SummaryMeiosis is a specialised form of the cell cycle that gives rise to haploid gametes. In Schizosaccharomyces pombe, the products of meiosis are four spores, which are formed by encapsulation of the four meiosis II nuclei within the cytoplasm of the zygote produced by fusion of the mating cells. The S. pombe spindle pole body is remodelled during meiosis II and membrane vesicles are then recruited there to form the forespore membrane, which encapsulates the haploid nucleus to form a prespore. Spore wall material is then deposited, giving rise to the mature spore. The septation initiation network is required to coordinate cytokinesis and mitosis in the vegetative cycle and for spore formation in the meiotic cycle. We have investigated the role of the SIN regulator dma1p in meiosis; we find that although both meiotic divisions occur in the absence of dma1p, asci frequently contain fewer than four spores, which are larger than in wild-type meiosis. Our data indicate that dma1p acts in parallel to the leading-edge proteins and septins to assure proper formation for the forespore membrane. Dma1p also contributes to the temporal regulation of the abundance of the meiosis-specific SIN component mug27p.

Cytokinesis in Schizosaccharomyces pombe begins at mitotic entry, when the site of division is defined by formation of the contractile acto-myosin ring (CAR) at the cell cortex. Contraction of the CAR and formation of the division septum are triggered at the end of mitosis by septation initiation network (SIN) proteins associated with the spindle pole body (SPB). SIN signalling requires activation of the GTPase Spg1p, which is regulated by the bipartite GTPase-activating protein (GAP) Byr4p-Cdc16p. We show that, for Spg1p to associate with the SPB, it must be bound to its GAP or to its mitotic effector, the protein kinase Cdc7p. Analysis of the GAP proteins reveals that the steady-state level of Byr4p reflects that of Spg1p. Furthermore, if the interaction of Byr4p with Spg1p is compromised, the level of Byr4p decreases dramatically. The adaptation of the level of Byr4p to that of Spg1p requires the presence of Cdc16p and is mediated by proteasome-dependent destruction. It requires neither association with the SPB nor an active SIN. We propose a mechanism that limits the amount of the Byr4p-Cdc16p GAP to the amount required to inhibit Spg1p signalling.Supplementary material available online at

The Schizosaccharomyces pombe septation initiation network (SIN) regulates cytokinesis, and asymmetric association of SIN proteins with the mitotic spindle pole bodies (SPBs) is important for its regulation. Here, we have used semi-automated image analysis to study SIN proteins in large numbers of wild-type and mutant cells. Our principal conclusions are: first, that the association of Cdc7p with the SPBs in early mitosis is frequently asymmetric, with a bias in favour of the new SPB; second, that the early association of Cdc7p-GFP to the SPB depends on Plo1p but not Spg1p, and is unaffected by mutations that influence its asymmetry in anaphase; third, that Cdc7p asymmetry in anaphase B is delayed by Pom1p and by activation of the spindle assembly checkpoint, and is promoted by Rad24p; and fourth, that the length of the spindle, expressed as a fraction of the length of the cell, at which Cdc7p becomes asymmetric is similar in cells dividing at different sizes. These data reveal that multiple regulatory mechanisms control the SIN in mitosis and lead us to propose a two-state model to describe the SIN.

In Saccharomyces cerevisiae, TBF1, an essential gene, influences telomere function but also has other roles in the global regulation of transcription. We have identified a new member of the tbf1 gene family in the mammalian pathogen Pneumocystis carinii. We demonstrate by transspecies complementation that its ectopic expression can provide the essential functions of Schizosaccharomyces pombe tbf1 but that there is no rescue between fission and budding yeast orthologues. Our findings indicate that an essential function of this family of proteins has diverged in the budding and fission yeasts and suggest that effects on telomere length or structure are not the primary cause of inviability in S. pombe tbf1 null strains.The phylogenetic pedigree of Pneumocystis species implies they are likely to employ conserved fungus-specific proteins in pathways that are essential for viability. Identification of such modules in the genome of the rat-specific pathogen P. carinii may help to identify novel targets for directed pharmaceutical intervention to treat opportunistic infection of immunocompromised human hosts by P. jirovecii. To identify conserved functional modules, we are assessing the ability of genes from P. carinii to function in the budding yeast Saccharomyces cerevisiae as well as in the fission yeast Schizosaccharomyces pombe, the closest evolutionary relationship for which molecular genetic analysis is well developed (25,36).In the course of this study, we identified a potential homologue of the S. pombe tbf1 (Sptbf1) and S. cerevisiae TBF1 (ScTBF1) genes. Sequence comparisons indicate that both SpTbf1p and ScTbf1p are members of a conserved fungusspecific family (50, 51). All the Tbf1 family members contain a C-terminal "telobox" DNA binding domain (8, 9) but bear additional significant homology throughout their coding sequences. The telobox, a particular variant of the Myb family motif, is also found in the mammalian telomere factors TRF1 and TRF2 as well as in the fission yeast telomeric protein Taz1p (13). The telobox recognizes sequences similar to the mammalian telomeric repeat TTAGGG. High-affinity binding sites for ScTbf1p have also been identified in the STARs (subtelomeric antisilencing regions) of the subtelomeric X and YЈ elements. Several studies have proposed a regulatory role for ScTbf1p at telomeres (2,6,19,33). The ScTBF1 gene is essential, but this is commonly assumed to be due to its function as a global transcriptional regulator that binds to many sites throughout chromatin rather than to direct effects on telomeres (10, 33).SpTbf1p was identified in S. pombe whole-cell extracts as one of several activities that exhibited differential affinity in vitro for tandem copies of the human and S. pombe telomere repeat sequences (55, 62). The Sptbf1 gene is essential, although its full range of cellular functions is unknown. Overexpression of Sptbf1 has been shown to slightly increase the mean length of telomeres in vivo (51).We have examined whether the P. carinii tbf1 homologue (Pctbf1) can rescue ...