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The fission yeast Schizosaccharomyces pombe is an important model organism for the study of eukaryotic molecular and cellular biology. Studies of S. pombe, together with studies of its distant cousin, Saccharomyces cerevisiae, have led to the discovery of genes involved in fundamental mechanisms of transcription, translation, DNA replication, cell cycle control, and signal transduction, to name but a few processes. However, since the divergence of the two species approximately 350 million years ago, S. pombe appears to have evolved less rapidly than S. cerevisiae so that it retains more characteristics of the common ancient yeast ancestor, causing it to share more features with metazoan cells. This Primer introduces S. pombe by describing the yeast itself, providing a brief description of the origins of fission yeast research, and illustrating some genetic and bioinformatics tools used to study protein function in fission yeast. In addition, a section on some key differences between S. pombe and S. cerevisiae is included for readers with some familiarity with budding yeast research but who may have an interest in developing research projects using S. pombe.

We identified the phhl ÷ gene that encodes a MAP kinase as the effector of Wisl MAP kinase kinase in fission yeast, which is highly homologous with HOG1 of S. cerevisiae. Heterothalic phhl dsiruptant is phenotypically indistinguishable from wisl deletion mutant, both displaying the same extent of partial sterility and enhanced sensitivity to a variety of stress. In phhl disruptant, nitrogen starvation-induced expression of stell +, a key controller of sexual differentiation, is markedly diminished. Ectopic expression of stell ÷ effectively restores fertility, but not stress resistance, to the phhl disruptant. These data show that stress signal, mediated by a MAP kinase, is required for efficient start of sexual differentiation.

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cdc1+ is required for cell cycle progression in Schizosaccharomyces pombe. Cells carrying temperature‐sensitive cdc1 mutants undergo cell cycle arrest when shifted to the restrictive temperature, becoming highly elongated. Here we describe the cloning and sequencing of cdc1+, which is shown to encode a 462 residue protein that displays significant sequence similarity to the small subunit of mammalian DNA polymerase delta. cdc1+ interacts genetically with pol3+, which encodes the large subunit of DNA polymerase delta in fission yeast, and the Cdc1 protein binds to Pol3 in vitro, strongly suggesting that Cdc1 is likely to be the small subunit of Pol delta. In addition, we show that cdc1+ overexpression is sufficient to rescue cells carrying temperature‐sensitive cdc27 alleles and that the Cdc1 and Cdc27 proteins interact in vivo and in vitro. Deletion of either cdc1+ or cdc27+ results in cell cycle arrest with the arrested cells having a single nucleus with 2C DNA content. No evidence was obtained for a cut phenotype, indicating that neither cdc1+ nor cdc27+ is required for checkpoint function. cdc1 mutant cells are supersensitive to the DNA synthesis inhibitor hydroxyurea and to the DNA damaging agent MMS, display increased frequency of mini‐chromosome loss and have an extended S phase.

The wis1+ gene encodes a newly identified mitotic control element in Schizosaccharomyces pombe. It was isolated by virtue of its interaction with the mitotic control genes cdc25, wee1 and win1. The wis1+ gene potentially encodes a 66 kDa protein with homology to the serine/threonine family of protein kinases. wis1+ plays an important role in the regulation of entry into mitosis, as it shares with cdc25+ and nim1+/cdr1+ the property of inducing mitosis in a dosage‐dependent manner. Increased levels of wis1+ expression cause mitotic initiation to occur at a reduced cell size. Loss of wis1+ function does not prevent vegetative growth and division, though wis1‐ cells show an elongated morphology, indicating that their entry into mitosis and cell division is delayed relative to wild type cells. wis1‐ cells undergo a rapid reduction of viability upon entry into stationary phase, suggesting a role for wis1+ in the integration of nutritional sensing with the control over entry into mitosis.

THERE is currently much interest in the mechanism which controls the timing of cell division. Certain features of the control have been found to be common to a variety of eukaryotes. In particular, the importance of cell size as a parameter affecting cell cycle progress has been reported for mammalian cells(1,2) and for several single-celled eukaryotes(3-6). Another feature common to several systems is that growth conditions have a direct effect on the timing of division cycle events(7-9), and on cell size(9,10). In the fission yeast Schizosaccharomyces pombe, both cell size(6) and nutritional conditions(9) have been shown to affect cycle kinetics. The organism has been used extensively as a model eukaryotic system, largely because of the ease of measuring cell size and because division occurs by binary fission(11). More recently, its genetic tractability has led to the isolation of cell division cycle (cdc) mutants(12), and also of wee mutants altered in the control coordinating growth with the division cycle(13-15). The existence of such control mutants allows a more direct approach to the investigation of the molecular basis of division control, in contrast to the indirect methods used in other systems(4,16-18). wee mutants are so far unique to S. pombe. The most conspicuous property of wee mutants is their reduced cell size(13,14). Analysis of these mutants(15,19) and other evidence(9) has shown that control over cell division timing normally acts at entry to mitosis. As the function of a number of cdc genes is specifically required for mitosis(12), interactions between wee and cdc mutants which affect mitosis might be expected. I report here that the mitotic defect caused by a defective cdc25 allele is suppressed in wee mutants. Suppression by wee1 mutants is almost complete, while the wee2.1 mutation is a less effective suppressor. The significance of these findings for genetic models of the control of mitosis is considered.

Fission yeast temperature‐sensitive cut5 (cell untimely torn) mutants are defective in initiation and/or elongation of DNA replication but allow mitosis and cell division at a restrictive temperature. We show that the cut5 protein (identical to rad4) (i) is an essential component of the replication checkpoint system but not the DNA damage checkpoint, and (ii) negatively regulates the activation of M phase kinase at mitotic entry. Even if the replication checkpoint has been activated previously, cut5 mutations allow mitosis and cell division after shift to 36 degrees C. Transcription of cut5+ is not under the control of the START gene cdc10+. The cut5 protein is enriched in the nucleus, consisting of repeating domains. An essential domain which resembles the proto‐oncoprotein Ect2 has a strong negative effect on the entry into mitosis when overexpressed. Expression of the cut5 mutant phenotype requires the function of the M phase regulator genes cdc2+, cdc25+ and cdc13+. The cut5 protein forms a novel, essential link between DNA synthesis and M phase activation in the replication checkpoint control pathway.