately 40 nm) is thus narrower than #10 ( approximately 60 nm). Since thin fibers are frequently seen to extend from one dense line to the next, the coiled-coil rod of Cep135 may span the light band. These results suggest that overexpressed Cep135 assemble into distinctive polymers in a domain-specific manner.

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).

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.

Functional domains of the Drosophila bicaudal-D protein

The localization of oocyte-specific determinants in the form of mRNAs to the pro-oocyte is essential for the establishment of oocyte identity. Localization of the Bicaudal-D (Bic-D) protein to the presumptive oocyte is required for the accumulation of Bic-D and other mRNAs to the pro-oocyte. The Bic-D protein contains four well-defined heptad repeat domains characteristic of intermediate filament proteins, and several of the mutations in Bic-D map to these conserved domains. We have undertaken a structure-function analysis of Bic-D by testing the function of mutant Bic-D transgenes (Bic-D(H)) deleted for each of the heptad repeat domains in a Bic-D null background. Our transgenic studies indicate that only the C-terminal heptad repeat deletion results in a protein that has lost zygotic and ovarian functions. The three other deletions result in proteins with full zygotic function, but with affected ovarian function. The functional importance of each domain is well correlated with its conservation in evolution. The analysis of females heterozygous for Bic-D(H) and the existing alleles Bic-D(PA66) or Bic-D(R26) reveals that Bic-D(R26) as well as some of Bic-D(H) transgenes have antimorphic effects. The yeast two-hybrid interaction assay shows that Bic-D forms homodimers. Furthermore, we found that Bic-D exists as a multimeric protein complex consisting of Egl and at least two Bic-D monomers.

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.

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.

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.

Chromosome size-dependent control of meiotic reciprocal recombination in Saccharomyces cerevisiae: the role of crossover interference

In the yeast Saccharomyces cerevisiae, small chromosomes undergo meiotic reciprocal recombination (crossing over) at rates (centimorgans per kilobases) greater than those of large chromosomes, and recombination rates respond directly to changes in the total size of a chromosomal DNA molecule. This phenomenon, termed chromosome size-dependent control of meiotic reciprocal recombination, has been suggested to be important for ensuring that homologous chromosomes cross over during meiosis. The mechanism of this regulation was investigated by analyzing recombination in identical genetic intervals present on different size chromosomes. The results indicate that chromosome size-dependent control is due to different amounts of crossover interference. Large chromosomes have high levels of interference while small chromosomes have much lower levels of interference. A model for how crossover interference directly responds to chromosome size is presented. In addition, chromosome size-dependent control was shown to lower the frequency of homologous chromosomes that failed to undergo crossovers, suggesting that this control is an integral part of the mechanism for ensuring meiotic crossing over between homologous chromosomes.

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.

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.

Meiotic behaviours of chromosomes and microtubules in budding yeast: relocalization of centromeres and telomeres during meiotic prophase

BACKGROUND

Meiosis is a process of universal importance in eukaryotic organisms, generating variation in the heritable haploid genome by recombination and re-assortment of chromosomes. The intranuclear movement of chromosomes is expected to achieve pairing and recombination of homologous chromosomes during meiosis. Meiosis in the budding yeast Saccharomyces cerevisiae has been extensively studied, both genetically and by molecular biology; here we report cytological observations of meiotic chromosomal events in this organism.

RESULTS

Using fluorescence microscopy, we have examined the behaviour of chromosomes and microtubules during meiosis in S. cerevisiae. We first observed the dynamic behaviour of nuclei in living cells using jellyfish green fluorescent protein (GFP) fused with nucleoplasmin, a Xenopus oocyte nuclear protein. The characterization of nuclear movement in living cells was extended by an analysis of chromosomes and microtubules in fixed specimens. In addition, the nuclear localization of centromeres and telomeres was determined by indirect immunofluorescence microscopy in synchronous populations of meiotic cells. While telomeres remain in clusters of 5-8 throughout meiosis, centromeres change their nuclear localization dramatically during the progression of meiosis: centromeres are first clustered at a single site near the spindle-pole body before the induction of meiosis, and become scattered during the meiotic prophase.

CONCLUSIONS

Our observations have demonstrated that nuclear and cytoskeletal reorganization take place with meiosis in S. cerevisiae. In particular, the distinct relocalization of centromeres during meiosis indicates a considerable movement of chromosomes within the meiotic prophase nucleus.

The meiosis-specific Hop2 protein of S. cerevisiae ensures synapsis between homologous chromosomes

The hop2 mutant of S. cerevisiae displays a novel phenotype: meiotic chromosomes form nearly wild-type amounts of synaptonemal complex, but most chromosomes are engaged in synapsis with nonhomologous partners. The meiosis-specific Hop2 protein localizes to chromosomes prior to and during synapsis and in the absence of the double-strand breaks that initiate recombination. hop2 strains sustain a wild-type level of meiotic double-strand breaks, but these breaks remain unrepaired. The hop2 mutant arrests at the pachytene stage of meiotic prophase with the RecA-like protein Dmc1 located at numerous sites along synapsed chromosomes. We propose that the Hop2 protein functions to prevent synapsis between nonhomologous chromosomes.

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.

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.

The yeast motor protein, Kar3p, is essential for meiosis I

The recognition and alignment of homologous chromosomes early in meiosis is essential for their subsequent segregation at anaphase I; however, the mechanism by which this occurs is unknown. We demonstrate here that, in the absence of the molecular motor, Kar3p, meiotic cells are blocked with prophase monopolar microtubule arrays and incomplete synaptonemal complex (SC) formation. kar3 mutants exhibit very low levels of heteroallelic recombination. kar3 mutants do produce double-strand breaks that act as initiation sites for meiotic recombination in yeast, but at levels severalfold reduced from wild-type. These data are consistent with a meiotic role for Kar3p in the events that culminate in synapsis of homologues.

Genetic interactions between HOP1, RED1 and MEK1 suggest that MEK1 regulates assembly of axial element components during meiosis in the yeast Saccharomyces cerevisiae

During meiosis, axial elements are generated by the condensation of sister chromatids along a protein core as precursors to the formation of the synaptonemal complex (SC). Functional axial elements are essential for wild-type levels of recombination and proper reductional segregation at meiosis I. Genetic and cytological data suggest that three meiosis-specific genes, HOP1, RED1 and MEK1, are involved in axial element formation in the yeast Saccharomyces cerevisiae. HOP1 and RED1 encode structural components of axial elements while MEK1 encodes a putative protein kinase. Using a partially functional allele of MEK1, new genetic interactions have been found between HOP1, RED1 and MEK1. Overexpression of HOP1 partially suppresses the spore inviability and recombination defects of mek1-974; in contrast, overexpression of RED1 exacerbates the mek1-974 spore inviability. Co-overexpression of HOP1 and RED1 in mek1-974 diploids alleviates the negative effect of overexpressing RED1 alone. Red1p/Red1p as well as Hop1p/Red1p interactions have been reconstituted in two hybrid experiments. Our results suggest a model whereby Mek1 kinase activity controls axial element assembly by regulating the affinity with which Hop1p and Red1p interact with each other.

Tam1, a telomere-associated meiotic protein, functions in chromosome synapsis and crossover interference

The TAM1 gene of Saccharomyces cerevisiae is expressed specifically during meiosis and encodes a protein that localizes to the ends of meiotic chromosomes. In a tam1 null mutant, there is an increase in the frequency of chromosomes that fail to recombine and an associated increase in homolog nondisjunction at meiosis I. The tam1 mutant also displays an increased frequency of precocious separation of sister chromatids and a reduced efficiency of distributive disjunction. The defect in distributive disjunction may be attributable to overloading of the distributive system by the increased number of nonrecombinant chromosomes. Recombination is not impaired in the tam1 mutant, but crossover interference is reduced substantially. In addition, chromosome synapsis is delayed in tam1 strains. The combination of a defect in synapsis and a reduction in interference is consistent with previous studies suggesting a role for the synaptonemal complex in regulating crossover distribution. tam1 is the only known yeast mutant in which the control of crossover distribution is impaired, but the frequency of crossing over is unaffected. We discuss here possibilities for how a telomere-associated protein might function in chromosome synapsis and crossover interference.

The transcriptional basis of chromosome pairing

Pairing between homologous chromosomes is essential for successful meiosis; generally only paired homologs recombine and segregate correctly into haploid germ cells. Homologs also pair in some somatic cells (e.g. in diploid and polytene cells of Drosophila). How homologs find their partners is a mystery. First, I review some explanations of how they might do so; most involve base-pairing (i.e. DNA-DNA) interactions. Then I discuss the remarkable fact that chromosomes only pair when they are transcriptionally active. Finally, I present a general model for pairing based upon the DNA-protein interactions involved in transcription. Each chromosome in the haploid set has a unique array of transcription units strung along its length. Therefore, each chromatin fibre will be folded into a unique array of loops associated with clusters of polymerases and transcription factors; only homologs share similar arrays. As these loops and clusters, or transcription factories, move continually, they make and break contact with others. Correct pairing would be nucleated when a promoter in a loop tethered to one factory binds to a homologous polymerizing site in another factory, before transcription stabilizes the association. This increases the chances that adjacent promoters will bind to their homologs, so that chromosomes eventually become zipped together with their partners. Pairing is then the inevitable consequence of transcription of partially-condensed chromosomes.

The yeast Red1 protein localizes to the cores of meiotic chromosomes

Mutants in the meiosis-specific RED1 gene of S. cerevisiae fail to make any synaptonemal complex (SC) or any obvious precursors to the SC. Using antibodies that specifically recognize the Red1 protein, Red1 has been localized along meiotic pachytene chromosomes. Red1 also localizes to the unsynapsed axial elements present in a zip1 mutant, suggesting that Red1 is a component of the lateral elements of mature SCs. Anti-Red1 staining is confined to the cores of meiotic chromosomes and is not associated with the loops of chromatin that lie outside the SC. Analysis of the spo11 mutant demonstrates that Red1 localization does not depend upon meiotic recombination. The localization of Red1 has been compared with two other meiosis-specific components of chromosomes, Hop1 and Zip1; Zip1 serves as a marker for synapsed chromosomes. Double labeling of wild-type meiotic chromosomes with anti-Zip1 and anti-Red1 antibodies demonstrates that Red1 localizes to chromosomes both before and during pachytene. Double labeling with anti-Hop1 and anti-Red1 antibodies reveals that Hop1 protein localizes only in areas that also contain Red1, and studies of Hop1 localization in a red1 null mutant demonstrate that Hop1 localization depends on Red1 function. These observations are consistent with previous genetic studies suggesting that Red1 and Hop1 directly interact. There is little or no Hop1 protein on pachytene chromosomes or in synapsed chromosomal regions.

Meiotic nuclear reorganization: switching the position of centromeres and telomeres in the fission yeast Schizosaccharomyces pombe

In fission yeast meiotic prophase, telomeres are clustered near the spindle pole body (SPB; a centrosome-equivalent structure in fungi) and take the leading position in chromosome movement, while centromeres are separated from the SPB. This telomere position contrasts with mitotic nuclear organization, in which centromeres remain clustered near the SPB and lead chromosome movement. Thus, nuclear reorganization switching the position of centromeres and telomeres must take place upon entering meiosis. In this report, we analyze the nuclear location of centromeres and telomeres in genetically well-characterized meiotic mutant strains. An intermediate structure for telomere-centromere switching was observed in haploid cells induced to undergo meiosis by synthetic mating pheromone; fluorescence in situ hybridization revealed that in these cells, both telomeres and centromeres were clustered near the SPB. Further analyses in a series of mutants showed that telomere-centromere switching takes place in two steps; first, association of telomeres with the SPB and, second, dissociation of centromeres from the SPB. The first step can take place in the haploid state in response to mating pheromone, but the second step does not take place in haploid cells and probably depends on conjugation-related events. In addition, a linear minichromosome was also co-localized with authentic telomeres instead of centromeres, suggesting that telomere clustering plays a role in organizing chromosomes within a meiotic prophase nucleus.

Spc110p: assembly properties and role in the connection of nuclear microtubules to the yeast spindle pole body

Spc110p is an essential component of the budding yeast spindle pole body (SPB). It binds calmodulin and contains a long central coiled-coil rod which acts as a spacer element between the central plaque of the SPB and the ends of the nuclear or spindle microtubules. This suggests that the essential function of Spc110p is to connect the nuclear microtubules to the SPB. To confirm this, we examined the phenotype of ts alleles of SPC110, one of which contains a mutation in the calmodulin binding site and was suppressed by overexpression of calmodulin. The alleles fail to form a functional mitotic spindle because spindle microtubules are not properly connected to the SPB. We also examined the phenotype of the toxic overexpression of either the wild-type or a truncated version of Spc110p containing a deletion of most of the coiled-coil domain. Both of these proteins form large ordered spheroidal polymers in the nucleus. The polymerization of the truncated Spc110p appears to be initiated inside the SPB from the position where Spc110p is normally located, and as the polymer grows in size it severs the connection between the nuclear microtubules and the SPB. The polymers were purified and are composed of Spc110p and calmodulin. A model for the structure of the polymer is proposed.

Analysis of meiotic recombination pathways in the yeast Saccharomyces cerevisiae

In the yeast, Saccharomyces cerevisiae, several genes appear to act early in meiotic recombination. HOP1 and RED1 have been classified as such early genes. The data in this paper demonstrate that neither a red nor a hop1 mutation can rescue the inviable spores produced by a rad52 spo13 strain; this phenotype helps to distinguish these two genes from other early meiotic recombination genes such as SPO11, REC104, or MEI4. In contrast, either a red1 or a hop1 mutation can rescue a rad50S spo13 strain; this phenotype is similar to that conferred by mutations in the other early recombination genes (e.g., REC104). These two different results can be explained because the data presented here indicate that a rad50S mutation does not diminish meiotic intrachromosomal recombination, similar to the mutant phenotypes conferred by red1 or hop1. Of course, RED1 and HOP1 do act in the normal meiotic interchromosomal recombination pathway; they reduce interchromosomal recombination to approximately 10% of normal levels. We demonstrate that a mutation in a gene (REC104) required for initiation of exchange is completely epistatic to a mutation in RED1. Finally, mutations in either HOP1 or RED1 reduce the number of double-strand breaks observed at the HIS2 meiotic recombination hotspot.

Localized chiasmata and meiotic nodules in the tetraploid onionAllium porrum

Allium porrum L. (cultivated leek) (2n = 4x = 32) is a fertile tetraploid that forms bivalents with pericentric chiasmata at metaphase I. To investigate the basis of this unusual behavior for a tetraploid, we describe the karyotype, axial cores, synaptonemal complexes (SCs), and meiotic nodules of A. porrum. The karyotype appears to be autotetraploid. This conclusion is also supported by presynaptic alignment of axial cores in groups of four and partner trades between pairs of SCs. Numerous early nodules are distributed all along axial cores and SCs during zygonema, but they are lost by late zygonema – early pachynema. Late (recombination) nodules (RNs) are present on SCs near kinetochores throughout the remainder of pachynema. This pattern of RNs corresponds to the pattern of pericentric chiasmata. Pachytene quadrivalents usually are resolved into bivalents because partner trades between SC lateral elements rarely occur between RNs on the same segment of SC. Thus, the patterns of crossing-over and partner trades promote balanced disjunction and high fertility in autotetraploid A. porrum. Rare quadrivalents observed at metaphase I must be due to infrequent partner trades between RNs. Polycomplexes, unusual in their number and size, were observed during zygonema. Key words : synaptonemal complex, recombination nodules, localized chiasmata, polycomplex, Allium porrum.

Synaptonemal complexes: structure and function

Synaptonemal complexes (SCs) are zipper-like structures which are assembled between homologous chromosomes during the prophase of the first meiotic division. Their assembly and disassembly correlate with the successive chromatin rearrangements of meiotic prophase, namely the condensation, pairing, recombination and disjunction of homologous chromosomes. It was originally thought that SCs created the preconditions for the homologous crossing over of chromosomes by bringing corresponding parts of homologous chromosomes in close apposition. However, this view has been gradually undermined during recent years, and ideas about the roles of SCs have radically changed. SCs are now considered to be structures that both control the number and distribution of reciprocal exchanges between homologous chromosomes (cross-overs) and convert cross-overs into functional chiasmata. How SCs fulfil these roles remains to be elucidated.

The perinuclear microtubule-organizing center and the synaptonemal complex of higher plants share a common antigen: its putative transfer and role in meiotic chromosomal ordering

Recognition of homologous chromosomes during meiotic prophase is associated in most cases with the formation of the synaptonemal complex along the length of the chromosome. Telomeres, located at the nuclear periphery, are preferential initiation sites for the assembly of the synaptonemal complex. In most eukaryotic cells, telomeres cluster in a restricted area, leading to the "bouquet" configuration in leptotene-zygotene, while this typical organization progressively disappears in late zygotene-pachytene. We wondered whether such striking changes in the intranuclear ordering and pairing of meiotic chromosomes during the progression of prophase I could be correlated with activity of the centrosome and/or microtubule-organizing center (MTOC). Plant cells may be used as a model of special interest for this study as the whole nuclear surface acts as an MTOC, unlike other cell types where MTOCs are restricted to centrosomes or spindle pole bodies. Using a monoclonal antibody (mAb 6C6) raised against isolated calf centrosomes we found that the 6C6 antigen is present over the entire surface of the plant meiotic nucleus, in early prophase I, before chromosomal pairing. At zygotene, short fragments of chromosomes become stained near the nuclear envelope and within the nucleus. At pachytene, after complete synapsis, the labeling specifically concentrates within the synaptonemal complexes, although the nuclear surface is no longer reactive. Ultrastructural localization using immunogold labeling indicates that the 6C6 antigen is colocalized with the synaptonemal complex structures. Later in metaphase I, the antigen is found at the kinetochores. Our data favor the idea that the 6C6 antigen may function as a particular "chromosomal passenger-like" protein. These observations shed new light on the molecular organization of the plant synaptonemal complex and on the redistribution of cytoskeleton-related antigens during initiation of meiosis. They suggest that antigens of MTOCs are relocated to chromosomes during the synapsis process starting at telomeres and contribute to the spatial arrangement of meiotic chromosomes. Such cytoskeleton-related antigens may acquire different functions depending on their localization, which is cell-cycle regulated.

Sex and the single cell: meiosis in yeast

Recent studies of Saccharomyces cerevisiae have significantly advanced our understanding of the molecular mechanisms of meiotic chromosome behavior. Structural components of the synaptonemal complex have been identified and studies of mutants defective in synapsis have provided insight into the role of the synaptonemal complex in homolog pairing, genetic recombination, crossover interference, and meiotic chromosome segregation. There is compelling evidence that most or all meiotic recombination events initiate with double-strand breaks. Several intermediates in the double-strand break repair pathway have been characterized and mutants blocked at different steps in the pathway have been identified. With the application of genetic, molecular, cytological, and biochemical methods in a single organism, we can expect an increasingly comprehensive and unified view of the meiotic process.

Roles for two RecA homologs in promoting meiotic chromosome synapsis

Previous studies have shown that the RAD51 and DMC1 genes of Saccharomyces cerevisiae encode homologs of the Escherichia coli RecA strand exchange enzyme. Results presented here demonstrate that the dmc1 and rad51 mutants undergo nearly complete chromosome synapsis, but synaptonemal complex formation is delayed substantially compared with wild type. In the zip1 mutant, chromosomes are paired homologously, but not synapsed, and the protein backbones (axial elements) of each pair of chromosomes are connected intimately to each other at a few sites referred to herein as axial associations. dmc1 zip1 and rad51 zip1 double mutants assemble axial elements that are not obviously associated, demonstrating that the Dmc1 and Rad51 proteins are required to establish or stabilize axial associations. We propose that axial associations serve to promote meiotic chromosome synapsis and that the absence of these associations accounts for the delayed and inefficient synapsis observed in dmc1 and rad51 strains. During meiosis in haploid yeast, chromosome synapsis takes place between nonhomologous chromosome segments. In a zip1 haploid, axial associations are not apparent, suggesting that these associations depend on interactions between homologous sequences.

Heteroduplex DNA formation and homolog pairing in yeast meiotic mutants

Previous studies of Saccharomyces cerevisiae have identified several meiosis-specific genes whose products are required for wild-type levels of meiotic recombination and for normal synaptonemal complex (SC) formation. Several of these mutants were examined in a physical assay designed to detect heteroduplex DNA (hDNA) intermediates in meiotic recombination. hDNA was not detected in the rec102, mei4 and hop1 mutants; it was observed at reduced levels in red1, mek1 and mer1 strains and at greater than the wild-type level in zip1. These results indicate that the REC102, MEI4, HOP1, RED1, MEK1 and MER1 gene products act before hDNA formation in the meiotic recombination pathway, whereas ZIP1 acts later. The same mutants assayed for hDNA formation were monitored for meiotic chromosome pairing by in situ hybridization of chromosome-specific DNA probes to spread meiotic nuclei. Homolog pairing occurs at wild-type levels in the zip1 and mek1 mutants, but is substantially reduced in mei4, rec102, hop1, red1 and mer1 strains. Even mutants that fail to recombine or to make any SC or SC precursors undergo a significant amount of meiotic chromosome pairing. The in situ hybridization procedure revealed defects in meiotic chromatin condensation in mer1, red1 and hop1 strains.

Bdf1, a yeast chromosomal protein required for sporulation

The BDF1 gene of Saccharomyces cerevisiae is required for sporulation. Under starvation conditions, most cells from the bdf1 null mutant fail to undergo one or both meiotic divisions, and there is an absolute defect in spore formation. The Bdf1 protein localizes to the nucleus throughout all stages of the mitotic and meiotic cell cycles. Analysis of spread meiotic nuclei reveals that the Bdf1 protein is localized fairly uniformly along chromosomes, except that it is excluded specifically from the nucleolus. A bdf1 null mutant displays a reduced rate of vegetative growth and sensitivity to a DNA-damaging agent. The BDF1 gene encodes a 77-kDa protein that contains two bromodomains, sequence motifs of unknown function. Separation-of-function alleles suggest that only one of the two bromodomains is required for sporulation, whereas both are required for Bdf1 function in vegetative cells. We propose that the Bdf1 protein is a component of chromatin and that the mitotic and meiotic defects of the bdf1 null mutant result from alterations in chromatin structure.