Identification of DNA regions required for mitotic and meiotic functions within the centromere of Schizosaccharomyces pombe chromosome I

Interspecies conservation of organisation and function between nonhomologous regional centromeres

Despite the conserved essential function of centromeres, centromeric DNA itself is not conserved. The histone-H3 variant, CENP-A, is the epigenetic mark that specifies centromere identity. Paradoxically, CENP-A normally assembles on particular sequences at specific genomic locations. To gain insight into the specification of complex centromeres, here we take an evolutionary approach, fully assembling genomes and centromeres of related fission yeasts. Centromere domain organization, but not sequence, is conserved between Schizosaccharomyces pombe, S. octosporus and S. cryophilus with a central CENP-A^Cnp1 domain flanked by heterochromatic outer-repeat regions. Conserved syntenic clusters of tRNA genes and 5S rRNA genes occur across the centromeres of S. octosporus and S. cryophilus, suggesting conserved function. Interestingly, nonhomologous centromere central-core sequences from S. octosporus and S. cryophilus are recognized in S. pombe, resulting in cross-species establishment of CENP-A^Cnp1 chromatin and functional kinetochores. Therefore, despite the lack of sequence conservation, Schizosaccharomyces centromere DNA possesses intrinsic conserved properties that promote assembly of CENP-A chromatin.

Repeat-Associated Fission Yeast-Like Regional Centromeres in the Ascomycetous Budding Yeast Candida tropicalis

The centromere, on which kinetochore proteins assemble, ensures precise chromosome segregation. Centromeres are largely specified by the histone H3 variant CENP-A (also known as Cse4 in yeasts). Structurally, centromere DNA sequences are highly diverse in nature. However, the evolutionary consequence of these structural diversities on de novo CENP-A chromatin formation remains elusive. Here, we report the identification of centromeres, as the binding sites of four evolutionarily conserved kinetochore proteins, in the human pathogenic budding yeast Candida tropicalis. Each of the seven centromeres comprises a 2 to 5 kb non-repetitive mid core flanked by 2 to 5 kb inverted repeats. The repeat-associated centromeres of C. tropicalis all share a high degree of sequence conservation with each other and are strikingly diverged from the unique and mostly non-repetitive centromeres of related Candida species--Candida albicans, Candida dubliniensis, and Candida lusitaniae. Using a plasmid-based assay, we further demonstrate that pericentric inverted repeats and the underlying DNA sequence provide a structural determinant in CENP-A recruitment in C. tropicalis, as opposed to epigenetically regulated CENP-A loading at centromeres in C. albicans. Thus, the centromere structure and its influence on de novo CENP-A recruitment has been significantly rewired in closely related Candida species. Strikingly, the centromere structural properties along with role of pericentric repeats in de novo CENP-A loading in C. tropicalis are more reminiscent to those of the distantly related fission yeast Schizosaccharomyces pombe. Taken together, we demonstrate, for the first time, fission yeast-like repeat-associated centromeres in an ascomycetous budding yeast.

The role of model organisms in the history of mitosis research

Mitosis is a cell-cycle stage during which condensed chromosomes migrate to the middle of the cell and segregate into two daughter nuclei before cytokinesis (cell division) with the aid of a dynamic mitotic spindle. The history of mitosis research is quite long, commencing well before the discovery of DNA as the repository of genetic information. However, great and rapid progress has been made since the introduction of recombinant DNA technology and discovery of universal cell-cycle control. A large number of conserved eukaryotic genes required for the progression from early to late mitotic stages have been discovered, confirming that DNA replication and mitosis are the two main events in the cell-division cycle. In this article, a historical overview of mitosis is given, emphasizing the importance of diverse model organisms that have been used to solve fundamental questions about mitosis.

Centromeric barrier disruption leads to mitotic defects in Schizosaccharomyces pombe

Centromeres are cis-acting chromosomal domains that direct kinetochore formation, enabling faithful chromosome segregation and preserving genome stability. The centromeres of most eukaryotic organisms are structurally complex, composed of nonoverlapping, structurally and functionally distinct chromatin subdomains, including the specialized core chromatin that underlies the kinetochore and pericentromeric heterochromatin. The genomic and epigenetic features that specify and preserve the adjacent chromatin subdomains critical to centromere identity are currently unknown. Here we demonstrate that chromatin barriers regulate this process in Schizosaccharomyces pombe. Reduced fitness and mitotic chromosome segregation defects occur in strains that carry exogenous DNA inserted at centromere 1 chromatin barriers. Abnormal phenotypes are accompanied by changes in the structural integrity of both the centromeric core chromatin domain, containing the conserved CENP-A^Cnp1 protein, and the flanking pericentric heterochromatin domain. Barrier mutant cells can revert to wild-type growth and centromere structure at a high frequency after the spontaneous excision of integrated exogenous DNA. Our results reveal a previously undemonstrated role for chromatin barriers in chromosome segregation and in the prevention of genome instability.

Minichromosome stability induced by partial genome duplication in Arabidopsis thaliana

Two partially reconstructed karyotypes (RK1 and RK2) of Arabidopsis thaliana have been established from a transformant, in which four structurally changed chromosomes (alpha, beta, gamma, and delta) were involved. Both karyotypes are composed of 12 chromosomes, 2n = 1" + 3" + 4" + 5" + alpha" + gamma" = 12 for RK1 and 2n = 3" + 4" + 5" + alpha" + beta" + gamma" = 12 for RK2, and these chromosome constitutions were relatively stable at least for three generations. Pairing at meiosis was limited to the homologues (1, 3, 4, 5, alpha, beta, or gamma), and no pairing occurred among non-homologous chromosomes in both karyotypes. For minichromosome alpha (mini alpha), precocious separation at metaphase I was frequently observed in RK2, as found for other minichromosomes, but was rare in RK1. This stable paring of mini alpha was possibly caused by duplication of the terminal tip of chromosome 1 that is characteristic of RK1.

Small RNAs and RNAi pathways in meiotic prophase I

RNA interference is involved in many aspects of cell biology, and the recent identification of germ-cell specific small RNAs has led to speculation that RNAi might also be involved in gametogenesis. Work in yeast indicates that RNAi is involved in establishing and maintaining heterochromatin at centromeres, an important component of yeast and mammalian meiosis. Here we review developments in the field of RNAi and relate these to possible roles in mammalian gametogenesis.

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.

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.

Kinetochore and heterochromatin domains of the fission yeast centromere

Fission yeast centromeres are composed of two distinctive chromatin domains. The central domain nucleosomes contain the histone H3-like protein CENP-A(Cnp1). In contrast, the flanking repeats are coated with silent chromatin in which Swi6 (HP1) binds histone H3 methylated on lysine 9 that is induced by the action of the RNA interference pathway on non-coding centromeric transcripts. The overall structure is similar to that of metazoan centromeres where the kinetochore is embedded in surrounding heterochromatin. Kinetochore specific proteins associate with the central domain and affect silencing in that region. The flanking heterochromatin is required to recruit cohesin and mediate tight physical cohesion between sister centromeres. The loss of silencing that accompanies defects in heterochromatin has been invaluable as a tool in the investigation of centromere function. Both the heterochromatin and kinetochore regions are required for the de novo assembly of a functional centromere on DNA constructs, suggesting that heterochromatin may provide an environment that promotes kinetochore assembly within the central domain. The process is clearly epigenetically regulated. Fission yeast kinetochores associate with 2-4 microtubules, and flanking heterochromatin may be required to promote the orientation of multiple microtubule binding sites on one kinetochore towards the same pole and thus prevent merotelic orientation.

Sim4: a novel fission yeast kinetochore protein required for centromeric silencing and chromosome segregation

Fission yeast centromeres are composed of two domains: the central core and the outer repeats. Although both regions are required for full centromere function, the central core has a distinct chromatin structure and is likely to underlie the kinetochore itself, as it is associated with centromere-specific proteins. Genes placed within either region are transcriptionally silenced, reflecting the formation of a functional kinetochore complex and flanking centromeric heterochromatin. Here, transcriptional silencing was exploited to identify components involved in central core silencing and kinetochore assembly or structure. The resulting sim (silencing in the middle of the centromere) mutants display severe chromosome segregation defects. sim2+ encodes a known kinetochore protein, the centromere-specific histone H3 variant Cnp1CENP-A. sim4+ encodes a novel essential coiled-coil protein, which is specifically associated with the central core region and is required for the unusual chromatin structure of this region. Sim4 coimmunoprecipitates with the central core component Mis6 and, like Mis6, affects Cnp1CENP-A association with the central domain. Functional Mis6 is required for Sim4 localization at the kinetochore. Our analyses illustrate the fundamental link between silencing, chromatin structure, and kinetochore function, and establish defective silencing as a powerful approach for identifying proteins required to build a functional kinetochore.

Fission yeast CENP-B homologs nucleate centromeric heterochromatin by promoting heterochromatin-specific histone tail modifications

Heterochromatin is a functionally important chromosomal component, especially at centromeres. In fission yeast, conserved heterochromatin-specific modifications of the histone H3 tail, involving deacetylation of Lys 9 and Lys 14 and subsequent methylation of Lys 9, promote the recruitment of a heterochromatin protein, Swi6, a homolog of the Drosophila heterochromatin protein 1. However, the primary determinants of the positioning of heterochromatin are still unclear. The fission yeast proteins Abp1, Cbh1, and Cbh2 are homologs of the human protein CENP-B that bind to centromeric alpha-satellite DNA and associate with centromeric heterochromatin. We show that the CENP-B homologs are functionally redundant at centromeres, and that Abp1 binds specifically to centromeric heterochromatin. In the absence of Abp1 or Cbh1, the centromeric association of Swi6 is diminished, resulting in a decrease in silencing of the region. CENP-B-homolog double disruptants show a synergistic reduction of Swi6 at centromeric heterochromatin, indicating that the three proteins are functionally redundant in the recruitment of Swi6. Furthermore, using chromatin immunoprecipitation assays, we show that disruption of CENP-B homologs causes a decrease in heterochromatin-specific modifications of histone H3. These results indicate that the CENP-B homologs act as site-specific nucleation factors for the formation of centromeric heterochromatin by heterochromatin-specific modifications of histone tails.

Chromatin assembly factor I and Hir proteins contribute to building functional kinetochores in S. cerevisiae

Budding yeast centromeres are comprised of approximately 125-bp DNA sequences that direct formation of the kinetochore, a specialized chromatin structure that mediates spindle attachment to chromosomes. We report here a novel role for the histone deposition complex chromatin assembly factor I (CAF-I) in building centromeric chromatin. The contribution of CAF-I to kinetochore function overlaps that of the Hir proteins, which have also been implicated in nucleosome formation and heterochromatic gene silencing. cacDelta hirDelta double mutant cells lacking both CAF-I and Hir proteins are delayed in anaphase entry in a spindle assembly checkpoint-dependent manner. Further, cacDelta and hirDelta deletions together cause increased rates of chromosome missegregation, genetic synergies with mutations in kinetochore protein genes, and alterations in centromeric chromatin structure. Finally, CAF-I subunits and Hir1 are enriched at centromeres, indicating that these proteins make a direct contribution to centromeric chromatin structures.

The model unicellular eukaryote, Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe has long been a model organism for studies of eukaryotic cells, winning renown especially for studies of the cell cycle. Now that its genome has been sequenced, S. pombe is ready to assume its rightful place in the pantheon of small eukaryotic giants.

The role of Drosophila CID in kinetochore formation, cell-cycle progression and heterochromatin interactions

Centromere function requires the coordination of many processes including kinetochore assembly, sister chromatid cohesion, spindle attachment and chromosome movement. Here we show that CID, the Drosophila homologue of the CENP-A centromere-specific H3-like proteins, colocalizes with molecular-genetically defined functional centromeres in minichromosomes. Injection of CID antibodies into early embryos, as well as RNA interference in tissue-culture cells, showed that CID is required for several mitotic processes. Deconvolution fluorescence microscopy showed that CID chromatin is physically separate from proteins involved in sister cohesion (MEI-S332), centric condensation (PROD), kinetochore function (ROD, ZW10 and BUB1) and heterochromatin structure (HP1). CID localization is unaffected by mutations in mei-S332, Su(var)2-5 (HP1), prod or polo. Furthermore, the localization of POLO, CENP-meta, ROD, BUB1 and MEI-S332, but not PROD or HP1, depends on the presence of functional CID. We conclude that the centromere and flanking heterochromatin are physically and functionally separable protein domains that are required for different inheritance functions, and that CID is required for normal kinetochore formation and function, as well as cell-cycle progression.

Fission yeast homologs of human CENP-B have redundant functions affecting cell growth and chromosome segregation

Two functionally important DNA sequence elements in centromeres of the fission yeast Schizosaccharomyces pombe are the centromeric central core and the K-type repeat. Both of these DNA elements show internal functional redundancy that is not correlated with a conserved DNA sequence. Specific, but degenerate, sequences in these elements are bound in vitro by the S. pombe DNA-binding proteins Abp1p (also called Cbp1p) and Cbhp, which are related to the mammalian centromere DNA-binding protein CENP-B. In this study, we determined that Abp1p binds to at least one of its target sequences within S. pombe centromere II central core (cc2) DNA with an affinity (K(s) = 7 x 10(9) M(-1)) higher than those of other known centromere DNA-binding proteins for their cognate targets. In vivo, epitope-tagged Cbhp associated with centromeric K repeat chromatin, as well as with noncentromeric regions. Like abp1(+)/cbp1(+), we found that cbh(+) is not essential in fission yeast, but a strain carrying deletions of both genes (Deltaabp1 Deltacbh) is extremely compromised in growth rate and morphology and missegregates chromosomes at very high frequency. The synergism between the two null mutations suggests that these proteins perform redundant functions in S. pombe chromosome segregation. In vitro assays with cell extracts with these proteins depleted allowed the specific assignments of several binding sites for them within cc2 and the K-type repeat. Redundancy observed at the centromere DNA level appears to be reflected at the protein level, as no single member of the CENP-B-related protein family is essential for proper chromosome segregation in fission yeast. The relevance of these findings to mammalian centromeres is discussed.

The Schizosaccharomyces pombe hst4(+) gene is a SIR2 homologue with silencing and centromeric functions

Although silencing is a significant form of transcriptional regulation, the functional and mechanistic limits of its conservation have not yet been established. We have identified the Schizosaccharomyces pombe hst4(+) gene as a member of the SIR2/HST silencing gene family that is defined in organisms ranging from bacteria to humans. hst4Delta mutants grow more slowly than wild-type cells and have abnormal morphology and fragmented DNA. Mutant strains show decreased silencing of reporter genes at both telomeres and centromeres. hst4(+) appears to be important for centromere function as well because mutants have elevated chromosome-loss rates and are sensitive to a microtubule-destabilizing drug. Consistent with a role in chromatin structure, Hst4p localizes to the nucleus and appears concentrated in the nucleolus. hst4Delta mutant phenotypes, including growth and silencing phenotypes, are similar to those of the Saccharomyces cerevisiae HSTs, and at a molecular level, hst4(+) is most similar to HST4. Furthermore, hst4(+) is a functional homologue of S. cerevisiae HST3 and HST4 in that overexpression of hst4(+) rescues the temperature-sensitivity and telomeric silencing defects of an hst3Delta hst4Delta double mutant. These results together demonstrate that a SIR-like silencing mechanism is conserved in the distantly related yeasts and is likely to be found in other organisms from prokaryotes to mammals.

Rice (Oryza sativa) centromeric regions consist of complex DNA

Rice bacterial artificial chromosome clones containing centromeric DNA were isolated by using a DNA sequence (pSau3A9) that is present in the centromeres of Gramineae species. Seven distinct repetitive DNA elements were isolated from a 75-kilobase rice bacterial artificial chromosome clone. All seven DNA elements are present in every rice centromere as demonstrated by fluorescence in situ hybridization. Six of the elements are middle repetitive, and their copy numbers range from approximately 50 to approximately 300 in the rice genome. Five of these six middle repetitive DNA elements are present in all of the Gramineae species, and the other element is detected only in species within the Bambusoideae subfamily of Gramineae. All six middle repetitive DNA elements are dispersed in the centromeric regions. The seventh element, the RCS2 family, is a tandem repeat of a 168-bp sequence that is represented approximately 6,000 times in the rice genome and is detected only in Oryza species. Fiber-fluorescence in situ hybridization analysis revealed that the RCS2 family is organized into long uninterrupted arrays and resembles previously reported tandem repeats located in the centromeres of human and Arabidopsis thaliana chromosomes. We characterized a large DNA fragment derived from a plant centromere and demonstrated that rice centromeres consist of complex DNA, including both highly and middle repetitive DNA sequences.

Targeted disruption of mouse centromere protein C gene leads to mitotic disarray and early embryo death

Centromere protein C (CENPC) is a key protein that has been localized to the inner kinetochore plate of active mammalian centromeres. Using gene targeting techniques, we have disrupted the mouse Cenpc gene and shown that the gene is essential for normal mouse embryonic development. Heterozygous mice carrying one functional copy of the gene are healthy and fertile, whereas homozygous embryos fail to thrive. In these embryos, mitotic arrest and gross morphological degeneration become apparent as early as the morula stage of development. The degenerating embryos demonstrate highly irregular cell and nuclear morphologies, including the presence of a large number of micronuclei. Mitotic chromosomes of these embryos display a scattered and often highly condensed configuration and do not segregate in an ordered fashion. These results describing the phenotype of the mutant mouse embryos indicate that CENPC has a direct role in the mitotic progression from metaphase to anaphase.

Centromere DNA dynamics: latent centromeres and neocentromere formation

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Purification and characterization of a CENP-B homologue protein that binds to the centromeric K-type repeat DNA of Schizosaccharomyces pombe

We have purified and characterized a novel 60-kDa protein that binds to centromeric K-type repeat DNA from Schizosaccharomyces pombe. This protein was initially purified by its ability to bind to the autonomously replicating sequence 3002 DNA. Cloning of the gene encoding this protein revealed that it possesses significant homology to the mammalian centromere DNA-binding protein CENP-B and S. pombe Abp1, and this gene was designated as cbh+ (CENP-B homologue). Cbh protein specifically interacts in vitro with the K-type repeat DNA, which is essential for centromere function. The Cbh-binding consensus sequence was determined by DNase I footprinting assays as PyPuATATPyPuTA, featuring an inverted repeat of the first four nucleotides. Based on its binding activity to centromeric DNA and homology to centromere proteins, we suggest that this protein may be a functional homologue of the mammalian CENP-B in S. pombe.

The centromere enhancer mediates centromere activation in Schizosaccharomyces pombe

The centromere enhancer is a functionally important DNA region within the Schizosaccharomyces pombe centromeric K-type repeat. We have previously shown that addition of the enhancer and cen2 centromeric central core to a circular minichromosome is sufficient to impart appreciable centromere function. A more detailed analysis of the enhancer shows that it is dispensable for centromere function in a cen1-derived minichromosome containing the central core and the remainder of the K-type repeat, indicating that the critical centromeric K-type repeat, like the central core, is characterized by functional redundancy. The centromeric enhancer is required, however, for a central core-carrying minichromosome to exhibit immediate centromere activity when the circular DNA is introduced via transformation into S. pombe. This immediate activation is probably a consequence of a centromere-targeted epigenetic system that governs the chromatin architecture of the region. Moreover, our studies show that two entirely different DNA sequences, consisting of elements derived from two native centromeres, can display centromere function. An S. pombe CENP-B-like protein, Abp1p/Cbp1p, which is required for proper chromosome segregation in vivo, binds in vitro to sites within and adjacent to the modular centromere enhancer, as well as within the centromeric central cores. These results provide direct evidence in fission yeast of a model, similar to one proposed for mammalian systems, whereby no specific sequence is necessary for centromere function but certain classes of sequences are competent to build the appropriate chromatin foundation upon which the centromere/kinetochore can be formed and activated.

A centromere DNA-binding protein from fission yeast affects chromosome segregation and has homology to human CENP-B

Genetic and biochemical strategies have been used to identify Schizosaccharomyces pombe proteins with roles in centromere function. One protein, identified by both approaches, shows significant homology to the human centromere DNA-binding protein, CENP-B, and is identical to Abp1p (autonomously replicating sequence-binding protein 1) (Murakami, Y., J.A. Huberman, and J. Hurwitz. 1996. Proc. Natl. Acad. Sci. USA. 93:502-507). Abp1p binds in vitro specifically to at least three sites in centromeric central core DNA of S. pombe chromosome II (cc2). Overexpression of abp1 affects mitotic chromosome stability in S. pombe. Although inactivation of the abp1 gene is not lethal, the abp1 null strain displays marked mitotic chromosome instability and a pronounced meiotic defect. The identification of a CENP-B-related centromere DNA-binding protein in S. pombe strongly supports the hypothesis that fission yeast centromeres are structurally and functionally related to the centromeres of higher eukaryotes.

Replication of centromere II of Schizosaccharomyces pombe

The centromeric DNAs of Schizosaccharomyces pombe chromosomes resemble those of higher eukaryotes in being large and composed predominantly of repeated sequences. To begin a detailed analysis of the mode of replication of a complex centromere, we examined whether any sequences within S. pombe centromere II (cen2) have the ability to mediate autonomous replication. We found a high density of segments with such activity, including at least eight different regions comprising most of the repeated and unique centromeric DNA elements. A physical mapping analysis using two-dimensional gels showed that autonomous replication initiated within the S. pombe sequences in each plasmid. A two-dimensional gel analysis of replication on the chromosomes revealed that the K and L repeat elements, which occur in multiple copies at all three centromeres and comprise approximately 70% of total centromeric DNA mass in S. pombe, are both sites of replication initiation. In contrast, the unique cen2 central core, which contains multiple segments that can support autonomous replication, appears to be repressed for initiation on the chromosome. We discuss the implications of these findings for our understanding of DNA replication and centromere function.

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.

A novel cis-acting centromeric DNA element affects S. pombe centromeric chromatin structure at a distance

The chromatin structure of the central core region of Schizosaccharomyces pombe centromeric DNA is unusual. This distinctive chromatin structure is associated only with central core sequences in a functional context and is modulated by a novel cis-acting DNA element (centromere enhancer) within the functionally critical K centromeric repeat, which is found in multiple copies in all three S. pombe centromeres. The centromere enhancer alters central core chromatin structure from a distance and in an orientation-independent manner without altering the nucleosomal packaging of sequences between the enhancer and the central core. These findings suggest a functionally relevant structural interaction between the enhancer and the centromeric central core brought about by DNA looping.

Fission yeast minichromosome loss mutants mis cause lethal aneuploidy and replication abnormality

Precise chromosome transmission in cell division cycle is maintained by a number of genes. The attempt made in the present study was to isolate temperature-sensitive (ts) fission yeast mutants that display high loss rates of minichromosomes at permissive or semipermissive temperature (designated mis). By colony color assay of 539 ts strains that contain a minichromosome, we have identified 12 genetic loci (mis1-mis12) and determined their phenotypes at restrictive temperature. Seven of them are related to cell cycle block phenotype at restrictive temperature, three of them in mitosis. Unequal distribution of regular chromosomes in the daughters is extensive in mis6 and mis12. Cells become inviable after rounds of cell division due to missegregation. The phenotype of mis5 is DNA replication defect and hypersensitivity to UV ray and hydroxyurea. mis5+ encodes a novel member of the ubiquitous MCM family required for the onset of replication. The mis5+ gene is essential for viability and functionally distinct from other previously identified members in fission yeast, cdc21+, nda1+, and nda4+. The mis11 mutant phenotype was the cell division block with reduced cell size. Progression of the G1 and G2 phases is blocked in mis11. The cloned mis11+ gene is identical to prp2+, which is essential for RNA splicing and similar to a mammalian splicing factor U2AF65.

The centromeric K-type repeat and the central core are together sufficient to establish a functional Schizosaccharomyces pombe centromere

The DNA requirements for centromere function in fission yeast have been investigated using a minichromosome assay system. Critical elements of Schizosaccharomyces pombe centromeric DNA are portions of the centromeric central core and sequences within a 2.1-kilobase segment found on all three chromosomes as part of the K-type (K/K"/dg) centromeric repeat. The S. pombe centromeric central core contains DNA sequences that appear functionally redundant, and the inverted repeat motif that flanks the central core in all native fission yeast centromeres is not essential for centromere function in circular minichromosomes. Tandem copies of centromeric repeat K", in conjunction with the central core, exert an additive effect on centromere function, increasing minichromosome mitotic stability with each additional copy. Centromeric repeats B and L, however, and parts of the central core and its core-associated repeat are dispensable and cannot substitute for K-type sequences. Several specific protein binding sites have been identified within the centromeric K-type repeat, consistent with a recently proposed model for centromere/kinetochore function in S. pombe.

Centromeres of the fission yeast Schizosaccharomyces pombe are highly variable genetic loci

Gross variations in the structure of the centromere of Schizosaccharomyces pombe chromosome III (cen3) were apparent following characterization of this centromeric DNA in strain Sp223 and comparison of the structure with that of cen3 in three other commonly used laboratory strains. Further differences in centromere structure were revealed when the structure of the centromere of S. pombe chromosome II (cen2) was compared among common laboratory strains and when the structures of cen2 and cen3 from our laboratory strains were compared with those reported from other laboratories. Differences observed in cen3 structure include variations in the arrangement of the centromeric K repeats and an inverted orientation of the conserved centromeric central core. In addition, we have identified two laboratory strains that contain a minimal cen2 repeat structure that lacks the tandem copies of the cen2-specific block of K-L-B-J repeats characteristic of Sp223 cen2. We have also determined that certain centromeric DNA structural motifs are relatively conserved among the four laboratory strains and eight additional wild-type S. pombe strains isolated from various food and beverage sources. We conclude that in S. pombe, as in higher eukaryotes, the centromere of a particular chromosome is not a defined genetic locus but can contain significant variability. However, the basic DNA structural motif of a central core immediately flanked by inverted repeats is a common parameter of the S. pombe centromere.

Centromeres of the fission yeast Schizosaccharomyces pombe are highly variable genetic loci

Gross variations in the structure of the centromere of Schizosaccharomyces pombe chromosome III (cen3) were apparent following characterization of this centromeric DNA in strain Sp223 and comparison of the structure with that of cen3 in three other commonly used laboratory strains. Further differences in centromere structure were revealed when the structure of the centromere of S. pombe chromosome II (cen2) was compared among common laboratory strains and when the structures of cen2 and cen3 from our laboratory strains were compared with those reported from other laboratories. Differences observed in cen3 structure include variations in the arrangement of the centromeric K repeats and an inverted orientation of the conserved centromeric central core. In addition, we have identified two laboratory strains that contain a minimal cen2 repeat structure that lacks the tandem copies of the cen2-specific block of K-L-B-J repeats characteristic of Sp223 cen2. We have also determined that certain centromeric DNA structural motifs are relatively conserved among the four laboratory strains and eight additional wild-type S. pombe strains isolated from various food and beverage sources. We conclude that in S. pombe, as in higher eukaryotes, the centromere of a particular chromosome is not a defined genetic locus but can contain significant variability. However, the basic DNA structural motif of a central core immediately flanked by inverted repeats is a common parameter of the S. pombe centromere.

Cell cycle-dependent specific positioning and clustering of centromeres and telomeres in fission yeast

Fluorescence in situ hybridization (FISH) shows that fission yeast centromeres and telomeres make up specific spatial arrangements in the nucleus. Their positioning and clustering are cell cycle regulated. In G2, centromeres cluster adjacent to the spindle pole body (SPB), while in mitosis, their association with each other and with the SPB is disrupted. Similarly, telomeres cluster at the nuclear periphery in G2 and their associations are disrupted in mitosis. Mitotic centromeres interact with the spindle. They remain undivided until the spindle reaches a critical length, then separate and move towards the poles. This demonstrated, for the first time, that anaphase A occurs in fission yeast. The mode of anaphase A and B is similar to that of higher eukaryotes. In nda3 and cut7 mutants defective in tubulin of a kinesin-related motor, cells are blocked in early stages of mitosis due to the absence of the spindle, and centromeres dissociate but remain close to the SPB, whereas in a metaphase-arrested nuc2 mutant, they reside at the middle of the spindle. FISH is therefore a powerful tool for analyzing mitotic chromosome movement and disjunction using various mutants. Surprisingly, in top2 defective in DNA topoisomerase II, while most chromatid DNAs remain undivided, sister centromeres are separated. Significance of this finding is discussed. In contrast, most chromatid DNAs are separated but telomeric DNAs are not in cut1 mutant. In cut1, the dependence of SPB duplication on the completion of mitosis is abolished. In crm1 mutant cells defective in higher-order chromosome organization, the interphase arrangements of centromeres and telomeres are disrupted.

The SEG1 element: a new DNA region promoting stable mitotic segregation of plasmids in the zygomycete Absidia glauca

A series of new vectors for the model zygomycete Absidia glauca was constructed on the basis of the structural neomycin resistance (Neor) gene controlled by the promoter of the gene for elongation factor 1 (TEF). In order to select for transformed colonies with a stable Neor phenotype, spores from primary transformants were pooled and grown for two sporulation cycles under non-selective conditions. Southern blot analysis of DNA from single spore isolates originating from independent transformant pools allowed the identification of two autonomously replicating plasmids. Retransformation of Escherichia coli and restriction analysis of the two plasmids provided evidence for spontaneous in vivo insertion of a new DNA element (SEG1) from the A. glauca genome. The inserted regions in both plasmids are essentially identical and do not represent repetitive DNA. Compared with other autonomously replicating vectors, these SEG1-containing plasmids are mitotically extremely stable and are passed on to the vegetative spore progeny of a retransformed A. glauca strain. We assume that SEG1 contains structural elements involved in partitioning and stable segregation of plasmids. For the construction of stable transformants of A. glauca, the SEG1 element may be regarded as a major breakthrough, because stabilization of transformed genetic traits by integration is difficult to achieve in all mucoraceous fungi and all known replicating plasmids are mitotically unstable.

A low copy number central sequence with strict symmetry and unusual chromatin structure in fission yeast centromere

Fission yeast centromeres vary in size but are organized in a similar fashion. Each consists of two distinct domains, namely, the approximately 15-kilobase (kb) central region (cnt+imr), containing chromosome-specific low copy number sequences, and 20- to 100-kb outer surrounding sequences (otr) with highly repetitive motifs common to all centromeres. The central region consists of an inner asymmetric sequence flanked by inverted repeats that exhibit strict identity with each other. Nucleotide changes in the left repeat are always accompanied with the same changes in the right. The chromatin structure of the central region is unusual. A nucleosomal nuclease digestion pattern formed on unstable plasmids but not on stable chromosome. DNase I hypersensitive sites correlate with the location of tRNA genes in the central region. Autonomously replicating sequences are also present in the central region. The behavior of truncated minichromosomes suggested that the central region is essential, but not sufficient, to confer transmission stability. A portion of the outer repetitive region is also required. A larger outer region is necessary to ensure correct meiotic behavior. Fluorescence in situ hybridization identified individual cens. In the interphase, they cluster near the nuclear periphery. The central sequence (cnt+imr) may play a role in positioning individual chromosomes within the nucleus, whereas the outer regions (otr) may interact with each other to form the higher-order complex structure.

Centromere protein B assembles human centromeric alpha-satellite DNA at the 17-bp sequence, CENP-B box

We purified 15,000-fold from HeLa cell nuclear extract the centromere antigen that reacts specifically with the 17-bp sequence, designated previously as CENP-B box, in human centromeric alpha-satellite (alphoid) DNA by a two-step procedure including an oligonucleotide affinity column. The purified protein was identified as the centromere protein B (CENP-B) by its mobility on SDS-PAGE (80 kD), and reactivities to a monoclonal antibody raised to CENP-B (bacterial fusion protein) and to anticentromere sera from patients with autoimmune diseases. Direct binding by CENP-B of the CENP-B box sequence in the alphoid DNA has been proved using the purified CENP-B by DNA mobility-shift assay, Southwestern blotting, and DNase I protection analysis. The binding constant of the antigen to the CENP-B box sequence is 6 x 10(8) M-1. DNA mobility-shift assays indicated that the major complex formed between the CENP-B and the DNA contains two DNA molecules, suggesting the importance of the CENP-B/CENP-B box interaction in organization of higher ordered chromatin structures in the centromere and/or kinetochore. Location of DNA binding and dimerization domains in CENP-B was discussed based on the DNA mobility-shift assays performed with a protein fraction containing intact and partial cleavage products of CENP-B.