Dikaryotic cell division of the fission yeast Schizosaccharomyces pombe

Dikaryons, cells with two haploid nuclei contributed by the members of a mating pair, are part of the life cycle of many filamentous fungi, but the molecular mechanisms underlying the division of dikaryons are largely unknown. We found that the fission yeast Schizosaccharomyces pombe has a latent ability to divide as a dikaryon. Cells capable of restarting the mitotic cycle with two nuclei were prepared by transient inactivation of the septation initiation network. Close pairing of the two nuclei before mitosis was dependent on minus-end-directed kinesin Klp2p and was essential for propagation as a dikaryon. The two spindles extended in opposite directions, keeping their old spindle pole bodies at the prospective site of cell division until the mid-anaphase. The spindles then overlapped, exchanging the inner nuclei. Finally, twin mitosis was followed by a single cytokinesis, producing two daughter dikaryons carrying copies of the original pair of nuclei.     

Uncontrolled septation in a cell division cycle mutant of the fission yeast Schizosaccharomyces pombe.

A temperature-sensitive Schizosaccharomyces pombe mutant, cdc16-116, has been isolated which undergoes uncontrolled septation during its cell division cycle. The mutant accumulates two types of cells after 3 h of growth at the restrictive temperature: (i) type I cells (85% of the population), which complete nuclear division and then form up to five septa between the divided nuclei; and (ii) type II cells (15% of the population), which form an asymmetrically situated septum in the absence of any nuclear division. cdc16-116 is a monogenic recessive mutation unlinked to any previously known cdc gene of S. pombe. It is not affected in a previously reported control by which septation is dependent upon completion of nuclear division. We propose the cdc16-116 is unable to complete septum formation and proceed to cell separation and is also defective in a control which prevents the manufacture of more than one septum in each cell cycle.     

Controlling cell cycle progress in the fission yeast Schizosaccharomyces pombe.

The suitability of fission yeast as a model for understanding the eukaryotic cell cycle has been validated in five years of exciting developments. We review recent advances in understanding the nature of the controls that regulate progression through the cell cycle and the coordination of DNA replication and mitosis.     

Mutants altered in the control co-ordinating cell division with cell growth in the fission yeast Schizosaccharomyces pombe

Summary The control co-ordinating cell division with cell growth has been investigated in the fission yeast Schizosaccharomyces pombe. Twenty-five mutants altered in this control have been isolated which have the same growth rate as wild type but divide at a smaller cell size. The mutants define two genes wee 1 and wee 2, both of which are involved in a control initiating mitosis when the cell attains a critical size.     

Effect of phenylarsine oxide on the fission yeast Schizosaccharomyces pombe cell cycle

Phosphotyrosyl turnover is an essential regulatory mechanism for many biological processes, and the balance between tyrosine kinases and phosphatases plays a major role in the control of cell proliferation. Phenylarsine oxide (PAO), a potent inhibitor of tyrosine phosphatases (PTPase), was used to investigate the involvement of PTPase in the growth and control of the cell cycle of the fission yeast Schizosaccharomyces pombe. Cell proliferation was arrested by treatment with PAO, which was found to inhibit cdc25 PTPase in vitro but appeared not to act in vivo on this mitosis inducer. The PAO-treated cells displayed a mono- or binucleated phenotype and a DNA content that was either 2C or 4C, indicating a cell cycle arrest with a failure to complete cytokinesis. Entry into the cell division cycle from the G0 quiescent stage was also delayed by treatment with PAO. These results suggest that a number of key events in the mitotic cell cycle are regulated by as yet unidentified PTPases.     

Programmed cell death in fission yeast Schizosaccharomyces pombe

Yeasts have proven to be invaluable, genetically tractable systems to study various fundamental biological processes including programmed cell death. Recent advances in the elucidation of the molecular pathways underlying apoptotic cell death in yeasts have revealed remarkable similarities to mammalian apoptosis at cellular, organelle and macromolecular levels, thus making a strong case for the relevance of yeast models of regulated cell death. Programmed cell death has been reported in fission yeast Schizosaccharomyces pombe, primarily in the contexts of perturbed intracellular lipid metabolism, defective DNA replication, improper mitotic entry, chronological and replicative aging. Here we review the current understanding of the programmed cell death in fission yeast, paying particular attention to lipid-induced cell death. We discuss our recent findings that fission yeast exhibits plasticity of apoptotic and non-apoptotic modes of cell death in response to different lipid stimuli and growth conditions, and that mitochondria, reactive oxygen species and novel cell death mediators including metacaspase Pca1, SpRad9 and Pck1 are involved in the lipotoxic cell death. We also present perspectives on how various aspects of the cell and molecular biology of this organism can be explored to shed light on the governing principles underlying lipid-mediated signaling and cell demise.     

Cold-sensitive nuclear division arrest mutants of the fission yeast Schizosaccharomyces pombe

Thirteen recessive cold sensitive nuclear division arrest mutants were isolated from the fission yeast Schizosaccharomyces pombe. Twelve unlinked genes were defined; six in chromosome I, three in chromosome II and two in chromosome III. The map positions of three nuclear division arrest genes (nda1, nda2 and nda3) in chromosome II were determined precisely. Together with the previously obtained temperature-sensitive cell division cycle mutations, at least 20 genes appear to control the nuclear division of the fission yeast. Physiological studies indicated that most cold sensitive nda mutants incubated previously at 22 degrees C proceeded with a synchronously normal cell-cycle after temperature shift-up. The morphology of the nuclei and nuclear chromatin region was studied by the 4',6-diamidino-2-phenylindole staining method and by electron microscopy. Each mutant exhibited characteristic nuclear morphology at 22 degrees C, showing the specific blockages. The nda genes seem to control a pathway of structural alterations in the nuclear chromatin region with the order hemisphere, condensed ellipsoid, segregating U-form and separating hemispheres. Two genes, nda2 and nda3, pleiotropically control nuclear division, nuclear location and cell shape. The terminal phenotype of nda2-KM52 is characterized by the nuclear displacement, the absence of a spindle and abnormal locations of spindle pole bodies. The cells of nda3-KM311 were aberrant in shape and contained a partially separated chromatin region with a long spindle. Together with the results of the accompanying paper, we conclude that nda2 and nda3 genes control nuclear and cytoplasmic microtubular organization.     

Synthesis of soluble protein during the cell cycle of the fission yeast Schizosaccharomyces pombe

Many enzymes show a pattern of increase in activity through the cell cycle which is different from the continuous exponential pattern of total protein synthesis. A group of proteins at an intermediate level between single enzymes and total protein, the soluble proteins, was examined to resolve this anomaly. The synthesis of the pH 8.1 soluble proteins of Schizosaccharomyces pombe through the cell cycle was followed by pulse labelling with ³H-leucine in synchronous cultures. The soluble proteins were analysed by electrophoresis on acrylamide gels. Soluble proteins represent 30% of the total proteins of S. pombe and the rates of synthesis showed a continuous increase through the cell cycle. Individual groups of proteins, represented by a single band after electrophoresis, showed a similar continuous increase in synthesis through the cell cycle. Any proteins which may be synthesised discontinuously, such as some enzymes, represent such a small proportion of any one protein group in the electrophoretic separation that their effect was not detectable. These results are different from those described for mammalian cells.     

Genetic control of the cell division cycle in the fission yeast Schizosaccharomyces pombe

Summary Twenty seven recessive temperature sensitive mutants have been isolated in Schizosaccharomyces pombe which are unable to complete the cell division cycle at the restrictive temperature. These mutants define 14 unlinked genes which are involved in DNA synthesis, nuclear division and cell plate formation. The products from most of these genes complete their function just before the cell cycle event in which they are involved. Physiological characterisation of the mutants has shown that DNA synthesis and nuclear division form a cycle of mutually dependent events which can operate in the absence of cell plate formation. Cell plate formation itself is usually dependent upon the completion of nuclear division.     

Site-specific mutagenesis of cdc2+, a cell cycle control gene of the fission yeast Schizosaccharomyces pombe.

The cdc2+ gene of Schizosaccharomyces pombe is homologous to the CDC28 gene of Saccharomyces cerevisiae. Both genes share limited homology with vertebrate protein kinases and have protein kinase activity. cdc2+ has been subjected to mutagenesis in vitro. A null allele of the gene, constructed by insertion of the S. cerevisiae LEU2 gene into a site within the gene, has a phenotype similar to that of many temperature-sensitive alleles of cdc2. Mutations within the predicted ATP-binding site and in a region which may be a site of phosphorylation result in loss of cdc2+ activity. A single substitution of Gly-146 to Asp-146 has been identified in cdc2-1w, a dominant activated allele of the gene. The four introns within the cdc2+ gene have been deleted. The resulting gene not only functions in fission yeast but also rescues cdc28(Ts) strains of S. cerevisiae, a property which is not shared by the genomic cdc2+ gene.     

Cyclins of the fission yeast Schizosaccharomyces pombe

Five cyclin-like genes, cig1, cig2/cyc17, mcs2, puc1 and cdc13, have been discovered in S. pombe to date. It is not yet clear what their functions are or even whether they are all involved with control of the cell cycle. Conflicting data for cig1 and cig2/cyc17 have obscured analysis of their function and cig1 remains largely uncharacterized, although clues to the role of cig2/cyc17 have emerged. There is genetic data available for the more distant cyclin homologue mcs2, which has an essential although as yet unspecified role. Puc1 may be involved in regulation of exit from the cell cycle. The first cyclin to be discovered, and the best understood, is cdc13 which with cdc2 promotes mitosis. Studies of the roles of cdc2 and cdc13 in the overall ordering of the cell cycle suggest that cdc13 and probably other cyclins are key regulators, maintaining the order of S phase and mitosis during the cell cycle.     

Genetic analysis of antisuppressor mutants in the fission yeast Schizosaccharomyces pombe.

Fourteen unlinked sin genes could be mutated to recessive antisuppressor alleles preventing the expression of suppressors in the fission yeast Schizosaccharomyces pombe. cyh1 alleles, resistant to the ribosomal inhibitor cycloheximide, also have some antisuppressor effect. The genetical and physiological characterization of these mutants is consistent with the hypothesis that they affect components of the messenger RNA translation machinery such as tRNA modifying enzymes or ribosomal proteins.     

Pheromone communication in the fission yeast Schizosaccharomyces pombe

Conjugation between two haploid yeast cells is generally controlled by the reciprocal action of diffusible mating pheromones, cells of each mating type releasing pheromones that induce mating-specific changes in cells of the opposite type. Recent studies into pheromone signalling in the fission yeast Schizosaccharomyces pombe have revealed significant parallels with processes in higher eukaryotes and could provide the opportunity for investigating communication in an organism that is amenable to both biochemical and genetic manipulation.