We have sequenced and annotated the genome of ®ssion yeast (Schizosaccharomyces pombe), which contains the smallest number of protein-coding genes yet recorded for a eukaryote: 4,824. The centromeres are between 35 and 110 kilobases (kb) and contain related repeats including a highly conserved 1.8-kb element. Regions upstream of genes are longer than in budding yeast (Saccharomyces cerevisiae), possibly re¯ecting more-extended control regions. Some 43% of the genes contain introns, of which there are 4,730. Fifty genes have signi®cant similarity with human disease genes; half of these are cancer related. We identify highly conserved genes important for eukaryotic cell organization including those required for the cytoskeleton, compartmentation, cell-cycle control, proteolysis, protein phosphorylation and RNA splicing. These genes may have originated with the appearance of eukaryotic life. Few similarly conserved genes that are important for multicellular organization were identi®ed, suggesting that the transition from prokaryotes to eukaryotes required more new genes than did the transition from unicellular to multicellular organization.We report here the completion of the fully annotated genome sequence of the simple eukaryote Schizosaccharomyces pombe, a ®ssion yeast. It becomes the sixth eukaryotic genome to be sequenced, following Saccharomyces cerevisiae 1 , Caenorhabditis elegans 2 , Drosophila melanogaster 3 , Arabidopsis thaliana 4 and Homo sapiens 5,6 . The entire sequence of the unique regions of the three chromosomes is complete, with gaps in the centromeric regions of about 40 kb, and about 260 kb in the telomeric regions. The completion of this sequence, the availability of sophisticated research methodologies, and the expanding community working on S. pombe, will accelerate the use of S. pombe for functional and comparative studies of eukaryotic cell processes.

A key question in cytokinesis is how the cell division plane is positioned. Whereas microtubules of the mitotic apparatus specify the division site in animal cells, we show here that the nucleus plays this role in the fission yeast Schizosaccharomyces pombe. By centrifuging cells to move the nucleus, we find that the nucleus (or a nuclear-associated structure) actively influences the position of contractile ring assembly during early mitosis. Displacement of the nucleus during this induction period can lead to formation of multiple rings. The nucleus signals its position in a microtubuleindependent manner by emitting the protein mid1p. Furthermore, movement of ring fragments together minimizes formation of multiple division sites. These dynamic mechanisms of ring positioning provide a robust coordination of nuclear and cell division.cytokinesis ͉ nucleus ͉ Schizosaccharomyces pombe

Dynamic properties of microtubules contribute to the establishment of spatial order within cells. In the fission yeast Schizosaccharomyces pombe, interphase cytoplasmic microtubules are organized into antiparallel bundles that attach to the nuclear envelope and are needed to position the nucleus at the geometric center of the cell. Here, we show that after the nucleus is displaced by cell centrifugation, these microtubule bundles efficiently push the nucleus back to the center. Asymmetry in microtubule number, length, and dynamics contributes to the generation of force responsible for this unidirectional movement. Notably, microtubules facing the distal cell tip are destabilized when the microtubules in the same bundle are pushing from the proximal cell tip. The CLIP-170-like protein tip1p and the microtubule-bundling protein ase1p are required for this asymmetric regulation of microtubule dynamics, indicating contributions of factors both at microtubule plus ends and within the microtubule bundle. Mutants in these factors are defective in nuclear movement. Thus, cells possess an efficient microtubule-based engine that produces and senses forces for centering the nucleus. These studies may provide insights into mechanisms of asymmetric microtubule behaviors and force sensing in other processes such as chromosome segregation and cell polarization.

In animal cells, cytokinesis occurs by constriction of an actomyosin ring. In fission yeast cells, ring constriction is triggered by the septum initiation network (SIN), an SPB-associated GTPase-regulated kinase cascade that coordinates exit from mitosis with cytokinesis. We have identified a novel protein, Etd1p, required to trigger actomyosin ring constriction in fission yeasts. This protein is localised at the cell tips during interphase. In mitosis, it relocates to the medial cortex region and, coincident with cytokinesis, it assembles into the actomyosin ring by association to Cdc15p. Relocation of Etd1p from the plasma membrane to the medial ring is triggered by SIN signalling and, reciprocally, relocation of the Sid2p-Mob1p kinase complex from the SPB to the division site, a late step in the execution of the SIN, requires Etd1p. These results suggest that Etd1p coordinates the mitotic activation of SIN with the initiation of actomyosin ring constriction. Etd1p peaks during cytokinesis and is degraded by the ubiquitin-dependent 26S-proteasome pathway at the end of septation, providing a mechanism to couple inactivation of SIN to completion of cytokinesis.

In Schizosaccharomyces pombe, Etd1 is a positive regulator of the septation initiation network (SIN), a conserved GTPase-regulated kinase cascade that triggers cytokinesis. Here we show that a mutation in the pab1 gene, which encodes the B-regulatory subunit of the protein phosphatase 2A (PP2A), suppresses mutations in the etd1 gene. Etd1 is required for the function of the GTPase Spg1, a key regulator of SIN signaling. Interestingly, the loss of Pab1 function restored the activity of Spg1 in Etd1-deficient cells. This result suggests that PP2A-Pab1-mediated dephosphorylation inhibits Spg1, thus antagonizing Etd1 function. The loss of pab1 function also rescues the lethality of mutants of other genes in the SIN cascade such as mob1, sid1, and cdc11. Two-hybrid assays indicate that Pab1 physically interacts with Mob1, Sid1, Sid2, and Cdc11, suggesting that the phosphatase 2A B-subunit is a component of the SIN complex. Together, our results indicate that PP2A-Pab1 plays a novel role in cytokinesis, regulating SIN activity at different levels. Pab1 is also required to activate polarized cell growth. Thus, PP2A-Pab1 may be involved in coordinating polar growth and cytokinesis.

Self-organization of cellular structures is an emerging principle underlying cellular architecture. Properties of dynamic microtubules and microtubule-binding proteins contribute to the self-assembly of structures such as microtubule asters. In the fission yeast Schizosaccharomyces pombe, longitudinal arrays of cytoplasmic microtubule bundles regulate cell polarity and nuclear positioning. These bundles are thought to be organized from the nucleus at multiple interphase microtubule organizing centres (iMTOCs). Here, we find that microtubule bundles assemble even in cells that lack a nucleus. These bundles have normal organization, dynamics and orientation, and exhibit anti-parallel overlaps in the middle of the cell. The mechanisms that are responsible for formation of these microtubule bundles include cytoplasmic microtubule nucleation, microtubule release from the equatorial MTOC (eMTOC), and the dynamic fusion and splitting of microtubule bundles. Bundle formation and organization are dependent on mto1p (gamma-TUC associated protein), ase1p (PRC1), klp2p (kinesin-14) and tip1p (CLIP-170). Positioning of nuclear fragments and polarity factors by these microtubules illustrates how self-organization of these bundles contributes to establishing global spatial order.

The chromosome complement of Danio rerio was investigated by Giemsa staining and C-banding, Ag-NORs and replication banding. The diploid number of this species is 2n = 50 and the arm number (NF) = 100. Constitutive heterochromatin was located at the centromeric position of all chromosome pairs. Nucleolus organizer regions appeared in the terminal position of the long arms of chromosomes 1, 2 and 8. Replication banding pattern allowed the identification of each chromosome pair.

from a single centrosome, differentiated cell types such 701 West 168th Street as muscle, neuronal, and epithelial cells adopt more New York, New York 10032 complex arrangements of MTOCs that organize nonra-2 Laboratory of Chromosome Structure and dial, linear MT arrays (Dammermann et al., 2003; Keating Function and Borisy, 1999; Mogensen, 1999). Kazusa DNA Research Institute Little is known about how the assembly, disassembly, 2-6-7 Kazusa-kamatari and activities of MTOCs are regulated. Experiments Kisarazu, Chiba 292-0818 showing de novo assembly of centrosomes and centri-Japan oles suggest that there must exist both global positive and negative regulation of centrosomal numbers (Khodjakov et al., 2002). Excess numbers of centrosomes are a common feature of cancer cells and may contribute Summaryto the development of cancer (Doxsey, 2001). Many centrosomal proteins reside outside of the centrosome Regulation of microtubule organizing centers (MTOCs) in the cytoplasm, often in the form of small particles, orchestrates the reorganization of the microtubule which may contribute to MTOC dynamics (Kubo et al., (MT) cytoskeleton. In the fission yeast Schizosacchar-1999; Moudjou et al., 1996; Paoletti et al., 1996; Zimmeromyces pombe, an equatorial MTOC (eMTOC) at the man and Doxsey, 2000). cell division site disassembles after cytokinesis, and S. pombe cells have three classes of MTOCs: the multiple interphase MTOCs (iMTOCs) appear on the spindle pole body (SPB), the equatorial MTOC (eMTOC), nucleus. Here, we show that, upon eMTOC disassemand multiple interphase MTOCs (iMTOCs) (Hagan, 1998; bly, small satellites carrying MTOC components such Hagan and Petersen, 2000; Heitz et al., 2001; Tran et as the ␥-tubulin complex travel in both directions along al., 2001). The SPB embeds into the nuclear envelope interphase MTs. We identify rsp1p, an MTOC protein and organizes the mitotic spindle and cytoplasmic astral required for eMTOC disassembly. In rsp1 loss-of-func-MTs during mitosis (Ding et al., 1993; Hagan, 1998). The tion mutants, the eMTOC persists and organizes an cytoplasmic face of the SPB may organize astral MT abnormal microtubule aster, while iMTOCs and satelbundles and at least one MT bundle in interphase (Hagan lites are greatly reduced. Conversely, rsp1p overexand Yanagida, 1995). pression inhibits eMTOC formation. Rsp1p is a J do-The eMTOC nucleates MTs from the medial cell divimain protein that interacts with an hsp70. Thus, our sion site during cytokinesis (Hagan, 1998; Heitz et al., findings suggest a model in which rsp1p is part of a 2001). This eMTOC assembles in midanaphase as a chaperone-based mechanism that disassembles the partial ring of ␥-tubulin at the contractile ring site. The eMTOC into satellites, contributing to the dynamic re-eMTOC ring closes with the contractile ring and then distribution of MTOC components for organization of forms one or two dots at the septum that disassemble interphase microtubules. at the end of cell division. One function of the eMTOC may be to maintain the positio...