Characterization of Schizosaccharomyces pombe minichromosome deletion derivatives and a functional allocation of their centromere

Fission yeast Cid12 has dual functions in chromosome segregation and checkpoint control

Fission yeast Cid12 is a member of the Cid1 family of specialized poly(A) polymerases. Like cells lacking cid1, cid12Delta mutants were shown to have checkpoint defects when DNA replication was inhibited. Here, we show that Cid12 is also required for faithful chromosome segregation and that mutation of amino acid residues predicted to be essential for poly(A) polymerase activity resulted in loss of Cid12 function in vivo. Cells lacking Cid12 had an increased chromosome segregation failure rate due to precocious loss of sister chromatid cohesion at the centromere but not along the chromosome arms. In keeping with a recently described function for Cid12 in RNA interference (RNAi)-mediated heterochromatin assembly, this was accompanied by an accumulation of polyadenylated transcripts corresponding to naturally silenced repeat elements within heterochromatic domains, with consequent defects in centromeric gene silencing. These cells also suffered increased meiotic defects, and their viability was dependent on the spindle checkpoint protein Bub1. To account for the effects of Cid12 on various aspects of DNA metabolism, including chromosome segregation and the checkpoint control, we suggest that Cid12 has dual functions in RNAi silencing and regulating mRNA stability.

Requirement of fission yeast Cid14 in polyadenylation of rRNAs

Polyadenylation in eukaryotes is conventionally associated with increased nuclear export, translation, and stability of mRNAs. In contrast, recent studies suggest that the Trf4 and Trf5 proteins, members of a widespread family of noncanonical poly(A) polymerases, share an essential function in Saccharomyces cerevisiae that involves polyadenylation of nuclear RNAs as part of a pathway of exosome-mediated RNA turnover. Substrates for this pathway include aberrantly modified tRNAs and precursors of snoRNAs and rRNAs. Here we show that Cid14 is a Trf4/5 functional homolog in the distantly related fission yeast Schizosaccharomyces pombe. Unlike trf4 trf5 double mutants, cells lacking Cid14 are viable, though they suffer an increased frequency of chromosome missegregation. The Cid14 protein is constitutively nucleolar and is required for normal nucleolar structure. A minor population of polyadenylated rRNAs was identified. These RNAs accumulated in an exosome mutant, and their presence was largely dependent on Cid14, in line with a role for Cid14 in rRNA degradation. Surprisingly, both fully processed 25S rRNA and rRNA processing intermediates appear to be channeled into this pathway. Our data suggest that additional substrates may include the mRNAs of genes involved in meiotic regulation. Polyadenylation-assisted nuclear RNA turnover is therefore likely to be a common eukaryotic mechanism affecting diverse biological processes.

The V260I mutation in fission yeast alpha-tubulin Atb2 affects microtubule dynamics and EB1-Mal3 localization and activates the Bub1 branch of the spindle checkpoint

We have identified a novel temperature-sensitive mutant of fission yeast alpha-tubulin Atb2 (atb2-983) that contains a single amino acid substitution (V260I). Atb2-983 is incorporated into the microtubules, and their overall structures are not altered noticeably, but microtubule dynamics is compromised during interphase. atb2-983 displays a high rate of chromosome missegregation and is synthetically lethal with deletions in a subset of spindle checkpoint genes including bub1, bub3, and mph1, but not with mad1, mad2, and mad3. During early mitosis in this mutant, Bub1, but not Mad2, remains for a prolonged period in the kinetochores that are situated in proximity to one of the two SPBs (spindle pole bodies). High dosage mal3(+), encoding EB1 homologue, rescues atb2-983, suggesting that Mal3 function is compromised. Consistently, Mal3 localization and binding between Mal3 and Atb2-983 are impaired significantly, and a mal3 single mutant, such as atb2-983, displays prolonged Bub1 kinetochore localization. Furthermore in atb2-983 back-and-forth centromere oscillation during prometaphase is abolished. Intriguingly, this oscillation still occurs in the mal3 mutant, indicating that there is another defect independent of Mal3. These results show that microtubule dynamics is important for coordinated execution of mitotic events, in which Mal3 plays a vital role.

Hip3 Interacts with the HIRA Proteins Hip1 and Slm9 and Is Required for Transcriptional Silencing and Accurate Chromosome Segregation

The fission yeast HIRA proteins Hip1 and Slm9 are members of an evolutionarily conserved family of histone chaperones that are implicated in nucleosome assembly. Here we have used single-step affinity purification and mass spectrometry to identify factors that interact with both Hip1 and Slm9. This analysis identified Hip3, a previously uncharacterized 187-kDa protein, with similarity to S. cerevisiae Hir3. Consistent with this, cells disrupted for hip3+ exhibit a range of growth defects that are similar to those associated with loss of Hip1 and Slm9. These include temperature sensitivity, a cell cycle delay, and synthetic lethality with cdc25-22. Furthermore, genetic analysis also indicates that disruption of hip3+ is epistatic with mutation of hip1+ and slm9+. Mutation of hip3+ alleviates transcriptional silencing at several heterochromatic loci, including in the outer (otr) centromeric repeats, indicating that Hip3 is required for the integrity of pericentric heterochromatin. As a result, loss of Hip3 function leads to high levels of minichromosome loss and an increased frequency of lagging chromosomes during mitosis. Importantly, the function of Hip1, Slm9, and Hip3 is not restricted to constitutive heterochromatic loci, since these proteins also repress the expression of a number of genes, including the Tf2 retrotransposons.

Schizosaccharomyces pombe mst2+ encodes a MYST family histone acetyltransferase that negatively regulates telomere silencing

Histone acetylation and deacetylation are associated with transcriptional activity and the formation of constitutively silent heterochromatin. Increasingly, histone acetylation is also implicated in other chromosome transactions, including replication and segregation. We have cloned the only Schizosaccharomyces pombe MYST family histone acetyltransferase genes, mst1(+) and mst2(+). Mst1p, but not Mst2p, is essential for viability. Both proteins are localized to the nucleus and bound to chromatin throughout the cell cycle. Deltamst2 genetically interacts with mutants that affect heterochromatin, cohesion, and telomere structure. Mst2p is a negative regulator of silencing at the telomere but does not affect silencing in the centromere or mating type region. We generated a census of proteins and histone modifications at wild-type telomeres. A histone acetylation gradient at the telomeres is lost in Deltamst2 cells without affecting the distribution of Taz1p, Swi6p, Rad21p, or Sir2p. We propose that the increased telomeric silencing is caused by histone hypoacetylation and/or an increase in the ratio of methylated to acetylated histones. Although telomere length is normal, meiosis is aberrant in Deltamst2 diploid homozygote mutants, suggesting that telomeric histone acetylation contributes to normal meiotic progression.

The fission yeast Crb2/Chk1 pathway coordinates the DNA damage and spindle checkpoint in response to replication stress induced by topoisomerase I inhibitor

Living organisms experience constant threats that challenge their genome stability. The DNA damage checkpoint pathway coordinates cell cycle progression with DNA repair when DNA is damaged, thus ensuring faithful transmission of the genome. The spindle assembly checkpoint inhibits chromosome segregation until all chromosomes are properly attached to the spindle, ensuring accurate partition of the genetic material. Both the DNA damage and spindle checkpoint pathways participate in genome integrity. However, no clear connection between these two pathways has been described. Here, we analyze mutants in the BRCT domains of fission yeast Crb2, which mediates Chk1 activation, and provide evidence for a novel function of the Chk1 pathway. When the Crb2 mutants experience damaged replication forks upon inhibition of the religation activity of topoisomerase I, the Chk1 DNA damage pathway induces sustained activation of the spindle checkpoint, which in turn delays metaphase-to-anaphase transition in a Mad2-dependent fashion. This new pathway enhances cell survival and genome stability when cells undergo replicative stress in the absence of a proficient G(2)/M DNA damage checkpoint.

Comparison of Dam tagging and chromatin immunoprecipitation as tools for the identification of the binding sites for S. pombe CENP-C

We have established the identity of the Schizosaccharomyces pombe homologue of vertebrate CENP-C and Saccharomyces cerevisiae MIF2p and have used it to compare Dam tagging and chromatin immunoprecipitation (ChiP)as tools for the mapping of protein binding sites on DNA. ChiP shows that S. pombe CENP-C binds to the central core and inner repeats of the S. pombe centromere. It binds weakly, however, to the outer repeats. The binding pattern is thus similar to that of S. pombe CENP-A. Dam-tagged S. pombe CENP-C, however, methylates the entire centromere and 5 kb of flanking DNA. This comparison suggests that Dam tagging is less precise as a tool for mapping DNA binding sites than ChiP. We have also used the Dam tagging technique to address the question of whether there is any CENP-C binding to the ribosomal DNA in S. pombe and find none.

The DASH complex and Klp5/Klp6 kinesin coordinate bipolar chromosome attachment in fission yeast

We identified a truncated allele of dam1 as a multicopy suppressor of the sensitivity of cdc13-117 (cyclin B) and mal3-1 (EB-1) cells to thiabendazole, a microtubule poison. We find that Dam1 binds to the plus end of spindle microtubules and kinetochores as cells enter mitosis and this is dependent on other components of the fission yeast DASH complex, including Ask1, Duo1, Spc34 and Dad1. By contrast, Dad1 remains bound to kinetochores throughout the cell cycle and its association is dependent on the Mis6 and Mal2, but not Mis12, Nuf2 or Cnp1, kinetochore proteins. In cells lacking Dam1, or other components of the DASH complex, anaphase is delayed due to activation of the spindle assembly checkpoint and lagging sister chromatids are frequently observed and occasionally sister chromatid pairs segregate to the same spindle pole. We find that the mitotic centromere-associated Klp5/Klp6 kinesin complex is essential in cells lacking components of the DASH complex. Cells lacking both Dam1 and Klp5 undergo a first cell cycle arrest in mitosis due to a failure to establish bipolar chromosome attachment.

Two distinct pathways responsible for the loading of CENP-A to centromeres in the fission yeast cell cycle

CENP-A is a centromere-specific histone H3 variant that is- essential for faithful chromosome segregation in all eukaryotes thus far investigated. We genetically identified two factors, Ams2 and Mis6, each of which is required for the correct centromere localization of SpCENP-A (Cnp1), the fission yeast homologue of CENP-A. Ams2 is a cell-cycle-regulated GATA factor that localizes on the nuclear chromatin, including on centromeres, during the S phase. Ams2 may be responsible for the replication-coupled loading of SpCENP-A by facilitating nucleosomal formation during the S phase. Consistently, overproduction of histone H4, but not that of H3, suppressed the defect of SpCENP-A localization in Ams2-deficient cells. We demonstrated the existence of at least two distinct phases for SpCENP-A loading during the cell cycle: the S phase and the late-G2 phase. Ectopically induced SpCENP-A was efficiently loaded onto the centromeres in G2-arrested cells, indicating that SpCENP-A probably undergoes replication-uncoupled loading after the completion of S phase. This G2 loading pathway of SpCENP-A may require Mis6, a constitutive centromere-binding protein that is also implicated in the Mad2-dependent spindle attachment checkpoint response. Here, we discuss the functional relationship between the flexible loading mechanism of CENP-A and the plasticity of centromere chromatin formation in fission yeast.

Basic mechanism of eukaryotic chromosome segregation

We now have firm evidence that the basic mechanism of chromosome segregation is similar among diverse eukaryotes as the same genes are employed. Even in prokaryotes, the very basic feature of chromosome segregation has similarities to that of eukaryotes. Many aspects of chromosome segregation are closely related to a cell cycle control that includes stage-specific protein modification and proteolysis. Destruction of mitotic cyclin and securin leads to mitotic exit and separase activation, respectively. Key players in chromosome segregation are SMC-containing cohesin and condensin, DNA topoisomerase II, APC/C ubiquitin ligase, securin-separase complex, aurora passengers, and kinetochore microtubule destabilizers or regulators. In addition, the formation of mitotic kinetochore and spindle apparatus is absolutely essential. The roles of principal players in basic chromosome segregation are discussed: most players have interphase as well as mitotic functions. A view on how the centromere/kinetochore is formed is described.

The role of heterochromatin in centromere function

Chromatin at centromeres is distinct from the chromatin in which the remainder of the genome is assembled. Two features consistently distinguish centromeres: the presence of the histone H3 variant CENP-A and, in most organisms, the presence of heterochromatin. In fission yeast, domains of silent "heterochromatin" flank the CENP-A chromatin domain that forms a platform upon which the kinetochore is assembled. Thus, fission yeast centromeres resemble their metazoan counterparts where the kinetochore is embedded in centromeric heterochromatin. The centromeric outer repeat chromatin is underacetylated on histones H3 and H4, and methylated on lysine 9 of histone H3, which provides a binding site for the chromodomain protein Swi6 (orthologue of Heterochromatin Protein 1, HP1). The remarkable demonstration that the assembly of repressive heterochromatin is dependent on the RNA interference machinery provokes many questions about the mechanisms of this process that may be tractable in fission yeast. Heterochromatin ensures that a high density of cohesin is recruited to centromeric regions, but it could have additional roles in centromere architecture and the prevention of merotely, and it might also act as a trigger for kinetochore assembly. In addition, we discuss an epigenetic model for ensuring that CENP-A is targeted and replenished at the kinetochore domain.

The roles of fission yeast ase1 in mitotic cell division, meiotic nuclear oscillation, and cytokinesis checkpoint signaling

The Ase1/Prc1 proteins constitute a conserved microtubule-associated protein family that is implicated in central spindle formation and cytokinesis. Here we characterize a role for fission yeast Ase1. Ase1 localizes to microtubule overlapping zones and displays dynamic alterations of localization during the cell cycle. In particular, its spindle localization during metaphase is reduced substantially, followed by robust appearance at the spindle midzone in anaphase. ase1 deletions are viable but defective in nuclear and septum positioning and completion of cytokinesis, which leads to diploidization and chromosome loss. Time-lapse imaging shows that elongating spindles collapse abruptly in the middle of anaphase B. Either absence or overproduction of Ase1 results in profound defects on microtubule bundling in an opposed manner, indicating that Ase1 is a dose-dependent microtubule-bundling factor. In contrast microtubule nucleating activities are not noticeably compromised in ase1 mutants. During meiosis astral microtubules are not bundled and oscillatory nuclear movement is impaired significantly. The Aurora kinase does not correctly localize to central spindles in the absence of Ase1. Finally Ase1 acts as a regulatory component in the cytokinesis checkpoint that operates to inhibit nuclear division when the cytokinesis apparatus is perturbed. Ase1, therefore, couples anaphase completion with cytokinesis upon cell division.

BAF53/Arp4 homolog Alp5 in fission yeast is required for histone H4 acetylation, kinetochore-spindle attachment, and gene silencing at centromere

Nuclear actin-related proteins play vital roles in transcriptional regulation; however, their biological roles remain elusive. Here, we characterize Alp5, fission yeast homolog of Arp4/BAF53. The temperature-sensitive mutant alp5-1134 contains a single amino acid substitution in the conserved C-terminal domain (S402N) and displays mitotic phenotypes, including chromosome condensation and missegregation. Alp5 forms a complex with Mst1-HAT (histone acetyltransferase). Consistently, inhibition of histone deacetylases (HDACs), by either addition of a specific inhibitor or a mutation in HDAC-encoding clr6+ gene, rescues alp5-1134. Immunoblotting with specific antibodies against acetylated histones shows that Alp5 is required for histone H4 acetylation at lysines 5, 8, and 12, but not histone H3 lysines 9 or 14, and furthermore Clr6 plays an opposing role. Mitotic arrest is ascribable to activation of the Mad2/Bub1 spindle checkpoint, in which both proteins localize to the mitotic kinetochores in alp5-1134. Intriguingly, alp5-1134 displays transcriptional desilencing at the core centromere without altering the overall chromatin structure, which also is suppressed by a simultaneous mutation in clr6+. This result shows that Alp5 is essential for histone H4 acetylation, and its crucial role lies in the establishment of bipolar attachment of the kinetochore to the spindle and transcriptional silencing at the centromere.

Role of the fission yeast SUMO E3 ligase Pli1p in centromere and telomere maintenance

Sumoylation represents a conserved mechanism of post-translational protein modification. We report that Pli1p, the unique fission yeast member of the SP-RING family, is a SUMO E3 ligase in vivo and in vitro. pli1Delta cells display no obvious mitotic growth defects, but are sensitive to the microtubule-destabilizing drug TBZ and exhibit enhanced minichromosome loss. The weakened centromeric function of pli1Delta cells may be related to the defective heterochromatin structure at the central core, as shown by the reduced silencing of an ura4 variegation reporter gene inserted at cnt and imr. Interestingly, pli1Delta cells also exhibit enhanced loss of the ura4 reporter at these loci, likely by gene conversion using homologous sequences as information donors. Moreover, pli1Delta cells exhibit consistent telomere length increase, possibly achieved by a similar process. Point mutations within the RING finger of Pli1p totally or partially reproduce the pli1 deletion phenotypes, thus correlating with their sumoylation activity. Altogether, these results strongly suggest that Pli1p, and by extension sumoylation, is involved in mechanisms that regulate recombination in particular heterochromatic repeated sequences.

Microtubule nucleation at non-spindle pole body microtubule-organizing centers requires fission yeast centrosomin-related protein mod20p

BACKGROUND

Many types of differentiated eukaryotic cells display microtubule distributions consistent with nucleation from noncentrosomal intracellular microtubule organizing centers (MTOCs), although such structures remain poorly characterized. In fission yeast, two types of MTOCs exist in addition to the spindle pole body, the yeast centrosome equivalent. These are the equatorial MTOC, which nucleates microtubules from the cell division site at the end of mitosis, and interphase MTOCs, which nucleate microtubules from multiple sites near the cell nucleus during interphase.

RESULTS

From an insertional mutagenesis screen we identified a novel gene, mod20+, which is required for microtubule nucleation from non-spindle pole body MTOCs in fission yeast. Mod20p is not required for intranuclear mitotic spindle assembly, although it is required for cytoplasmic astral microtubule growth during mitosis. Mod20p localizes to MTOCs throughout the cell cycle and is also dynamically distributed along microtubules themselves. We find that mod20p is required for the localization of components of the gamma-tubulin complex to non-spindle pole body MTOCs and physically interacts with the gamma-tubulin complex in vivo. Database searches reveal a family of eukaryotic proteins distantly related to mod20p; these are found in organisms ranging from fungi to mammals and include Drosophila centrosomin.

CONCLUSIONS

Mod20p appears to act by recruiting components of the gamma-tubulin complex to non-spindle pole body MTOCs. The identification of mod20p-related proteins in higher eukaryotes suggests that this may represent a general mechanism for the organization of noncentrosomal MTOCs in eukaryotic cells.

The Schizosaccharomyces pombe HIRA-like protein Hip1 is required for the periodic expression of histone genes and contributes to the function of complex centromeres

HIRA-like (Hir) proteins are evolutionarily conserved and are implicated in the assembly of repressive chromatin. In Saccharomyces cerevisiae, Hir proteins contribute to the function of centromeres. However, S. cerevisiae has point centromeres that are structurally different from the complex centromeres of metazoans. In contrast, Schizosaccharomyces pombe has complex centromeres whose domain structure is conserved with that of human centromeres. Therefore, we examined the functions of the fission yeast Hir proteins Slm9 and the previously uncharacterised protein Hip1. Deletion of hip1(+) resulted in phenotypes that were similar to those described previously for slm9 Delta cells: a cell cycle delay, synthetic lethality with cdc25-22, and poor recovery from nitrogen starvation. However, while it has previously been shown that Slm9 is not required for the periodic expression of histone H2A, we found that loss of Hip1 led to derepression of core histone genes expression outside of S phase. Importantly, we found that deletion of either hip1(+) or slm9(+) resulted in increased rates of chromosome loss, increased sensitivity to spindle damage, and reduced transcriptional silencing in the outer centromeric repeats. Thus, S. pombe Hir proteins contribute to pericentromeric heterochromatin, and our data thus suggest that Hir proteins may be required for the function of metazoan centromeres.

Centromere Silencing and Function in Fission Yeast Is Governed by the Amino Terminus of Histone H3

BACKGROUND

Centromeric domains often consist of repetitive elements that are assembled in specialized chromatin, characterized by hypoacetylation of histones H3 and H4 and methylation of lysine 9 of histone H3 (K9-MeH3). Perturbation of this underacetylated state by transient treatment with histone deacetylase inhibitors leads to defective centromere function, correlating with delocalization of the heterochromatin protein Swi6/HP1. Likewise, deletion of the K9-MeH3 methyltransferase Clr4/Suvar39 causes defective chromosome segregation. Here, we create fission yeast strains retaining one histone H3 and H4 gene; the creation of these strains allows mutation of specific N-terminal tail residues and their role in centromeric silencing and chromosome stability to be investigated.

RESULTS

Reduction of H3/H4 gene dosage to one-third does not affect cell viability or heterochromatin formation. Mutation of lysines 9 or 14 or serine 10 within the amino terminus of histone H3 impairs centromere function, leading to defective chromosome segregation and Swi6 delocalization. Surprisingly, silent centromeric chromatin does not require the conserved lysine 8 and 16 residues of histone H4.

CONCLUSIONS

To date, mutation of conserved N-terminal residues in endogenous histone genes has only been performed in budding yeast, which lacks the Clr4/Suvar39 histone methyltransferase and Swi6/HP1. We demonstrate the importance of conserved residues within the histone H3 N terminus for the maintenance of centromeric heterochromatin in fission yeast. In sharp contrast, mutation of two conserved lysines within the histone H4 tail has no impact on the integrity of centromeric heterochromatin. Our data highlight the striking divergence between the histone tail requirements for the fission yeast and budding yeast silencing pathways.

Nonrandom homolog segregation at meiosis I in Schizosaccharomyces pombe mutants lacking recombination

Physical connection between homologous chromosomes is normally required for their proper segregation to opposite poles at the first meiotic division (MI). This connection is generally provided by the combination of reciprocal recombination and sister-chromatid cohesion. In the absence of meiotic recombination, homologs are predicted to segregate randomly at MI. Here we demonstrate that in rec12 mutants of the fission yeast Schizosaccharomyces pombe, which are devoid of meiosis-induced recombination, homologs segregate to opposite poles at MI 63% of the time. Residual, Rec12-independent recombination appears insufficient to account for the observed nonrandom homolog segregation. Dyad asci are frequently produced by rec12 mutants. More than half of these dyad asci contain two viable homozygous-diploid spores, the products of a single reductional division. This set of phenotypes is shared by other S. pombe mutants that lack meiotic recombination, suggesting that nonrandom MI segregation and dyad formation are a general feature of meiosis in the absence of recombination and are not peculiar to rec12 mutants. Rec8, a meiosis-specific sister-chromatid cohesin, is required for the segregation phenotypes displayed by rec12 mutants. We propose that S. pombe possesses a system independent of recombination that promotes homolog segregation and discuss possible mechanisms.

A cell cycle-regulated GATA factor promotes centromeric localization of CENP-A in fission yeast

CENP-A, the centromere-specific histone H3 variant, plays a crucial role in organizing kinetochore chromatin for precise chromosome segregation. We have isolated Ams2, a Daxx-like motif-containing GATA factor, and histone H4, as multicopy suppressors of cnp1-1, an S. pombe CENP-A mutant. While depletion of Ams2 results in the reduction of CENP-A binding to the centromere and chromosome missegregation, increasing its dosage restores association of a CENP-A mutant protein with centromeres. Conversely, overexpression of CENP-A or histone H4 suppresses an ams2 disruptant. The intracellular amount of Ams2 thus affects centromeric nucleosomal constituents. Ams2 is abundant in S phase and associates with chromatin, including the central centromeres through binding to GATA-core sequences. Ams2 is thus a cell cycle-regulated GATA factor that is required for centromere function.

Spindle-kinetochore attachment requires the combined action of Kin I-like Klp5/6 and Alp14/Dis1-MAPs in fission yeast

Fission yeast Klp5 and Klp6 belong to the microtubule-destabilizing Kin I family. In klp5 mutants, spindle checkpoint proteins Mad2 and Bub1 are recruited to mitotic kinetochores for a prolonged duration, indicating that these kinetochores are unattached. Further analysis shows that there are kinetochores to which only Bub1, but not Mad2, localizes. These kinetochores are likely to have been captured, yet lack tension. Thus Klp5 and Klp6 play a role in a spindle- kinetochore interaction at dual steps, capture and generation of tension. The TOG/XMAP215 family, Alp14 and Dis1 are known to stabilize microtubules and be required for the bivalent attachment of the kinetochore to the spindle. Despite apparent opposing activities towards microtubule stability, Klp5/Klp6 and Alp14/Dis1 share an essential function, as either dis1klp or alp14klp mutants are synthetically lethal, like alp14dis1. Defective phenotypes are similar to each other, characteristic of attachment defects and chromosome mis-segregation. Furthermore Alp14 is of significance for kinetochore localization of Klp5. We propose that Klp5/Klp6 and Alp14/Dis1 play a collaborative role in bipolar spindle formation during prometaphase through producing spindle dynamism.

Two kinesin-like Kin I family proteins in fission yeast regulate the establishment of metaphase and the onset of anaphase A

BACKGROUND

Metaphase is thought to be a force-equilibrium state of "tug of war," in which poleward forces are pulling kinetochores and counteracting the cohesive forces between the centromeres. Unlike conventional kinesins, members of the Kin I family are microtubule-depolymerizing enzymes, which are expected to be molecules that could generate poleward forces.

RESULTS

We have characterized mitotic roles of two Kin I homologs, Klp5 and Klp6, in fission yeast. Klp5 and Klp6 colocalize to the mitotic kinetochores and the spindle midzone. These two proteins form a heterocomplex, but not a homocomplex. Albeit not essential, both proteins are required for accurate chromosome segregation and normal morphology of interphase microtubules. Time-lapse live analysis using GFP-alpha-tubulin indicates that these mutants spend a much longer time (2-fold) in mitosis before the initiation of anaphase B. Further observation using kinetochore and centromere markers shows that, in these mutants, sister centromeres move back and forth between the two poles, indicating that entry into anaphase A is delayed. This is supported by live image analysis showing that Cut2 securin is retained during the prolonged mitosis. Furthermore, the mitotic extension is dependent upon the Mad2 spindle checkpoint.

CONCLUSIONS

We discuss two models of Kin I function in fission yeast. One proposes that Klp5 and Klp6 are required for efficient capturing of kinetochores by the spindles, while the other proposes that they are required to generate tension upon kinetochore capturing. Kin I, therefore, plays a fundamental role in the establishment of metaphase, probably by generating poleward forces at the kinetochores.

Fission yeast F-box protein Pof3 is required for genome integrity and telomere function

The Skp1-Cullin-1/Cdc53-F-box protein (SCF) ubiquitin ligase plays an important role in various biological processes. In this enzyme complex, a variety of F-box proteins act as receptors that recruit substrates. We have identified a fission yeast gene encoding a novel F-box protein Pof3, which contains, in addition to the F-box, a tetratricopeptide repeat motif in its N terminus and a leucine-rich-repeat motif in the C terminus, two ubiquitous protein-protein interaction domains. Pof3 forms a complex with Skp1 and Pcu1 (fission yeast cullin-1), suggesting that Pof3 functions as an adaptor for specific substrates. In the absence of Pof3, cells exhibit a number of phenotypes reminiscent of genome integrity defects. These include G2 cell cycle delay, hypersensitivity to UV, appearance of lagging chromosomes, and a high rate of chromosome loss. pof3 deletion strains are viable because the DNA damage checkpoint is continuously activated in the mutant, and this leads to G2 cell cycle delay, thereby preventing the mutant from committing lethal mitosis. Pof3 localizes to the nucleus during the cell cycle. Molecular analysis reveals that in this mutant the telomere is substantially shortened and furthermore transcriptional silencing at the telomere is alleviated. The results highlight a role of the SCF(Pof3) ubiquitin ligase in genome integrity via maintaining chromatin structures.

Recruitment of cohesin to heterochromatic regions by Swi6/HP1 in fission yeast

Fission yeast centromeres, like those of higher eukaryotes, are composed of repeated DNA structures and associated heterochromatin protein complexes, that have a critical function in the faithful segregation of chromosomes during cell division. Cohesin protein complexes, which are essential for sister-chromatid cohesion and proper chromosome segregation, are enriched at centromeric repeats. We have identified a functional and physical link between heterochromatin and cohesin. We find that the preferential localization of cohesins at the centromeric repeats is dependent on Swi6, a conserved heterochromatin protein that is required for proper kinetochore function. Cohesin is also enriched at the mating-type heterochromatic region in a manner that depends on Swi6 and is required to preserve the genomic integrity of this locus. We provide evidence that a cohesin subunit Psc3 interacts with Swi6 and its mouse homologue HP1. These data define a conserved function of Swi6/HP1 in recruitment of cohesin to heterochromatic regions, promoting the proper segregation of chromosomes.

Conserved Wat1/Pop3 WD-repeat protein of fission yeast secures genome stability through microtubule integrity and may be involved in mRNA maturation

Accurate chromosome segregation is dependent upon the integrity of mitotic spindles, which pull each pair of sister chromatids towards opposite poles. In this study, we have characterised fission yeast pop3-5235, a diploidising mutant that is impaired in genome stability. Pop3 is the same as Wat1, a conserved protein containing 7 WD repeats. Pop3/Wat1 has also been isolated from a two-hybrid screen as a binding partner to Prp2, the large subunit of the essential splicing factor U2AF. In wat1 mutants, the cellular amount of α-tubulin is decreased to very low levels, which results in compromised microtubules and spindles, consequently leading to unequal chromosome separation. Further analysis shows that, in spite of the binding between Wat1 and Prp2, Wat1 may not be involved directly in splicing reactions per se. Instead, we find that Wat1 is required for the maintenance of α-tubulin mRNA levels; moreover, transcript levels of genes other than the α-tubulin gene are also equally decreased in this mutant. Wild-type Wat1, but not the mutant protein, forms a large complex in the cell with several other proteins, suggesting that Wat1 functions as a structural linker in the complex. The results suggest that Wat1 plays a role in mRNA maturation as a coupling protein between splicing and synthesis and/or stabilisation.

Meiotic recombination and chromosome segregation in Schizosaccharomyces pombe

In most organisms homologous recombination is vital for the proper segregation of chromosomes during meiosis, the formation of haploid sex cells from diploid precursors. This review compares meiotic recombination and chromosome segregation in the fission yeast Schizosaccharomyces pombe and the distantly related budding yeast Saccharomyces cerevisiae, two especially tractable microorganisms. Certain features, such as the occurrence of DNA breaks associated with recombination, appear similar, suggesting that these features may be common in eukaryotes. Other features, such as the role of these breaks and the ability of chromosomes to segregate faithfully in the absence of recombination, appear different, suggesting multiple solutions to the problems faced in meiosis.

Fission yeast Ras1 effector Scd1 interacts with the spindle and affects its proper formation

Ras1 GTPase is the Schizosaccharomyces pombe homolog of the mammalian Ha-Ras proto-oncoprotein. Ras1 interacts with Scd1 (aka Ral1), a presumptive guanine nucleotide exchange factor for Cdc42sp, to control organization of the cytoskeleton. In this study, we demonstrated that the scd1 deletion (scd1Delta) induced hypersensitivity to microtubule destabilizing drugs and instability of the minichromosome. Overexpression of scd1 induced formation of abnormal spindles and chromosome missegregation. The scd1 deletion worsened the defects of spindle formation in tubulin mutants; by contrast, it did not induce lethality in mutants defective in the spindle pole bodies. These genetic data suggest that Scd1 can interact with tubulin with substantial specificity to affect proper spindle formation and chromosome segregation. Subcellular localization data further illustrated that a GFP-Scd1 fusion protein can associate with the spindle. Finally, we showed that unlike ras1Delta and scd1Delta, byr2Delta (affecting the Ras1 effector for mating) is not synthetically lethal with the tubulin mutations. These data collectively suggest that the Ras1 pathway can impinge upon microtubules through Scd1, but not Byr2, to affect proper spindle formation and chromosome segregation.

Conservation of heterochromatin protein 1 function

Heterochromatin represents a cytologically visible state of heritable gene repression. In the yeast, Schizosaccharomyces pombe, the swi6 gene encodes a heterochromatin protein 1 (HP1)-like chromodomain protein that localizes to heterochromatin domains, including the centromeres, telomeres, and the donor mating-type loci, and is involved in silencing at these loci. We identify here the functional domains of swi6p and demonstrate that the chromodomain from a mammalian HP1-like protein, M31, can functionally replace that of swi6p, showing that chromodomain function is conserved from yeasts to humans. Site-directed mutagenesis, based on a modeled three-dimensional structure of the swi6p chromodomain, shows that the hydrophobic amino acids which lie in the core of the structure are critical for biological function. Gel filtration, gel overlay experiments, and mass spectroscopy show that HP1 proteins can self-associate, and we suggest that it is as oligomers that HP1 proteins are incorporated into heterochromatin complexes that silence gene activity.

Separation anxiety at the centromere

During mitosis, replicated sister-chromatids must maintain cohesion as they attach to the mitotic spindle. At anaphase, cohesion is lost simultaneously along the entire chromosome, releasing sisters from one another and allowing them to segregate to opposite poles. During meiosis, sisters separate in a two-step process. At anaphase of meiosis I, cohesion is lost along the chromosome arms but is maintained at centromeric regions. Not until meiosis II are sister chromatids able to break the connection at the centromere and separate away from one another. Recent studies suggest that the centromere exhibits dynamics that are very different compared with those of the chromatid arms during both mitosis and meiosis. This review discusses the nature of the specialized chromatid cohesion seen at the centromere.

Telomere-led bouquet formation facilitates homologous chromosome pairing and restricts ectopic interaction in fission yeast meiosis

A polarized chromosomal arrangement with clustered telomeres in a meiotic prophase nucleus is often called bouquet and is thought to be important for the pairing of homologous chromosomes. Fluorescence in situ hybridization in fission yeast indicated that chromosomal loci are positioned in an ordered manner as anticipated from the bouquet arrangement. Blocking the formation of the telomere cluster with the kms1 mutation created a disorganized chromosomal arrangement, not only for the regions proximal to the telomere but also for interstitial regions. The kms1 mutation also affected the positioning of a linear minichromosome. Consistent with this cytological observation, the frequency of ectopic homologous recombination between a linear minichromosome and a normal chromosome increased in the kms1 background. Intragenic recombination between allelic loci is reduced in the kms1 mutant, but those between non-allelic loci are unaffected or slightly increased. Thus, telomere-led chromosome organization facilitates homologous pairing and also restricts irregular chromosome pairing during meiosis.

Four chromo-domain proteins of Schizosaccharomyces pombe differentially repress transcription at various chromosomal locations

Transcription is repressed in regions of the fission yeast genome close to centromeres, telomeres, or the silent mating-type cassettes mat2-P and mat3-M. The repression involves the chromo-domain proteins Swi6 and Clr4. We report that two other chromo-domain proteins, Chp1 and Chp2, are also important for these position effects. Chp1 showed a specificity for centromeric regions. Its essentiality for the transcriptional repression of centromeric markers correlates with its importance for chromosome stability. Chp2 appeared more pleiotropic. Its effects on centromeric silencing were less pronounced than those of Chp1, and it participated in telomeric position effects and transcriptional silencing in the mating-type region. We also found that PolII-transcribed genes were repressed when placed in one of the Schizosaccharomyces pombe rDNA clusters, a situation analogous to that in the budding yeast Saccharomyces cerevisiae. Chp2, Swi6, Clr4, and, to a lesser extent, Chp1 participated in that repression.

Characterization of a fission yeast SUMO-1 homologue, pmt3p, required for multiple nuclear events, including the control of telomere length and chromosome segregation

Unlike ubiquitin, the ubiquitin-like protein modifier SUMO-1 and its budding yeast homologue Smt3p have been shown to be more important for posttranslational protein modification than for protein degradation. Here we describe the identification of the SUMO-1 homologue of fission yeast, which we show to be required for a number of nuclear events including the control of telomere length and chromosome segregation. A disruption of the pmt3(+) gene, the Schizosaccharomyces pombe homologue of SMT3, was not lethal, but mutant cells carrying the disrupted gene grew more slowly. The pmt3Delta cells showed various phenotypes such as aberrant mitosis, sensitivity to various reagents, and high-frequency loss of minichromosomes. Interestingly, we found that pmt3(+) is required for telomere length maintenance. Loss of Pmt3p function caused a striking increase in telomere length. When Pmt3p synthesis was restored, the telomeres became gradually shorter. This is the first demonstration of involvement of one of the Smt3p/SUMO-1 family proteins in telomere length maintenance. Fusion of Pmt3p to green fluorescent protein (GFP) showed that Pmt3p was predominantly localized as intense spots in the nucleus. One of the spots was shown to correspond to the spindle pole body (SPB). During prometaphase- and metaphase, the bright GFP signals at the SPB disappeared. These observations suggest that Pmt3p is required for kinetochore and/or SPB functions involved in chromosome segregation. The multiple functions of Pmt3p described here suggest that several nuclear proteins are regulated by Pmt3p conjugation.

Fission yeast mutants that alleviate transcriptional silencing in centromeric flanking repeats and disrupt chromosome segregation

In the fission yeast Schizosaccharomyces pombe genes are transcriptionally silenced when placed within centromeres, within or close to the silent mating-type loci or adjacent to telomeres. Factors required to maintain mating-type silencing also affect centromeric silencing and chromosome segregation. We isolated mutations that alleviate repression of marker genes in the inverted repeats flanking the central core of centromere I. Mutations csp1 to 13 (centromere: suppressor of position effect) defined 12 loci. Ten of the csp mutants have no effect on mat2/3 or telomere silencing. All csp mutants allow some expression of genes in the centromeric flanking repeat, but expression in the central core is undetectable. Consistent with defective centromere structure and function, chromosome loss rates are elevated in all csp mutants. Mutants csp1 to 6 are temperature-sensitive lethal and csp3 and csp6 cells are defective in mitosis at 36 degrees. csp7 to 13 display a high incidence of lagging chromosomes on late anaphase spindles. Thus, by screening for mutations that disrupt silencing in the flanking region of a fission yeast centromere a novel collection of mutants affecting centromere architecture and chromosome segregation has been isolated.

Rad18 is required for DNA repair and checkpoint responses in fission yeast

To survive damage to the genome, cells must respond by activating both DNA repair and checkpoint responses. Using genetic screens in the fission yeast Schizosaccharomyces pombe, we recently isolated new genes required for DNA damage checkpoint control. We show here that one of these strains defines a new allele of the previously described rad18 gene, rad18-74. rad18 is an essential gene, even in the absence of extrinsic DNA damage. It encodes a conserved protein related to the structural maintenance of chromosomes proteins. Point mutations in rad18 lead to defective DNA repair pathways responding to both UV-induced lesions and, as we show here, double-stranded breaks. Furthermore, rad18p is required to maintain cell cycle arrest in the presence of DNA damage, and failure of this leads to highly aberrant mitoses. A gene encoding a BRCT-containing protein, brc1, was isolated as an allele-specific high-copy suppressor of rad18-74. brc1 is required for mitotic fidelity and for cellular viability in strains with rad18 mutations but is not essential for DNA damage responses. Mutations in rad18 and brc1 are synthetically lethal with a topoisomerase II mutant (top2-191), indicating that these proteins play a role in chromatin organization. These studies show a role for chromatin organization in the maintenance or activation of responses to DNA damage.

Genetic control of telomere integrity in Schizosaccharomyces pombe: rad3(+) and tel1(+) are parts of two regulatory networks independent of the downstream protein kinases chk1(+) and cds1(+)

The Schizosaccharomyces pombe checkpoint gene named rad3(+) encodes an ATM-homologous protein kinase that shares a highly conserved motif with proteins involved in DNA metabolism. Previous studies have shown that Rad3 fulfills its function via the regulation of the Chk1 and Cds1 protein kinases. Here we describe a novel role for Rad3 in the control of telomere integrity. Mutations in the rad3(+) gene alleviated telomeric silencing and produced shortened lengths in the telomere repeat tracts. Genetic analysis revealed that the other checkpoint rad mutations rad1, rad17, and rad26 belong to the same phenotypic class with rad3 with regard to control of the telomere length. Of these mutations, rad3 and rad26 have a drastic effect on telomere shortening. tel1(+), another ATM homologue in S. pombe, carries out its telomere maintenance function in parallel with the checkpoint rad genes. Furthermore, either a single or double disruption of cds1(+) and chk1(+) caused no obvious changes in the telomeric DNA structure. Our results demonstrate a novel role of the S. pombe ATM homologues that is independent of chk1(+) and cds1(+).

Defects in components of the proteasome enhance transcriptional silencing at fission yeast centromeres and impair chromosome segregation

Fission yeast centromeres are transcriptionally silent and form a heterochromatin-like structure essential for normal centromere function; this appears analogous to heterochromatin and position effect variegation in other eukaryotes. Conditional mutations in three genes designated cep (centromere enhancer of position effect) were found to enhance transcriptional silencing within centromeres. Cloning of the cep1(+) and cep2(+) genes by functional complementation revealed that they are identical to the previously described genes pad1(+) and mts2(+), respectively, which both encode subunits of the proteasome 19S cap. Like Mts2 and Mts4, epitope-tagged Cep1/Pad1 localizes to or near the nuclear envelope throughout the cell cycle. The cep mutants display a range of phenotypes depending on the temperature. Silencing within the central domain of centromeres is increased at 36 degrees C. This suggests that the proteasome is involved in regulating silencing and thus centromeric chromatin architecture, possibly by lowering the level of some chromatin-associated protein by ubiquitin-dependent degradation. This is the first report of defective proteasome function affecting heterochromatin-mediated transcriptional silencing. At 36 and 32 degrees C, the cep mutants lose chromosomes at an elevated rate, and at 18 degrees C, the mutants are cryosensitive for growth. Cytological analysis at 18 degrees C revealed a defect in sister chromatid separation while other mitotic events occurred normally, indicating that cep mutations might interfere specifically with the degradation of inhibitor(s) of sister chromatid separation. These observations suggest that 19S subunits confer a level of substrate specificity on the proteasome and raise the possibility of a link between components involved in centromere architecture and sister chromatid cohesion.

A Schizosaccharomyces pombe artificial chromosome large DNA cloning system

The feasibility of using the fission yeast, Schizosaccharomyces pombe , as a host for the propagation of cloned large fragments of human DNA has been investigated. Two acentric vector arms were utilized; these carry autonomously replicating sequences ( ars elements), selectable markers ( ura4(+) or LEU2 ) and 250 bp of S. pombe terminal telomeric repeats. All cloning was performed between the unique sites in both vector arms for the restriction endonuclease Not I. Initially the system was tested by converting six previously characterized cosmids from human chromosome 11p13 into a form that could be propagated in S.pombe as linear episomal elements of 50-60 kb in length. In all transformants analysed these cosmids were maintained intact. To test if larger fragments of human DNA could also be propagated total human DNA was digested with Not I and size fractionated by pulsed field gel electrophoresis (PFGE). Fractions of 100-1000 kb were ligated to Not I-digested vector arms and transformed into S.pombe protoplasts in the presence of lipofectin. Prototrophic ura+leu+transformants were obtained which upon examination by PFGE were found to contain additional linear chromosomes migrating at between 100 and 500 kb with a copy number of 5-10 copies/cell. Hybridization analyses revealed that these additional bands contained human DNA. Fluorescent in situ hybridization (FISH) analyses of several independent clones indicated that the inserts were derived from single loci within the human genome. These analyses clearly demonstrate that it is possible to clone large fragments of heterologous DNA in fission yeast using this S.p ombe artificial chromosome system which we have called SPARC. This vector-host system will complement the various other systems for cloning large DNA fragments.

Control of meiotic recombination in Schizosaccharomyces pombe

Homologous recombination occurs at high frequency during meiosis and is essential for the proper segregation of chromosomes and the generation of genetic diversity. Meiotic recombination is controlled in numerous ways. In the fission yeast Schizosaccharomyces pombe nutritional starvation induces meiosis and high-level expression of many genes, including numerous recombination (rec) genes, whose products are required for recombination. Accompanying the two meiotic divisions are profound changes in nuclear and chromosomal structure and movement, which may play an important role in meiotic recombination. Although recombination occurs throughout the genome, it occurs at high frequency in some intervals (hotspots) and at low frequency in others (coldspots). The well-characterized hotspot M26 is activated by the Mts1/Mts2 protein; this site and its binding proteins interact with the local chromosomal structure to enhance recombination. A coldspot between the silent mating-type loci is repressed by identified proteins, which may also alter local chromatin. We discuss in detail the rec genes and the possible functions of their products, some but not all of which share homology with other identified proteins. Although some of the rec gene products are required for recombination throughout the genome, others demonstrate regional specificity and are required in certain genomic regions but not in others. Throughout the review contrasts are made with meiotic recombination in the more thoroughly studied budding yeast Saccharomyces cerevisiae.

Meiotic telomeres: a matchmaker for homologous chromosomes

Telomeres, with their special structures and special schemes of synthesis, are essential for protecting the ends of eukaryotic linear chromosomes during cell proliferation. In addition to this basic function, the meiosis-specific functions of telomeres have long been inferred from the cytological observations of characteristic chromosome configurations in meiotic prophase. Recent studies in the fission yeast Schizosaccharomyces pombe have provided deeper insights into the role of meiotic telomeres in the pairing of homologous chromosomes. Here I have summarized our current understanding of the meiotic behaviour of telomeres in S. pombe, and discuss the role of telomeres in meiosis.

Regulation of telomere length by checkpoint genes in Schizosaccharomyces pombe

We have studied telomere length in Schizosaccharomyces pombe strains carrying mutations affecting cell cycle checkpoints, DNA repair, and regulation of the Cdc2 protein kinase. Telomere shortening was found in rad1, rad3, rad17, and rad26 mutants. Telomere lengths in previously characterized rad1 mutants paralleled the replication checkpoint proficiency of those mutants. In contrast, rad9, chk1, hus1, and cds1 mutants had intact telomeres. No difference in telomere length was seen in mutants affected in the regulation of Cdc2, whereas some of the DNA repair mutants examined had slightly longer telomeres than did the wild type. Overexpression of the rad1(+) gene caused telomeres to elongate slightly. The kinetics of telomere shortening was monitored by following telomere length after disruption of the rad1(+) gene; the rate was approximately 1 nucleotide per generation. Wild-type telomere length could be restored by reintroduction of the wild-type rad1(+) gene. Expression of the Saccharomyces cerevisiae RCK1 protein kinase gene, which suppresses the radiation and hydroxyurea sensitivity of Sz. pombe checkpoint mutants, was able to attenuate telomere shortening in rad1 mutant cells and to increase telomere length in a wild-type background. The functional effects of telomere shortening in rad1 mutants were assayed by measuring loss of a linear and a circular minichromosome. A minor increase in loss rate was seen with the linear minichromosome, and an even smaller difference compared with wild-type was detected with the circular plasmid.

Mis6, a fission yeast inner centromere protein, acts during G1/S and forms specialized chromatin required for equal segregation

Disorder in sister chromatid separation can lead to genome instability and cancer. A temperature-sensitive S. pombe mis6-302 frequently loses a minichromosome at 26 degrees C and abolishes equal segregation of regular chromosomes at 36 degrees C. The mis6+ gene is essential for viability, and its deletion results in missegregation identical to mis6-302. Mis6 acts before or at the onset of S phase, and mitotic missegregation defects are produced only after the passage of G1/S at 36 degrees C. Mis6 locates at the centromeres throughout the cell cycle. In the mutant, positioning of the centromeres becomes abnormal, and specialized chromatin in the inner centromeres, which give the smear micrococcal nuclease pattern in wild type, is disrupted. The ability to establish correct biorientation of sister centromeres in metaphase cells requires the Mis6-containing chromatin and originates during the passage of G1/S.

There is an upper limit of chromosome size for normal development of an organism

A clearly definable upper tolerance limit for chromosome arm length has been found. As a rule we postulate that, for normal development of an organism, the longest chromosome arm must not exceed half of the average length of the spindle axis at telophase. Above this length, fertility and viability of the carrier individuals become severely impaired due to increasingly incomplete separation of the longest chromatids during mitosis, resulting finally in the loss of DNA. The experimental work that points to a limit in genome plasticity has been carried out on a series of field bean lines with karyotypes of considerable variation in length of individual chromosomes.

Assembly of CENP-A into centromeric chromatin requires a cooperative array of nucleosomal DNA contact sites

We investigated the requirements for targeting the centromeric histone H3 homologue CENP-A for assembly at centromeres in human cells by transfection of epitope-tagged CENP-A derivatives into HeLa cells. Centromeric targeting is driven solely by the conserved histone fold domain of CENP-A. Using the crystal structure of histone H3 as a guide, a series of CENP-A/histone H3 chimeras was constructed to test the role of discrete structural elements of the histone fold domain. Three elements were identified that are necessary for efficient targeting to centromeres. Two correspond to contact sites between histone H3 and nucleosomal DNA. The third maps to a homotypic H3-H3 interaction site important for assembly of the (H3/H4)2 heterotetramer. Immunoprecipitation confirms that CENP-A self-associates in vivo. In addition, targeting requires that CENP-A expression is uncoupled from histone H3 synthesis during S phase. CENP-A mRNA accumulates later in the cell cycle than histone H3, peaking in G2. Isolation of the gene for human CENP-A revealed a regulatory motif in the promoter region that directs the late S/G2 expression of other cell cycle-dependent transcripts such as cdc2, cdc25C, and cyclin A. Our data suggest a mechanism for molecular recognition of centromeric DNA at the nucleosomal level mediated by a cooperative series of differentiated CENP-A-DNA contact sites arrayed across the surface of a CENP-A nucleosome and a distinctive assembly pathway occurring late in the cell cycle.

Mutations in the fission yeast silencing factors clr4+ and rik1+ disrupt the localisation of the chromo domain protein Swi6p and impair centromere function

Transcriptional silencing is known to occur at centromeres, telomeres and the mating type region in the nucleus of fission yeast, Schizosaccharomyces pombe. Mating-type silencing factors have previously been shown also to affect transcriptional repression within centromeres and to some extent at telomeres. Mutations in the clr4+, rik1+ and swi6+ genes dramatically reduce silencing at certain centromeric regions and cause elevated chromosome loss rates. Recently, Swi6p was found to co-localise with the three silent chromosomal regions. Here the involvement of clr4+, rik1+ and swi6+ in centromere function is investigated in further detail. Fluorescence in situ hybridisation (FISH) was used to show that, as in swi6 mutant cells, centromeres lag on late anaphase spindles in clr4 and rik1 mutant cells. This phenotype is consistent with a role for these three gene products in fission yeast centromere function. The Swi6 protein was found to be delocalised from all three silent chromosomal regions, and dispersed within the nucleus, in both clr4 and rik1 mutant cells. The phenotypic similarity observed in all three mutants is consistent with the products of both the clr4+ and rik1+ genes being required to recruit Swi6p to the centromere and other silent regions. Mutations in clr4, rik1 and swi6 also result in elevated sensitivity to reagents which destabilise microtubules and show a synergistic interaction with a mutation in the beta-tubulin gene (nda3). These observations suggest that clr4+ and rik1+ must play a role in the assembly of Swi6p into a transcriptionally silent, inaccessible chromatin structure at fission yeast centromeres which is required to facilitate interactions with spindle microtubules and to ensure normal chromosome segregation.

Fission yeast sta mutations that stabilize an unstable minichromosome are novel cdc2-interacting suppressors and are involved in regulation of spindle dynamics

Cytological observations have shown that the presence of unstable minichromosomes can delay progression through the early stages of mitosis in fission yeast (Schizosaccharomyces pombe), suggesting that such minichromosomes may provide a useful tool for examining the system that regulates the coordinated segregation of chromosomes. One such unstable minichromosome is a large circular minichromosome. We previously showed that the mitotic instability of this minichromosome is probably due to the frequent occurrence of catenated forms of DNA after replication. To identify genes involved in the regulation of chromosome behavior in mitosis, we isolated mutants which stabilized this minichromosome. Three loci (sta1, sta2, and sta3) were identified. Two of them were found to be suppressors of temperature-sensitive mutations in cdc2, which encodes the catalytic subunit of muturation promoting factor (MPF). They show no linkage to, and are thus different from, suc1, and cdc13, previously identified as genes that interact with cdc2. The other mutation mapped to a gene previously identified as being required for the correct formation of the mitotic spindle. Data provided in this study suggest that the sta genes are involved in the regulation of spindle dynamics to ensure proper chromosome segregation during mitosis.

Elements of chromosome structure and function in fission yeast

The investigation of fission yeast chromosome structure and function has moved rapidly over the past 10 years. The isolation of replication origins, telomeres and centromeres has allowed the development of minichromosomes, a yeast artificial chromosome (YAC)-like cloning system and investigations into chromosome segregation and behaviour during mitosis and meiosis. Many mutants have been isolated which are defective in chromosome segregation. The development of the fluorescent in-situ hybridization (FISH) technique for use in S. pombe has allowed the localization of centromeres and telomeres throughout mitosis and meiosis. In combination with indirect immunofluorescence to detect spindle and chromosomal proteins, the FISH technique should further advance our understanding of fission yeast chromosome structure and function. The recent discovery of a heterochromatin-like structure mediating transcriptional repression at centromeres reinforces the notion that fission yeast centromeres are similar to those of larger eukaryotes. Further characterization of such phenomena will accelerate the genetic dissection of this important chromosomal element.

A large circular minichromosome of Schizosaccharomyces pombe requires a high dose of type II DNA topoisomerase for its stabilization

We have constructed circular minichromosomes, ranging in size from 36 to 110 kb, containing the centromeric repeats of Schizosaccharomyces pombe cen3. Comparison of their mitotic stability showed that the circular minichromosomes became more unstable with increasing in size, however, a linear cen3 minichromosome, which is almost the same size as the largest circular one tested, does not show such instability. High levels of expression of the top2+ (type II DNA topoisomerase; topo II) but not top1+ gene (type I DNA topoisomerase) suppressed the instability of the largest circular minichromosome, whereas partial inactivation of topo II dramatically destabilized the minichromosome. A mutant topo II, defective in nuclear localization but still retaining its in vitro relaxation activity, did not stabilize the circular minichromosome. These results indicate that endogenous type II DNA topoisomerase is insufficient for accurate segregation of the circular minichromosome. In addition, the replication of the minichromosomal DNA appears to proceed normally, because the presence of the unstable minichromosome did not cause G2 delay. A likely cause of the instability is intertwining of the minichromosome DNA possibly occurring after DNA replication. An interaction between topo II and the centromeric repeats is implied by the finding that multiple copies of the centromeric repeat, dg-dh, affect stability of the minichromosome similarly to top2+ gene dosage.

Mutations derepressing silent centromeric domains in fission yeast disrupt chromosome segregation

The ura4+ gene displays phenotypes consistent with variegated expression when inserted at 11 sites throughout fission yeast centromere 1. An abrupt transition occurs between the zone of centromeric repression and two adjacent expressed sites. Mutations in six genes alleviate repression of the silent-mating type loci and of ura4+ expressed from a site adjacent to the silent locus, mat3-M. Defects at all six loci affect repression of the ura4+ gene adjacent to telomeres and at the three centromeric sites tested. The clr4-S5 and rik1-304 mutations cause the most dramatic derepression at two out of three sites within cen1. All six mutations had only slight or intermediate effects on a third site in the center of cen1 or on telomeric repression. Strains with lesions at the clr4, rik1, and swi6 loci have highly elevated rates of chromosome loss. We propose that the products of these genes are integral in the assembly of a heterochromatin-like structure, with distinct domains, enclosing the entire centromeric region that reduces or excludes access to transcription factors. The formation of this heterochromatic structure may be an absolute requirement for the formation of a fully functional centromere.

Unusual chromosome structure of fission yeast DNA in mouse cells

Chromosomes from the fission yeast Schizosaccharomyces pombe have been introduced into mouse cells by protoplast fusion. In most cell lines the yeast DNA integrates into a single site within a mouse chromosome and results in striking chromosome morphology at metaphase. Both light and electron microscopy show that the yeast chromosome region is narrower than the flanking mouse DNA. Regions of the yeast insert stain less intensely with propidium iodide than surrounding DNA and bear a morphological resemblance to fragile sites. We investigate the composition of the yeast transgenomes and the modification and chromatin structure of this yeast DNA in mouse cells. We suggest that the underlying basis for the structure we see lies above the level of DNA modification and nucleosome assembly, and may reflect the attachment of the yeast DNA to the rodent cell nucleoskeleton. The yeast integrant replicates late in S phase at a time when G bands of the mouse chromosomes are being replicated, and participates in sister chromatid exchanges at a high frequency. We discuss the implications of these studies to the understanding of how chromatin folding relates to metaphase chromosome morphology and how large stretches of foreign DNA behave when introduced into mammalian cells.

Localization of DNA sequences required for human centromere function through an analysis of rearranged Y chromosomes

We have localized the DNA sequences required for mitotic centromere function on the human Y chromosome. Analysis of 33 rearranged Y chromosomes allowed the centromere to be placed in interval 8 of a 24-interval deletion map. Although this interval is polymorphic in size, it can be as small as approximately 500kb. It contains alphoid satellite DNA and approximately 300kb of adjacent Yp sequences. Chromosomes with rearrangements in this region were analysed in detail. Two translocation chromosomes and one monocentric isochromosome had breakpoints within the alphoid array. Of 12 suppressed Y centromeres on translocation chromosomes and dicentric isochromosomes that were also analysed two showed deletions one of which only removed alphoid DNA. These results indicate that alphoid DNA is a functional part of the Y chromosome centromere.