Two enzymes involved in the synthesis of pyrimidine and purine nucleotides, CTP synthase (CTPS) and IMP dehydrogenase (IMPDH), can assemble into a single or very few large filaments called rods and rings (RR) or cytoophidia. Most recently, asymmetric cytoplasmic distribution of organelles during cell division has been described as a decisive event in hematopoietic stem cell fate. We propose that cytoophidia, which could be considered as membrane‐less organelles, may also be distributed asymmetrically during mammalian cell division as previously described for (...) Schizosaccharomyces pombe. Furthermore, because each type of nucleotide intervenes in distinct processes (e.g., membrane synthesis, glycosylation, and G protein‐signaling), alterations in the rate of synthesis of specific nucleotide types could influence cell differentiation in multiple ways. Therefore, we hypothesize that whether a daughter cell inherits or not CTPS or IMPDH filaments determines its fate and that this asymmetric inheritance, together with the dynamic nature of these structures enables plasticity in a cell population. (shrink)

We analyzed the expressivity of recombinant proteins by using data mining methods. The expression technique of recombinant protein is a key step towards elucidating the functions of genes discovered through genomic sequence projects. We have studied the productive efficiency of recombinant proteins in fission yeast, Schizosaccharomyces pombe, by mining the expression results. We gathered 57 proteins whose expression levels were known roughly in the host. Correlation analysis, principal component analysis and decision tree analysis were applied to these expression (...) data. Analysis featuring codon usage and amino acid composition clarified that the amino acid composition affected to the expression levels of a recombinant protein strongly than the effect of codon usage. Furthermore, analysis of amino acid composition showed that protein solubility and the metabolism cost of amino acids correlated with a protein expressivity. Codon usage was often interesting in the field of recombinant expressions. However, our analysis found the weak correlation codon features with expressivities. These results indicated that ready-made indices of codon bias were irrelevant ones for modeling the expressivities of recombinant proteins. Our data driven approach was an easy and powerful method to improve recombinant protein expression, and this approach should be concentrated attention with the huge amount of expression data accumulating through the post-genome era. (shrink)

In a recent issue of Nature Communications Ukleja and co‐workers reported a cryo‐EM 3D reconstruction of the Ccr4‐Not complex from Schizosaccharomyces pombe with an immunolocalization of the different subunits. The newly gained architectural knowledge provides cues to apprehend the functional diversity of this major eukaryotic regulator. Indeed, in the cytoplasm alone, Ccr4‐Not regulates translational repression, decapping and deadenylation, and the Not module additionally plays a positive role in translation. The spatial distribution of the subunits within the structure is (...) compatible with a model proposing that the Ccr4‐Not complex interacts with the 5′ and 3′ ends of target mRNAs, allowing different functional modules of the complex to act at different stages of the translation process, possibly within a circular constellation of the mRNA. This work opens new avenues, and reveals important gaps in our understanding regarding structure and mode of function of the Ccr4‐Not complex that need to be addressed in the future. (shrink)

The dynamics of eukaryotic DNA polymerases has been difficult to establish because of the difficulty of tracking them along the chromosomes during DNA replication. Recent work has addressed this problem in the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae through the engineering of replicative polymerases to render them prone to incorporating ribonucleotides at high rates. Their use as tracers of the passage of each polymerase has provided a picture of unprecedented resolution of the organization of replicons and replication origins (...) in the two yeasts and has uncovered important differences between them. Additional studies have found an overlapping distribution of DNA polymorphisms and the junctions of Okazaki fragments along mononucleosomal DNA. This sequence instability is caused by the premature release of polymerase δ and the retention of non proof‐read DNA tracts replicated by polymerase α. The possible implementation of these new experimental approaches in multicellular organisms opens the door to the analysis of replication dynamics under a broad range of genetic backgrounds and physiological or pathological conditions. (shrink)

The transition from mitotic cell division to meiosis in yeast is governed by both the mating‐type genes and signals from the environment. Analysis of mutants that are unable to regulate entry into meiosis has identified many genes that function in this process and in some cases, the biochemical activity of their protein products has been described. At least two of the the mating‐type genes of Saccharomyces cerevisiae encode DNA binding proteins that regulate transcription of unlinked genes required for entry into (...) meiosis. Meiotic development of the distantly related yeast, Schizosaccharomyces pombe, is also controlled by the mating‐type genes but in this yeast, their role is to regulate expression of a protein that acts as an inhibitor of a protein kinase. The ability to use the powerful tool of genetics in yeast has provided us with many new insights into the problem of meiotic development. (shrink)

Homologous chromosome pairing is required for proper chromosome segregation and recombination during meiosis. The mechanism by which a pair of homologous chromosomes contact each other to establish pairing is not fully understood. When pairing occurs during meiotic prophase in the fission yeast, Schizosaccharomyces pombe, the nucleus oscillates between the cell poles and telomeres remain clustered at the leading edge of the moving nucleus. These meiosis‐specific activities produce movements of telomere‐bundled chromosomes. Several lines of evidence suggest that these movements (...) facilitate homologous chromosome pairing by aligning homologous chromosomes and promoting contact between homologous regions. Since telomere clustering and nuclear or chromosome movements in meiotic prophase have been observed in a wide range of eukaryotic organisms, it is suggested that telomere‐mediated chromosome movements are general activities that facilitate homologous chromosome pairing. BioEssays 23:526–533, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

Dependency relationships within the cell cycle allow cells to arrest the cycle reversibly in response to agents or conditions that interfere with specific aspects of its normal progression. In addition, overlapping pathways exist which also arrest the cell cycle in response to DNA damage. Collectively, these control mechanisms have become known as checkpoints. Analysis of checkpoints is facilitated by the fact that dependency relationships within the cell cycle, such as the dependency of mitosis on the completion of DNA synthesis, and (...) the DNA damage checkpoint can be separated genetically. In fission yeast, Schizosaccharomyces pombe, the dependency of mitosis on prior completion of DNA synthesis is mediated through tyrosine‐15 phosphorylation of the ubiquitous mitotic regulator p34cdc2. In contrast, the arrest of mitosis caused by DNA damage acts through a separate mechanism that appears to be independent of tyrosine‐15 phosphorylation. Despite these distinct interactions with the mitotic machinery, the majority of fission yeast mutants that are deficient in mitotic arrest after DNA damage are also unable to respond to inhibition of DNA synthesis. In this essay we survey the current knowledge concerning feedback controls and checkpoints within fission yeast and relate this to information derived from other systems. (shrink)

In the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, the initiation of DNA replication is controlled at a point called START. At this point, the cellular environment is assessed; only if conditions are appropriate do cells traverse START, thus becoming committed to initiate DNA replication and complete the remainder of the cell cycle. The cdc2+ / CDC28+ gene, encoding the protein kinase p34, is a key element in this complex control. The identification of structural and functional homologues of p34 (...) suggests that it has a role in the control of DNA replication in all eukaryotes. The WHI1+, CLN1+ and CLN2+ gene products, identified in S. cerevisiae, are positive regulators that function at START and may interact with p34. Determining how passing the START control point leads to the initiation of DNA replication is a major outstanding challenge in cell cycle studies. (shrink)

Period homeostasis is the defining characteristic of a biological clock. Strict period homeostasis is found for the ultradian clocks of eukaryotic microbes. In addition to being temperature-compensated, the period of these rhythms is unaffected by differences in nutrient composition or changes in other environmental variables. The best-studied examples of ultradian clocks are those of the ciliates Paramecium tetraurelia and Tetrahymena sp. and of the fission yeast, Schizosaccharomyces pombe. In these single cell eukaryotes, up to seven different parameters display (...) ultradian rhythmicity with the same, species- and strain-specific period. In fission yeast, the molecular genetic analysis of ultradian clock mechanisms has begun with the systematic analysis of mutants in identified candidate genes. More than 40 “clock mutants” have already been identified, most of them affected in components of major regulatory and signalling pathways. These results indicate a high degree of complexity for a eukaryotic clock mechanism. BioEssays 22:16–22, 2000. ©2000 John Wiley & Sons, Inc. (shrink)

Spontaneous polarization without spatial cues, or symmetry breaking, is a fundamental problem of spatial organization in biological systems. This question has been extensively studied using yeast models, which revealed the central role of the small GTPase switch Cdc42. Active Cdc42‐GTP forms a coherent patch at the cell cortex, thought to result from amplification of a small initial stochastic inhomogeneity through positive feedback mechanisms, which induces cell polarization. Here, I review and discuss the mechanisms of Cdc42 activity self‐amplification and dynamic turnover. (...) A robust Cdc42 patch is formed through the combined effects of Cdc42 activity promoting its own activation and active Cdc42‐GTP displaying reduced membrane detachment and lateral diffusion compared to inactive Cdc42‐GDP. I argue the role of the actin cytoskeleton in symmetry breaking is not primarily to transport Cdc42 to the active site. Finally, negative feedback and competition mechanisms serve to control the number of polarization sites. (shrink)

In eukaryotic cells, heterochromatin is typically composed of tandem DNA repeats and plays crucial roles in gene expression and genome stability. It has been reported that silencing at individual units within tandem heterochromatin repeats exhibits a position‐dependent variation. However, how the heterochromatin is organized at an individual repeat level remains poorly understood. Using a novel genetic approach, our recent study identified a conserved protein Rex1BD required for position‐dependent silencing within heterochromatin repeats. We further revealed that Rex1BD interacts with the 14‐3‐3 (...) protein to regulate heterochromatin silencing by linking RNAi and HDAC pathways. In this review, we discuss how Rex1BD and the 14‐3‐3 protein coordinate to modulate heterochromatin organization at the individual repeat level, and comment on the biological significance of the position‐dependent effect in heterochromatin repeats. We also identify the knowledge gaps that still need to be unveiled in the field. (shrink)

The three cycles of cell division immediately following theformation of the cellular blastoderm during Drosophila embryogenesis display an invariant pattern(1,2). Bursts of transcription of a gene called string are required and sufficient to trigger mitosis at this time during development(3). The activator of mitosis encoded by the string gene is a positive regulator of cdc2 kinase and a Drosophila homologue of the Saccharomyces pombe cdc25 tyrosine phosphatase(4,5). Evidence presented in a recent paper(6) demonstrates that transcription of string, and hence (...) the timing and pattern of mitosis in the postblastoderm embryo, is under complex developmental control. Several lines of evidence support this interpretation, including the analysis of string transcription in pattern formation mutants, cell cycle arrest mutants, and the preliminary characterization of an extensive cis‐acting regulatory region. (shrink)

Unlike the most well‐characterized prokaryotic polymerase, E. Coli DNA pol I, none of the eukaryotic polymerases have their own 5′ to 3′ exonuclease domain for nick translation and Okazaki fragment processing. In eukaryotes, FEN‐1 is an endo‐and exonuclease that carries out this function independently of the polymerase molecules. Only seven nucleases have been cloned from multicellular eukaryotic cells. Among these, FEN‐1 is intriguing because it has complex structural preferences; specifically, it cleaves at branched DNA structures. The cloning of FEN‐1 permitted (...) establishment of the first eukaryotic nuclease family, predicting that S. cerevisiae RAD2 (S. pombe Rad13) and its mammalian homolog, XPG, would have similar structural specficity. The FEN‐1 nuclease family includes several similar enzymes encoded by bacteriophages. The crystal structures of two enzymes in the FEN‐1 nuclease family have been solved and they provide a structural basis for the interesting steric requirements of FEN‐1 substrates. Because of their unique structural specificities, FEN‐1 and its family members have important roles in DNA replication, repair and, potentially, recombination. Recently, FEN‐1 was found to specifically associate with PCNA, explaining some aspects of FEN‐1 function during DNA replication and potentially in DNA repair. (shrink)

Although alterations in Ras signalling are found in about 30% of human cancers, the transforming activity of oncogenic Ras is not fully understood. In a recent paper, a putative Ras1 effector in S. pombe, named Scd1, was reported to localize to mitotic apindies. Scd1 physically associates with Moe1, a factor that may contribute to the inherent inatability of microtubules (MTs) and appears to be needed for proper apindle function. Altered MT dynamics within the spindle are likely to affect spindle (...) assembly and chromosome capture, processes that need to be delicately controlled if cells are to guard against genome instability adn transformation. BloEssays 23: 307‐310,2001.©2001 John Willey & Sons, Inc. (shrink)