Telomere-led premeiotic chromosome movement in fission yeast

The leptotene-zygotene transition of meiosis

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

Localization of the 26S proteasome during mitosis and meiosis in fission yeast

The 26S proteasome is a large multisubunit complex involved in degrading both cytoplasmic and nuclear proteins. We have investigated the localization of this complex in the fission yeast, Schizosaccharomyces pombe. Immunofluorescence microscopy shows a striking localization pattern whereby the proteasome is found predominantly at the nuclear periphery, both in interphase and throughout mitosis. Electron microscopic analysis revealed a concentration of label near the inner side of the nuclear envelope. The localization of green fluorescent protein (GFP)-tagged 26S proteasomes was analyzed in live cells during mitosis and meiosis. Throughout mitosis the proteasome remained predominantly at the nuclear periphery. During meiosis the proteasome was found to undergo dramatic changes in its localization. Throughout the first meiotic division, the signal is more dispersed over the nucleus. During meiosis II, there was a dramatic re-localization, and the signal became restricted to the area between the separating DNA until the end of meiosis when the signal dispersed before returning to the nuclear periphery during spore formation. These findings strongly imply that the nuclear periphery is a major site of protein degradation in fission yeast both in interphase and throughout mitosis. Furthermore they raise interesting questions as to the spatial organization of protein degradation during meiosis.

Dynamics of centromeres during metaphase-anaphase transition in fission yeast: Dis1 is implicated in force balance in metaphase bipolar spindle

In higher eukaryotic cells, the spindle forms along with chromosome condensation in mitotic prophase. In metaphase, chromosomes are aligned on the spindle with sister kinetochores facing toward the opposite poles. In anaphase A, sister chromatids separate from each other without spindle extension, whereas spindle elongation takes place during anaphase B. We have critically examined whether such mitotic stages also occur in a lower eukaryote, Schizosaccharomyces pombe. Using the green fluorescent protein tagging technique, early mitotic to late anaphase events were observed in living fission yeast cells. S. pombe has three phases in spindle dynamics, spindle formation (phase 1), constant spindle length (phase 2), and spindle extension (phase 3). Sister centromere separation (anaphase A) rapidly occurred at the end of phase 2. The centromere showed dynamic movements throughout phase 2 as it moved back and forth and was transiently split in two before its separation, suggesting that the centromere was positioned in a bioriented manner toward the poles at metaphase. Microtubule-associating Dis1 was required for the occurrence of constant spindle length and centromere movement in phase 2. Normal transition from phase 2 to 3 needed DNA topoisomerase II and Cut1 but not Cut14. The duration of each phase was highly dependent on temperature.

Faithful anaphase is ensured by Mis4, a sister chromatid cohesion molecule required in S phase and not destroyed in G1 phase

The loss of sister chromatid cohesion triggers anaphase spindle movement. The budding yeast Mcd1/Scc1 protein, called cohesin, is required for associating chromatids, and proteins homologous to it exist in a variety of eukaryotes. Mcd1/Scc1 is removed from chromosomes in anaphase and degrades in G1. We show that the fission yeast protein, Mis4, which is required for equal sister chromatid separation in anaphase is a different chromatid cohesion molecule that behaves independent of cohesin and is conserved from yeast to human. Its inactivation in G1 results in cell lethality in S phase and subsequent premature sister chromatid separation. Inactivation in G2 leads to cell death in subsequent metaphase-anaphase progression but missegregation occurs only in the next round of mitosis. Mis4 is not essential for condensation, nor does it degrade in G1. Rather, it associates with chromosomes in a punctate fashion throughout the cell cycle. mis4 mutants are hypersensitive to hydroxyurea (HU) and UV irradiation but retain the ability to restrain cell cycle progression when damaged or sustaining a block to replication. The mis4 mutation results in synthetic lethality with a DNA ligase mutant. Mis4 may form a stable link between chromatids in S phase that is split rather than removed in anaphase.

Two modes of survival of fission yeast without telomerase

Deletion of the telomerase catalytic subunit gene trt1+ in Schizosaccharomyces pombe results in death for the majority of cells, but a subpopulation survives. Here it is shown that most survivors have circularized all of their chromosomes, whereas a smaller number maintain their telomeres presumably through recombination. When the telomeric DNA-binding gene taz1+ is also deleted, trt1- taz1- survivors use the recombinational mode more frequently. Moreover, the massive elongation of telomeres in taz1- cells is absent in the double mutant. Thus, Taz1p appears to regulate telomeric recombination as well as telomerase activity in fission yeast.

RNA-assisted nuclear transport of the meiotic regulator Mei2p in fission yeast

Fission yeast Mei2p is an RNA-binding protein required for both premeiotic DNA synthesis and meiosis I. Mei2p binds to a polyadenylated RNA molecule, meiRNA, loss of which blocks meiosis I. Mei2p forms a dot in meiotic prophase nuclei. Here, we show that meiRNA is required for the nuclear localization of Mei2p and is detectable in the dot. However, Mei2p carrying a nuclear localization signal can produce a nuclear dot and promote meiosis I in the absence of meiRNA. Mei2p expressed in cultured mammalian cells stays in the cytoplasm, but it accumulates in the nucleolus if meiRNA is coexpressed. These results indicate that meiRNA contributes to the promotion of meiosis I exclusively as a cofactor that assists nuclear transport of Mei2p.

Circular chromosome formation in a fission yeast mutant defective in two ATM homologues

Telomeres, found at chromosomal ends, are essential for stable maintenance of linear chromosomes in eukaryotes. The ATM family of genes, including budding yeast TEL1 (refs 1,2), fission yeast rad3+ (ref. 3) and human ATM (ref. 4), have been reported to be involved in telomere length regulation, although the significance of the telomere phenotypes observed with the mutated genes remains elusive. We have cloned tel1+, another fission yeast ATM homologue, and found that a tel1rad3 double mutant lost all telomeric DNA sequences. Thus, the ATM homologues are essential in telomere maintenance. The mutant grew poorly and formed irregular-shaped colonies, probably due to chromosome instability, however, during prolonged culture of the double mutant, cells forming normal round-shaped colonies arose at a relatively high frequency. All three chromosomes in these derivative cells were circular and lacked telomeric sequences. To our knowledge, this is the first report of eukaryotic cells whose chromosomes are all circular. Upon meiosis, these derivative cells produced few viable spores. Therefore, the exclusively circular genome lacking telomeric sequences is proficient for mitotic growth, but does not permit meiosis.

cut11(+): A gene required for cell cycle-dependent spindle pole body anchoring in the nuclear envelope and bipolar spindle formation in Schizosaccharomyces pombe

The "cut" mutants of Schizosaccharomyces pombe are defective in spindle formation and/or chromosome segregation, but they proceed through the cell cycle, resulting in lethality. Analysis of temperature-sensitive alleles of cut11(+) suggests that this gene is required for the formation of a functional bipolar spindle. Defective spindle structure was revealed with fluorescent probes for tubulin and DNA. Three-dimensional reconstruction of mutant spindles by serial sectioning and electron microscopy showed that the spindle pole bodies (SPBs) either failed to complete normal duplication or were free floating in the nucleoplasm. Localization of Cut11p tagged with the green fluorescent protein showed punctate nuclear envelope staining throughout the cell cycle and SPBs staining from early prophase to mid anaphase. This SPB localization correlates with the time in the cell cycle when SPBs are inserted into the nuclear envelope. Immunoelectron microscopy confirmed the localization of Cut11p to mitotic SPBs and nuclear pore complexes. Cloning and sequencing showed that cut11(+) encodes a novel protein with seven putative membrane-spanning domains and homology to the Saccharomyces cerevisiae gene NDC1. These data suggest that Cut11p associates with nuclear pore complexes and mitotic SPBs as an anchor in the nuclear envelope; this role is essential for mitosis.

The role of topoisomerase II in meiotic chromosome condensation and segregation in Schizosaccharomyces pombe

Topoisomerase II is able to break and rejoin double-strand DNA. It controls the topological state and forms and resolves knots and catenanes. Not much is known about the relation between the chromosome segregation and condensation defects as found in yeast top2 mutants and the role of topoisomerase II in meiosis. We studied meiosis in a heat-sensitive top2 mutant of Schizosaccharomyces pombe. Topoisomerase II is not required until shortly before meiosis I. The enzyme is necessary for condensation shortly before the first meiotic division but not for early meiotic prophase condensation. DNA replication, prophase morphology, and dynamics of the linear elements are normal in the top2 mutant. The top2 cells are not able to perform meiosis I. Arrested cells have four spindle pole bodies and two spindles but only one nucleus, suggesting that the arrest is nonregulatory. Finally, we show that the arrest is partly solved in a top2 rec7 double mutant, indicating that topoisomerase II functions in the segregation of recombined chromosomes. We suggest that the inability to decatenate the replicated DNA is the primary defect in top2. This leads to a loss of chromatin condensation shortly before meiosis I, failure of sister chromatid separation, and a nonregulatory arrest.

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.

Telomere structure, replication and length maintenance

Telomeres are the termini of linear eukaryotic chromosomes consisting of tandem repeats of DNA and proteins that bind to these repeat sequences. Telomeres ensure the complete replication of chromosome ends, impart protection to ends from nucleolytic degradation, end-to-end fusion, and guide the localization of chromosomes within the nucleus. In addition, a combination of genetic, biochemical, and molecular biological approaches have implicated key roles for telomeres in diverse cellular processes such as regulation of gene expression, cell division, cell senescence, and cancer. This review focuses on recent advances in our understanding of the organization of telomeres, telomere replication, proteins that bind telomeric DNA, and the establishment of telomere length equilibrium.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Telomere-mediated chromosome pairing during meiosis in budding yeast

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

Aspects of three-dimensional chromosome reorganization during the onset of human male meiotic prophase

The three-dimensional morphology and distribution of human chromosomes 3 were studied in nuclei of spermatogonia and spermatocytes I from formaldehyde-fixed human testis sections. Chromosome arms, pericentromeres and telomeric regions were painted by a three-color, five-probe fluorescence in situ hybridization protocol. Light optical serial sections of premeiotic and meiotic nuclei obtained by confocal laser scanning microscopy revealed that premeiotic chromosomes 3 are separate from each other and occupy variably shaped territories, which are sectored in distinct 3 p- and q-arm domains. Three-dimensional reconstructions of the painted chromosome domains by a Voronoi tessellation approach showed that mean chromosome volumes did not differ significantly among the premeiotic and meiotic stages investigated. A significant increase in surface area and reduction of dimensionless ‘roundness factor’ estimates of arm domains indicated that the restructuring of spatially separate chromosome territories initiates during preleptotene. Telomeric regions, which in meiotic stem cells located predominantly in arm-domain chromatin, showed a redistribution towards the domain surface during this stage. At leptotene homologues were generally misaligned and displayed intimate intermingling of non-homologous chromatin. Pairing initiated at the ends of bent zygotene chromosomes, which displayed a complex surface structure with discernible sister chromatids. The results indicate that, in mammals, homology search is executed during leptotene, after remodeling of chromosome territories.

Pleiotropic effects of HIV-1 protein R (Vpr) on morphogenesis and cell survival in fission yeast and antagonism by pentoxifylline

Expression of HIV-1 Vpr causes cell cycle G2 arrest, change in cell shape, and cell death over a large evolutionary distance ranging from human to yeast cells. As a step toward understanding these highly conserved Vpr functions, we have examined the effect of Vpr on cytoskeletal elements and the viability of fission yeast. We demonstrate that the changes in cell morphology induced by Vpr in fission yeast are caused by several underlying cellular abnormalities, including increased biosynthesis of chitin in the cell wall, disruption of the actin cytoskeleton, and altered polarity for cell growth. The extent of these cellular alterations and cell survival correlates with the level of vpr expression. Accompanying cell death, Vpr induces aberrant nuclear morphologies in fission yeast which are similar to those found during the apoptosis induced by Vpr in mammalian cells. The Vpr-induced cytopathic effects and cell death can be suppressed by treatment with pentoxifylline, a compound that inhibits HIV-1 viral replication and suppresses Vpr-induced cell cycle G2 arrest in human and fission yeast cells. The results presented here suggest that pentoxifylline suppresses the effects of Vpr by blocking interactions of Vpr with cellular proteins. Given that pentoxifylline has potential therapeutic value in blocking the effects of Vpr in HIV-infected patients, understanding the molecular mechanisms by which pentoxifylline antagonizes Vpr may have general implications for HIV therapy.

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.

The fission yeast microtubule cytoskeleton

The Schizosaccharomyces pombe genome sequencing project (http://www.sanger.ac.uk/Projects/S_pombe/) is nearly complete, and this is likely to generate interest in fission yeast as a model system beyond its traditional strongholds in the study of the cell cycle and sexual differentiation. In many fields S. pombe will offer a useful complement to the more widely studied Saccharomyces cerevisiae, but in some areas the impact of S. pombe may well rival or exceed that of this budding yeast in terms of relevance to higher systems. Because of the considerable differences from the S. cerevisiae microtubule cytoskeleton, studying microtubules in S. pombe is likely to enhance the contribution of model systems to our understanding of the principles and practices of microtubule organisation in eukaryotes in general.

Organelle transport and molecular motors in fungi

Polarized growth, secretion of exoenzymes, organelle inheritance, and organelle positioning require vectorial transport along cytoskeletal elements. The discovery of molecular motors and intensive studies on their biological function during the past 3 years confirmed a central role of these mechanoenzymes in morphogenesis and development of yeasts and filamentous fungi. Saccharomyces cerevisiae proved to be an excellent model system, in which the complete set of molecular motors is presumed to be known. Genetic studies combined with cell biological methods revealed unexpected functional relationships between these motors and has greatly improved our understanding of nuclear migration, exocytosis, and endocytosis in yeasts. Tip growth of elongated hyphae, compared to budding, however, does require vectorial transport over long distances. The identification of ubiquitous motors that are not present in yeast indicates that studies on filamentous fungi might be helpful to elucidate the role of motors in long-distance organelle transport within higher eukaryotic cells. Copyright 1998 Academic Press.

FH3, a domain found in formins, targets the fission yeast formin Fus1 to the projection tip during conjugation

Formins are involved in diverse aspects of morphogenesis, and share two regions of homology: FH1 and FH2. We describe a new formin homology region, FH3. FH3 is an amino-terminal domain that differs from the Rho binding site identified in Bni1p and p140mDia. The Schizosaccharomyces pombe formin Fus1 is required for conjugation, and is localized to the projection tip in cells of mating pairs. We replaced genomic fus1+ with green fluorescent protein (GFP)- tagged versions that lacked either the FH1, FH2, or FH3 domain. Deletion of any FH domain essentially abolished mating. FH3, but neither FH1 nor FH2, was required for Fus1 localization. An FH3 domain-GFP fusion protein localized to the projection tips of mating pairs. Thus, the FH3 domain alone can direct protein localization. The FH3 domains of both Fus1 and the S. pombe cytokinesis formin Cdc12 were able to localize GFP to the spindle pole body in half of the late G2 cells in a vegetatively growing population. Expression of both FH3-GFP fusions also affected cytokinesis. Overexpression of the spindle pole body component Sad1 altered the distribution of both Sad1 and the FH3-GFP domain. Together these data suggest that proteins at multiple sites can interact with FH3 domains.

Cut1 is loaded onto the spindle by binding to Cut2 and promotes anaphase spindle movement upon Cut2 proteolysis

BACKGROUND

The Cut1 and Cut2 proteins of the fission yeast Schizosaccharomyces pombe form a complex and are required for the separation of sister chromatids during anaphase. Polyubiquitinated Cut2 degrades at the onset of anaphase and this degradation, like that of mitotic cyclin, is dependent on the anaphase-promoting complex/cyclosome. Expression of Cut2 that cannot be degraded blocks sister chromatid separation and anaphase spindle elongation. Here, we have investigated the role of the Cut1-Cut2 interaction in sister chromatid separation.

RESULTS

The carboxyl terminus of Cut2 interacts with the amino terminus of Cut1, and temperature-sensitive Cut2 mutants expressed Cut2 proteins that contain substitutions in the carboxyl terminus and fail to interact with Cut1, resulting in aberrant anaphase. Localization of Cut1 alters dramatically during the cell cycle. Cut1 is retained in the cytoplasm during interphase and moves to the mitotic spindle pole bodies and the spindle upon entry into prophase, when spindles are formed. The association between Cut2 and Cut1 is needed for the localization of Cut1 to the spindles, as Cut1 remains unbound to the spindle if complex formation is impaired. Cut2 degrades during anaphase, but Cut1 remains bound to the anaphase spindle. This association with the anaphase spindle requires the conserved carboxyl terminus of Cut1.

CONCLUSIONS

Complex formation between Cut1 and Cut2 is needed for the onset of normal anaphase. Cut2 is required for loading Cut1 onto the spindle at prophase and Cut2 proteolysis is needed for the active participation of Cut1 in sister chromatid separation.

Fission yeast Taz1 protein is required for meiotic telomere clustering and recombination

The alignment of homologous chromosomes during meiosis is essential for their recombination and segregation. Telomeres form and protect the ends of eukaryotic linear chromosomes, and are composed of tandem repeats of a simple DNA sequence and the proteins that bind to these repeats. A role for telomeres in meiosis was suspected from observations of telomere clustering in meiotic cells, and has now been supported experimentally by the dramatic rearrangement of telomere locations during premeiotic stages in fission yeast. Here we show that the fission yeast telomere protein, Taz1, is required for stable association between telomeres and spindle pole bodies during meiotic prophase. In the absence of Taz1, telomere clustering at the spindle pole bodies is disrupted, meiotic recombination is reduced, and both spore viability and the ability of zygotes to re-enter mitosis are impaired to a level that would be expected if chromosome segregation were occurring randomly. Such telomeric association mediated by telomere-specific proteins may also be important for proper chromosome alignment and recombination during meiosis in humans.

Defective meiosis in telomere-silencing mutants of Schizosaccharomyces pombe

During meiotic prophase, chromosomes frequently adopt a bouquet-like arrangement, with their telomeres clustered close to the nuclear periphery. A dramatic example of this occurs in the fission yeast, Schizosaccharomyces pombe, where all telomeres aggregate adjacent to the spindle pole body (SPB). Nuclei then undergo rapid traverses of the cell, known as 'horsetail' movement, which is led by the SPB dragging telomeres and chromosomes behind. This process may initiate or facilitate chromosome pairing before recombination and meiosis. With the aim of identifying components involved in telomere structure and function, we report here the isolation of S. pombe mutants defective in the ability to impose transcriptional silencing on genes placed near telomeres. Two of these mutants, lot2-s17 and lot3-uv3, also display a dramatic lengthening of telomeric repeats. lot3-uv3 carries a mutation in Taz1, a telomere-binding protein containing a Myb-like motif similar to two human telomere-binding proteins. Meiosis is aberrant in these mutant yeast strains, and our analysis demonstrates a decreased association of telomeres with the SPB in meiotic prophase. This results in defective 'horsetail' movement, a significant reduction in recombination, low spore viability and chromosome missegregation through meiosis.

Homologous chromosome pairing in Drosophila melanogaster proceeds through multiple independent initiations

The dynamics by which homologous chromosomes pair is currently unknown. Here, we use fluorescence in situ hybridization in combination with three-dimensional optical microscopy to show that homologous pairing of the somatic chromosome arm 2L in Drosophila occurs by independent initiation of pairing at discrete loci rather than by a processive zippering of sites along the length of chromosome. By evaluating the pairing frequencies of 11 loci on chromosome arm 2L over several timepoints during Drosophila embryonic development, we show that all 11 loci are paired very early in Drosophila development, within 13 h after egg deposition. To elucidate whether such pairing occurs by directed or undirected motion, we analyzed the pairing kinetics of histone loci during nuclear cycle 14. By measuring changes of nuclear length and correlating these changes with progression of time during cycle 14, we were able to express the pairing frequency and distance between homologous loci as a function of time. Comparing the experimentally determined dynamics of pairing to simulations based on previously proposed models of pairing motion, we show that the observed pairing kinetics are most consistent with a constrained random walk model and not consistent with a directed motion model. Thus, we conclude that simple random contacts through diffusion could suffice to allow pairing of homologous sites.

F-actin distribution and function during sexual differentiation in Schizosaccharomyces pombe

Sexual differentiation in Schizosaccharomyces pombe is induced from the G1 phase of the cell cycle by nitrogen starvation and the presence of mating pheromones. We describe the distribution of F-actin during sexual differentiation. Cortical F-actin dots have previously been shown to be restricted to one end of the rod shaped cell during the G1 phase of the cell cycle. Within half an hour of nitrogen starvation the distribution of cortical F-actin dots switched from being monopolar to bipolar. This was then reversed as the F-actin cytoskeleton repolarized so that cortical F-actin dots accumulated towards the projection tip at one end of the cell. Following cell fusion, F-actin dots were randomly scattered during the horsetail movement that precedes meiosis I and remained scattered until prometaphase or metaphase of meiosis II, when they concentrated around the nucleus. F-actin was seen on the lagging face of the nuclei which faced the partner nucleus during anaphase B of meiosis II. Early on in this anaphase F-actin was also seen on the opposite side of the nucleus, near the spindle pole body. F-actin accumulated within the spores in the mature ascus. Treatment with the actin depolymerising drug Latrunculin A showed that F-actin is required for cell fusion and spore formation. Latrunculin A treatment extended all stages from karyogamy to meiosis I. The S. pombe homologue of the actin binding protein profilin, Cdc3, was shown to be required for conjugation. Cdc3 co-localized with the formin related molecule Fus1 at the projection tip. The polarization of F-actin cortical dots to the projection tip was unaffected in the cdc3.124 mutant, but cdc3.124 mutant cells were unable to break down the cell walls between the two cells following agglutination.

Oscillatory nuclear movement in fission yeast meiotic prophase is driven by astral microtubules, as revealed by continuous observation of chromosomes and microtubules in living cells

Using a computerized fluorescence microscope system to observe fluorescently stained cellular structures in vivo, we have examined the dynamics of chromosomes and microtubules during the process of meiosis in the fission yeast Schizosaccharomyces pombe. Fission yeast meiotic prophase is characterized by a distinctive type of nuclear movement that is led by telomeres clustered at the spindle-pole body (the centrosome-equivalent structure in fungi): the nucleus oscillates back and forth along the cell axis, moving continuously between the two ends of the cell for some hours prior to the meiotic divisions. To obtain a dynamic view of this oscillatory nuclear movement in meiotic prophase, we visualized microtubules and chromosomes in living cells using jellyfish green fluorescent protein fused with alpha-tubulin and a DNA-specific fluorescent dye, Hoechst 33342, respectively. Continuous observation of chromosomes and microtubules in these cells demonstrated that the oscillatory nuclear movement is mediated by dynamic reorganization of astral microtubules originating from the spindle-pole body. During each half-oscillatory period, the microtubules extending rearward from the leading edge of the nucleus elongate to drive the nucleus to one end of the cell. When the nucleus reversed direction, its motion during the second half of the oscillation was not driven by the same microtubules that drove its motion during the first half, but rather by newly assembled microtubules. Reversible inhibition of nuclear movement by an inhibitor of microtubule polymerization, thiabendazole, confirmed the involvement of astral microtubules in oscillatory nuclear movement. The speed of the movement fluctuated within a range 0 to 15 micron/minute, with an average of about 5 microm/minute. We propose a model in which the oscillatory nuclear movement is mediated by dynamic instability and selective stabilization of astral microtubules.

Cell cycle-dependent duplication and bidirectional migration of SeqA-associated DNA-protein complexes in E. coli

Using immunofluorescence microscopy, we have found that SeqA protein, a regulator of replication initiation, is localized as discrete fluorescent foci in E. coli wild-type cells. Surprisingly, SeqA foci were observed also in an oriC deletion mutant. Statistical analysis revealed that a SeqA focus is localized at midcell in newborn cells. The SeqA focus is duplicated and tethered at midcell until an FtsZ ring is formed. Subsequently, these foci migrate in opposite directions toward cell quarter sites and remain tethered there until the cell divides. The cell cycle-dependent bidirectional migration of SeqA-DNA complexes is quite different from the migration pattern of oriC Dna copies. MukB protein is required for correct localization of SeqA complexes by an unknown mechanism.

A novel fission yeast gene, tht1+, is required for the fusion of nuclear envelopes during karyogamy

We have isolated a fission yeast karyogamy mutant, tht1, in which nuclear congression and the association of two spindle pole bodies occurs but the subsequent fusion of nuclear envelopes is blocked. The tht1 mutation does not prevent meiosis, so cells execute meiosis with two unfused nuclei, leading to the production of aberrant asci. The tht1(+) gene was cloned and sequenced. Predicted amino acid sequence has no significant homology to previously known proteins but strongly suggests that it is a type I membrane protein. The tht1(+) gene is dispensable for vegetative growth and expressed only in conjugating cells. Tht1p is a glycoprotein susceptible to endoglycosilase H digestion. Site- directed mutagenesis showed that the N-glycosylation site, as well as the COOH-terminal region of Tht1p, is essential for its function. A protease protection assay indicated that the COOH terminus is cytoplasmic. Immunocytological analysis using a HA-tagged Tht1p suggested that the protein is localized in nuclear envelopes and in the ER during karyogamy and that its levels are reduced in cells containing fused nuclei.

Characterization and cell cycle regulation of the related human telomeric proteins Pin2 and TRF1 suggest a role in mitosis

Telomeres are essential for preserving chromosome integrity during the cell cycle and have been specifically implicated in mitotic progression, but little is known about the signaling molecule(s) involved. The human telomeric repeat binding factor protein (TRF1) is shown to be important in regulating telomere length. However, nothing is known about its function and regulation during the cell cycle. The sequence of PIN2, one of three human genes (PIN1-3) we previously cloned whose products interact with the Aspergillus NIMA cell cycle regulatory protein kinase, reveals that it encodes a protein that is identical in sequence to TRF1 apart from an internal deletion of 20 amino acids; Pin2 and TRF1 may be derived from the same gene, PIN2/TRF1. However, in the cell Pin2 was found to be the major expressed product and to form homo- and heterodimers with TRF1; both dimers were localized at telomeres. Pin2 directly bound the human telomeric repeat DNA in vitro, and was localized to all telomeres uniformly in telomerase-positive cells. In contrast, in several cell lines that contain barely detectable telomerase activity, Pin2 was highly concentrated at only a few telomeres. Interestingly, the protein level of Pin2 was highly regulated during the cell cycle, being strikingly increased in G2+M and decreased in G1 cells. Moreover, overexpression of Pin2 resulted in an accumulation of HeLa cells in G2+M. These results indicate that Pin2 is the major human telomeric protein and is highly regulated during the cell cycle, with a possible role in mitosis. The results also suggest that Pin2/TRF1 may connect mitotic control to the telomere regulatory machinery whose deregulation has been implicated in cancer and aging.

The yeast silent information regulator Sir4p anchors and partitions plasmids

Circular plasmids containing telomeric TG1-3 arrays or the HMR E silencer segregate efficiently between dividing cells of the yeast Saccharomyces cerevisiae. Subtelomeric X repeats augment the TG1-3 partitioning activity by a process that requires the SIR2, SIR3, and SIR4 genes, which are also required for silencer-based partitioning. Here we show that targeting Sir4p to DNA directly via fusion to the bacterial repressor LexA confers efficient mitotic segregation to otherwise unstable plasmids. The Sir4p partitioning activity resides within a 300-amino-acid region (residues 950 to 1262) which precedes the coiled-coil dimerization motif at the extreme carboxy end of the protein. Using a topology-based assay, we demonstrate that the partitioning domain also retards the axial rotation of LexA operators in vivo. The anchoring and partitioning properties of LexA-Sir4p chimeras persist despite the loss of the endogenous SIR genes, indicating that these functions are intrinsic to Sir4p and not to a complex of Sir factors. In contrast, inactivation of the Sir4p-interacting protein Rap1p reduces partitioning by a LexA-Sir4p fusion. The data are consistent with a model in which the partitioning and anchoring domain of Sir4p (PAD4 domain) attaches to a nuclear component that divides symmetrically between cells at mitosis; DNA linked to Sir4p by LexA serves as a reporter of protein movement in these experiments. We infer that the segregation behavior of telomere- and silencer-based plasmids is, in part, a consequence of these Sir4p-mediated interactions. The assays presented herein illustrate two novel approaches to monitor the intracellular dynamics of nuclear proteins.

The telomere-associated DNA from human chromosome 20p contains a pseudotelomere structure and shares sequences with the subtelomeric regions of 4q and 18p

The human chromosome 20p telomere has been cloned on a yeast artificial chromosome (YAC). The telomere-associated DNA contains an interstitial tract of (TTAGGG)n telomeric repeats 60 kb in from the chromosome end. Frequent truncation of the YAC clone was observed due to resolution of the internal telomeric array into a telomere. The 20p internal telomeric repeat tract is flanked on its centromeric side by telomere-associated repeated sequences that have previously been found adjacent to terminal telomeric repeat arrays. The pseudotelomere structure of the 20p subtelomeric region is similar to the structure of some yeast subtelomeric regions where these sequences act as substrates for recombinational repair of chromosome ends that have lost their terminal telomeric repeat arrays. Sequences flanking the telomeric end of the internal (TTAGGG)n repeat array on 20p are found adjacent to three other subtelomeric (TTAGGG)n tracts on 4q, 18p, and an unknown chromosome end, respectively. These shared sequences provide evidence of exchange between nonhomologous chromosomes in humans.

Identification and characterization of a telomerase activity from Schizosaccharomyces pombe

A telomerase-like primer extension activity has been detected in chromatographic fractions derived from Schizosaccharomyces pombe extracts. This primer extension activity acts preferentially on dG-rich oligodeoxynucleotides, is sensitive to RNase A pretreatment and requires all four deoxynucleotides for optimal polymerization. The extension products are also truncated by the inclusion of any one of the four dideoxynucleotides, consistent with the presence of all four bases in the S.pombe telomeric repeats. The intensity distribution of the extension products and the dideoxynucleotide termination pattern suggest that nucleotide addition is template directed, and that telomere-like sequences are added to the primers. In particular, the sequence d(CGGTTA), a variant of the S.pombe telomeric repeat, can be added directly by the in vitro activity. Partially purified S.pombe telomerase sediments as a 35S particle, suggesting that it exists in vivo as part of a large multi-protein complex.

Functionally interacting telomerase RNAs in the yeast telomerase complex

The ribonucleoprotein (RNP) enzyme telomerase from Saccharomyces cerevisiae adds telomeric DNA to chromosomal ends in short increments both in vivo and in vitro. Whether or not telomerase functions as a multimer has not been addressed previously. Here we show, first, that following polymerization, the telomerase RNP remains stably bound to its telomeric oligonucleotide reaction product. We then exploit this finding and a previously reported mutant telomerase RNA to demonstrate that, unexpectedly, the S. cerevisiae telomerase complex contains at least two functionally interacting RNA molecules that both act as templates for DNA polymerization. Here, functional telomerase contains at least two active sites.

Preclinical and clinical strategies for development of telomerase and telomere inhibitors

BACKGROUND

Telomerase is an important enzyme whose activity has been convincingly demonstrated in humans recently. It is required for maintenance of ends of chromosomes (telomeres) during cell division. Since its presence has been selectively demonstrated in dividing cells including tumor cells, it has generated considerable excitement as a potential anti-cancer strategy.

DESIGN

In this article, we review the current relevant biology of the enzyme, the challenges encountered in the preclinical phase of target development and the current efforts that focus on telomeres and telomerase as therapeutic targets. We also speculate on the potential toxicities and mechanisms of resistance that may be encountered during use of such therapies.

The transcriptional basis of chromosome pairing

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

Telomere-telomere interactions and candidate telomere binding protein(s) in mammalian sperm cells

We have used fluorescent in situ hybridization to localize telomeres within the nuclei of sperm from six mammals (human, rat, mouse, stallion, boar, and bull). In minimally swollen sperm of mouse and rat, most of the telomeres are clustered within a limited area in the posterior part of nuclei. In sperm of other species, telomeres associate into tetrameres and dimers. On swelling of sperm cells with heparin/dithiotriethol, telomere associations disperse, and hybridization signals become smaller in size and their numbers approach or correspond to the number of chromosome ends in a haploid genome. Quantitation of telomere loci indicates that dimeric associations are prominent features of mammalian sperm nuclear architecture. Higher order telomere-telomere interactions and organization develop during meiotic stages of human spermatogenesis. At this stage, telomeres also become associated with the nuclear membrane. In an attempt to elucidate the molecular mechanisms underlying telomere interactions in sperm, we have identified a novel protein activity that binds to the double-stranded telomeric repeat (TTAGGG)n. Sperm telomere binding protein(s) (STBP) was extracted from human and bull sperm by 0.5 M NaCl. STBP does not bind single-stranded telomeric DNA and is highly specific for single base substitutions in a duplex DNA sequence. Depending on the conditions of binding, we observed the formation of several nucleoprotein complexes. We have shown that there is a transition between complexes, which indicates that the slower migrating complex is a multimer of the higher mobility one. We propose that STBP participates in association between the telomere domains which were microscopically observed in mammalian spermatozoa.

Telomeres cluster de novo before the initiation of synapsis: a three-dimensional spatial analysis of telomere positions before and during meiotic prophase

We have analyzed the progressive changes in the spatial distribution of telomeres during meiosis using three-dimensional, high resolution fluorescence microscopy. Fixed meiotic cells of maize (Zea mays L.) were subjected to in situ hybridization under conditions that preserved chromosome structure, allowing identification of stage-dependent changes in telomere arrangements. We found that nuclei at the last somatic prophase before meiosis exhibit a nonrandom, polarized chromosome organization resulting in a loose grouping of telomeres. Quantitative measurements on the spatial arrangements of telomeres revealed that, as cells passed through premeiotic interphase and into leptotene, there was an increase in the frequency of large telomere-to-telomere distances and a decrease in the bias toward peripheral localization of telomeres. By leptotene, there was no obvious evidence of telomere grouping, and the large, singular nucleolus was internally located, nearly concentric with the nucleus. At the end of leptotene, telomeres clustered de novo at the nuclear periphery, coincident with a displacement of the nucleolus to one side. The telomere cluster persisted throughout zygotene and into early pachytene. The nucleolus was adjacent to the cluster at zygotene. At the pachytene stage, telomeres rearranged again by dispersing throughout the nuclear periphery. The stage-dependent changes in telomere arrangements are suggestive of specific, active telomere-associated motility processes with meiotic functions. Thus, the formation of the cluster itself is an early event in the nuclear reorganizations associated with meiosis and may reflect a control point in the initiation of synapsis or crossing over.

The meiotic recombination hot spot created by the single-base substitution ade6-M26 results in remodeling of chromatin structure in fission yeast

The G -->T transversion mutation, ade6-M26, creates the heptanucleotide sequence ATGACTG, which lies close to the 5' end of the open reading frame of the ade6 gene in Schizosaccharomyces pombe. The mutation generates a meiosis-specific recombination hot spot and a binding site for the Mts1/Mts2 protein. We examined the chromatin structure at the ade6 locus in the M26 strain and compared it to that of the wild-type and hot spot-negative control M375. Micrococcal nuclease (MNase) digestion and indirect end-labeling methods were applied. In the M26 strain, we detected a new MNase-hypersensitive site at the position of the M26 mutation and no longer observed the phasing of nucleosomes seen in the wild-type and the M375 strains. Quantitative comparison of MNase sensitivity of the chromatin in premeiotic and meiotic cultures revealed a small meiotic induction of MNase hypersensitivity in the ade6 promoter region of the wild-type and M375 strains. The meiotic induction of MNase hypersensitivity was enhanced significantly in the ade6 promoter region of the M26 strain and also occurred at the M26 mutation site. The formation of the MNase-sensitive region around the heptamer sequence was abolished by the introduction of single-nucleotide substitutions in the heptamer sequence, which also abolish hot spot activity and binding of Mts1/Mts2. These data suggest that Mts1/Mts2 binding to the heptamer sequence results in a chromatin structure suitable for the recruitment of a meiosis-specific recombination function or functions.

Deconstructing the nucleus: global architecture from local interactions

Recent advances in fluorescence in situ hybridization and three-dimensional microscopy have revealed a high degree of large-scale order in the nucleus, indicating that the position of each gene within the nucleus is not random. As with any other biological phenomenon, this large-scale organization must ultimately be specified by molecular interactions. Biochemical and molecular investigations have revealed a small set of local molecular-scale interactions that can be used together in a combinatorial fashion to establish a global large-scale nuclear architecture.

Aspergillus nidulans maintains short telomeres throughout development

We report the identification and cloning of the telomeres of the filamentous fungus,Aspergillus nidulans. We have identified three classes of cloned chromosomal ends based on the telomere-associated sequences (TASs) and demonstrated that the telomeric repeat sequence is TTAGGG, identical to that found in vertebrates, including humans, and some lower eukaryotes. One category of telomere clones was found to contain internal, variant TAAGGG repeats. The A.nidulans telomeric tract length is strikingly short (4-22 repeats). We demonstrate that telomere length is remarkably stable in different cell types and at altered growth temperatures, suggesting a highly regulated mechanism for length control.

Phosphorylation of RNA-binding protein controls cell cycle switch from mitotic to meiotic in fission yeast

Meiosis generates haploid gametes from diploid cells and is an almost universal feature of eukaryotic organisms. But little is known about how the switch from mitotic to meiotic cell cycles is molecularly controlled. In the fission yeast Schizosaccharomyces pombe, inactivation of the protein kinase Pat1(Ran1) upon nutrient deprivation triggers entry into the meiotic cell cycle. Here we show that the RNA-binding protein Mei2 is a substrate of Pat1 kinase and that dephosphorylation of Mei2 is sufficient to switch cells from the mitotic cell cycle into meiosis. Mei2 is localized mainly in the cytoplasm of proliferating cells but is seen as a single spot close to the microtubule organizing centre in prophase nuclei during meiosis. Our results, and others from a metazoan, emphasize the crucial role of RNA-binding proteins in the initiation and execution of meiosis.

Regulation of telomere length and function by a Myb-domain protein in fission yeast

Telomeres, the specialized nucleoprotein structures that comprise the ends of eukaryotic chromosomes, are essential for complete replication, and regulation of their length has been a focus of research on tumorigenesis. In the budding yeast Saccharomyces cerevisiae, the protein Rap1p binds to telomeric DNA and functions in the regulation of telomere length. A human telomere protein, hTRF (human TTAGGG repeat factor) binds the telomere sequence in vitro and localizes to telomeres cytologically, but its functions are not yet known. Here we use a genetic screen to identify a telomere protein in fission yeast, Taz1p (telomere-associated in Schizosaccharomyces pombe), that shares homology to the Myb proto-oncogene DNA-binding domain with hTRF. Disruption or deletion of the taz1+ gene causes a massive increase in telomere length. Taz1p is required for the repression of telomere-adjacent gene expression and for normal meiosis or sporulation. It may be a negative regulator of the telomere-replicating enzyme, telomerase, or may protect against activation of telomerase-independent pathways of telomere elongation.

Telomerase RNA mutations in Saccharomyces cerevisiae alter telomerase action and reveal nonprocessivity in vivo and in vitro

The ribonucleoprotein enzyme telomerase adds telomeric DNA to chromosomal ends. In most eukaryotes the telomeric repeat units are repeated precisely, consistent with the action of a telomerase that faithfully copies its RNA template. In contrast, Saccharomyces cerevisiae telomeric repeats are degenerate, suggesting that its telomerase has unusual mechanistic properties. We mutated the S. cerevisiae telomerase RNA (TLC1) with a series of 3-base (GUG) substitutions in and next to the 17-nucleotide templating domain. All mutant telomerases were active in TLC1/tlc1 diploids and synthesized patterns of mixed wild-type and mutant telomeric repeats into telomeric DNA, consistent with nonprocessive action. Telomerase isolated from cells containing each mutated tlc1 allele by itself had altered reaction properties in vitro. One mutant template enzyme, 476GUG, was active in vivo and in vitro in the presence of wild-type TLC1 RNA but lacked detectable activity in its absence. Haploid tlc1-476GUG cells containing only this mutant tlc1 allele underwent senescence. Other tlc1 template region mutations allowed maintenance of shortened telomeres in vivo but altered specific enzymatic properties of telomerase in vitro, including induction of primer-template slippage (472GUG) or alteration of the 5' boundary of the template (467GUG). These data demonstrate that telomerase RNA bases influence enzyme activity profoundly, suggesting that their roles are not confined to serving simply as the template for this specialized reverse transcriptase.

Yeast telomeric sequences function as chromosomal anchorage points in vivo

Site-specific recombination in Saccharomyces cerevisiae was used to generate non-replicative DNA rings containing yeast telomeric sequences. In topoisomerase mutants expressing Escherichia coli topoisomerase I, the rings adopted a novel DNA topology consistent with the ability of yeast telomeric DNA to block or retard the axial rotation of DNA. DNA fragments bearing portions of the terminal repeat sequence C1-3 A/TG1-3 were both necessary and sufficient to create a barrier to DNA rotation. Synthetic oligonucleotide sequences containing Rap1p binding sites, a well represented motif in naturally occurring C1-3A arrays, also conferred immobilization; mutant Rap1p binding sites and telomeric sequences from other organisms were not sufficient. DNA anchoring was diminished by addition of competing telomeric sequences, implicating a role for an as yet unidentified limiting trans-acting factor. Though Rap1p is a likely protein constituent of the DNA anchor, deletion of the non-essential C-terminal domain did not affect the topology of telomeric DNA rings. Similarly, disruption of SIR2, SIR3 and SIR4, genes which influence a variety of telomere functions in yeast, also had no effect. We propose that telomeric DNA supports the formation of a SIR-independent macromolecular protein-DNA assembly that hinders the motion of DNA because of its linkage to an insoluble nuclear structure. Potential roles for DNA anchoring in telomere biology are discussed.

FISH technology in chromosome and genome research

Fluorescent in situ hybridization technology is one of the most exciting and versatile research tools to be developed in recent years. It has enabled research to progress at a phenomenal rate in diverse areas of basic research as well as in clinical medicine. Fluorescent in situ hybridization has applications in physical mapping, the study of nuclear architecture and chromatin packaging, and the investigation of fundamental principles of biology such as DNA replication, RNA processing, gene amplification, gene integration and chromatin elimination. This review highlights some of these areas and provides source material for the reader who seeks more information on a specific field.

Genome downsizing during ciliate development: nuclear division of labor through chromosome restructuring

The ciliated protozoa divide the labor of germline and somatic genetic functions between two distinct nuclei. The development of the somatic (macro-) nucleus from the germinal (micro-) nucleus occurs during sexual reproduction and involves large-scale, genetic reorganization including site-specific chromosome breakage and DNA deletion. This intriguing process has been extensively studied in Tetrahymena thermophila. Characterization of cis-acting sequences, putative protein factors, and possible reaction intermediates has begun to shed light on the underlying mechanisms of genome rearrangement. This article summarizes the current understanding of this phenomenon and discusses its origin and biological function. We postulate that ciliate nuclear restructuring serves to segregate the two essential functions of chromosomes: the transmission and expression of genetic information.