Molecular cloning and characterization of the Schizosaccharomyces pombe his3 gene for use as a selectable marker

A role for chromatin remodeling in transcriptional termination by RNA polymerase II

Chromatin remodeling can facilitate the recruitment of RNA polymerase II (Pol II) to targeted promoters, as well as enhancing the level of transcription. Here, we describe a further key role for chromatin remodeling in transcriptional termination. Using a genetic screen in S. pombe, we identified the CHD-Mi2 class chromatin remodeling ATPase, Hrp1, as a termination factor. In S. cerevisiae, we show that transcriptional termination and chromatin structure at the 3' ends of three genes all depend on the activity of the Hrp1 homolog, Chd1p, either alone or redundantly with the ISWI ATPases, Isw1p, and Isw2p. We suggest that chromatin remodeling of termination regions is a necessary prelude to efficient Pol II termination.

Two non-complementing genes encoding enzymatically active methylenetetrahydrofolate reductases control methionine requirement in fission yeast Schizosaccharomyces pombe

By transforming two methionine auxotrophic mutants from fission yeast Schizosaccharomyces pombe with a wild-type gene library, we defined two genes, met9 and met11, which both encode a methylenetetrahydrofolate reductase. The genes cannot complement each other. We detected single transcripts for both. In vitro measurements of enzymatic activities showed that the met11-encoded enzyme was responsible for only 15-20% of the total methylenetetrahydrofolate reductase activity. A strain in which gene met9 was disrupted required significantly more methionine for full growth and efficient mating and sporulation than the strain disrupted for gene met11. The in vitro and in vivo data thus indicated that met9 was the major expressed gene. Our results are in accordance with the assumption that the two methylenetetrahydrofolate reductases generate the methyl groups necessary for methionine synthetase to convert homocysteine to methionine, and suggest that expression of the two genes is an important parameter in the control of methionine biosynthesis.

Schizosaccharomyces pombe pfh1+ encodes an essential 5' to 3' DNA helicase that is a member of the PIF1 subfamily of DNA helicases

The Saccharomyces cerevisiae Pif1p DNA helicase is the prototype member of a helicase subfamily conserved from yeast to humans. S. cerevisiae has two PIF1-like genes, PIF1 itself and RRM3, that have roles in maintenance of telomeric, ribosomal, and mitochondrial DNA. Here we describe the isolation and characterization of pfh1+, a Schizosaccharomyces pombe gene that encodes a Pif1-like protein. Pfh1p was the only S. pombe protein with high identity to Saccharomyces Pif1p. Unlike the two S. cerevisiae Pif1 subfamily proteins, the S. pombe Pfh1p was essential. Like Saccharomyces Pif1p, a truncated form of the S. pombe protein had 5' to 3' DNA helicase activity. Point mutations in an invariant lysine residue in the ATP binding pocket of Pfh1p had the same phenotype as deleting pfh1+, demonstrating that the ATPase/helicase activity of Pfh1p was essential. Although mutant spores depleted for Pfh1p proceeded through S phase, they arrested with a terminal cellular phenotype consistent with a postinitiation defect in DNA replication. Telomeric DNA was modestly shortened in the absence of Pfh1p. However, genetic analysis demonstrated that maintenance of telomeric DNA was not the sole essential function of S. pombe Pfh1p.

Marker construction and cloning of a cut1-like sequence with ARS activity in the fission yeast Schizosaccharomyces japonicus

The dimorphic fission yeast Schizosaccharomyces japonicus has proved to be an excellent experimental model for the investigation of the eukaryotic cell. Here we show that it has a haplontic life cycle, in which the diploid phase is confined to the zygote. To make it amenable to genetic and molecular analysis, we generated genetic markers and cloned a genomic sequence which acts as ars when integrated into a plasmid. Diploids suitable for testing complementation and recombination between markers can be formed by protoplast fusion. The complementation tests and the recombination frequencies determined in octads of spores identified 28 non-allelic groups (genes) of mutations of the auxotrophic and mycelium-negative mutants. Two groups of linked markers were also identified. The cloned fragment, which expresses ars activity, encodes a putative amino acid sequence highly similar to a conserved domain of proteins Cut1 (Schizosaccharomyces pombe), BimB (Aspergillus nidulans) and Esp1 (Saccharomyces cerevisiae).

Transformation systems of non-Saccharomyces yeasts

This review describes the transformation systems including vectors, replicons, genetic markers, transformation methods, vector stability, and copy numbers of 13 genera and 31 species of non-Saccharomyces yeasts. Schizosaccharomyces pombe was the first non-Saccharomyces yeast studied for transformation and genetics. The replicons of non-Saccharomyces yeast vectors are from native plasmids, chromosomal DNA, and mitochondrial DNA of Saccharomyces cerevisiae, non-Saccharomyces yeasts, protozoan, plant, and animal. Vectors such as YAC, YCp, YEp, YIp, and YRp were developed for non-Saccharomyces yeasts. Forty-two types of genes from bacteria, yeasts, fungi, and plant were used as genetic markers that could be classified into biosynthetic, dominant, and colored groups to construct non-Saccharomyces yeasts vectors. The LEU2 gene and G418 resistance gene are the two most popular markers used in the yeast transformation. All known transformation methods such as spheroplast-mediating method, alkaline ion treatment method, electroporation, trans-kingdom conjugation, and biolistics have been developed successfully for non-Saccharomyces yeasts, among which the first three are most widely used. The highest copy number detected from non-Saccharomyces yeasts is 60 copies in Kluyveromyces lactis. No general rule is known to illustrate the transformation efficiency, vector stability, and copy number, although factors such as vector composition, host strain, transformation method, and selective pressure might influence them.

Redundant control of rereplication in fission yeast

The initiation of DNA replication at replication origins in eukaryotic cells is tightly controlled to ensure that the genome is duplicated only once each cell cycle. We present evidence that in fission yeast, independent regulation of two essential components of the initiation complex, Cdc18 and Cdt1, contributes to the prevention of reinitiation of DNA replication. Cdc18 is negatively controlled by cyclin-dependent kinase (CDK) phosphorylation, but low level expression of a mutant form of Cdc18 lacking CDK phosphorylation sites (Cdc18(CDK)) is not sufficient to induce rereplication. Similar to Cdc18, Cdt1 is expressed periodically in the cell cycle, accumulating in the nucleus in G(1) and declining in G(2). When Cdt1 is expressed constitutively from an ectopic promoter, it accumulates in the nucleus throughout the cell cycle but does not promote reinitiation. However, constitutive expression of Cdt1, together with Cdc18(CDK), is sufficient to induce extra rounds of DNA replication in the absence of mitosis. Significantly greater levels of rereplication can be induced by coexpression of Cdc18(CDK) and a Cdt1 mutant lacking a conserved C-terminal motif. In contrast, uncontrolled DNA replication does not occur when either mutant protein is expressed in the absence of the other. Constitutive expression of wild-type or mutant Cdt1 also leads to an increase in the levels of Cdc18(CDK), possibly as a result of increased protein stability. Our data are consistent with the hypothesis that control of rereplication depends on a redundant mechanism in which negative regulation of Cdt1 functions in parallel with the negative regulation of Cdc18.

BRCT domain-containing protein TopBP1 functions in DNA replication and damage response

Topoisomerase IIbeta-binding protein (TopBP1), a human protein with eight BRCT domains, is similar to Saccharomyces cerevisiae Dpb11 and Schizosaccharomyces pombe Cut5 checkpoint proteins and closely related to Drosophila Mus101. We show that human TopBP1 is required for DNA replication and that it interacts with DNA polymerase epsilon. In S phase TopBP1 colocalizes with Brca1 to foci that do not represent sites of ongoing DNA replication. Inhibition of DNA synthesis leads to relocalization of TopBP1 together with Brca1 to replication forks, suggesting a role in rescue of stalled forks. DNA damage induces formation of distinct TopBP1 foci that colocalize with Brca1 in S phase, but not in G(1) phase. We also show that TopBP1 interacts with the checkpoint protein hRad9. Thus, these results implicate TopBP1 in replication and checkpoint functions.

Transcriptional termination factors for RNA polymerase II in yeast

The molecular connections between mRNA 3' end processing and transcriptional termination have been investigated in S. pombe using a genetic screen. By this approach, we have identified a RNAP II termination domain in the well-defined cleavage polyadenylation factor called CstF-64 in metazoans and Rna15p in S. cerevisiae. Furthermore, this C-terminal domain interacts with Res2, previously identified as a component of the G1/S transition-specific transcription factor MBF. Deletion of res2 in both fission and budding yeast results in a defect in 3' end formation. This raises the possibility that RNAP II transcriptional termination may in some situations be integrated with cell cycle events.

Specificity of Cdk activation in vivo by the two Caks Mcs6 and Csk1 in fission yeast

Activating phosphorylation of cyclin-dependent kinases (Cdks) is mediated by at least two structurally distinct types of Cdk-activating kinases (Caks): the trimeric Cdk7-cyclin H-Mat1 complex in metazoans and the single-subunit Cak1 in budding yeast. Fission yeast has both Cak types: Mcs6 is a Cdk7 ortholog and Csk1 a single-subunit kinase. Both phosphorylate Cdks in vitro and rescue a thermosensitive budding yeast CAK1 strain. However, this apparent redundancy is not observed in fission yeast in vivo. We have identified mutants that exhibit phenotypes attributable to defects in either Mcs6-activating phosphorylation or in Cdc2-activating phosphorylation. Mcs6, human Cdk7 and budding yeast Cak1 were all active as Caks for Cdc2 when expressed in fission yeast. Although Csk1 could activate Mcs6, it was unable to activate Cdc2. Biochemical experiments supported these genetic results: budding yeast Cak1 could bind and phosphorylate Cdc2 from fission yeast lysates, whereas fission yeast Csk1 could not. These results indicate that Mcs6 is the direct activator of Cdc2, and Csk1 only activates Mcs6. This demonstrates in vivo specificity in Cdk activation by Caks.

bgs2+, a sporulation-specific glucan synthase homologue is required for proper ascospore wall maturation in fission yeast

The formation of the ascospore cell wall of Schizosaccharomyces pombe requires the co-ordinated activity of enzymes involved in the biosynthesis of its components, such as glucans. We have cloned the bgs2+ gene. bgs2+ belongs to the glucan synthase family of S. pombe and is homologous to the Saccharomyces cerevisiae FKS1 and FKS2 genes. Deletion or overexpression of this gene does not lead to any apparent defect during vegetative growth, but homozygous bgs2Delta diploids do show a sporulation defect. Although meiosis takes place normally, ascospores are unable to mature, and their wall differs from that of wild-type ascospores. Moreover, bgs2Delta zygotes were not able to release ascospores spontaneously, and the ascospores were unable to germinate. We show that expression of bgs2+ is restricted to sporulation and that a bgs2-green fluorescent protein (GFP) fusion protein localizes to the ascospore envelope. The glucan synthase activity in sporulating diploids bearing a bgs2 deletion was diminished in comparison with that of the wild-type diploids, a fact that underscores the importance of the bgs2+ gene and glucan synthesis for the proper formation and maturation of the ascospore wall.

Construction of fission yeast vectors with a novel selection strategy that allows their use in wild-type fission yeasts

Novel vectors that use the Pichia pastoris INO1 gene as a selectable marker and exploit the natural inositol auxotrophy of the fission yeast are described. These plasmids also contained other features desirable in a plasmid cloning vector. These plasmids were evaluated in other species of Schizosaccharomyces and found to replicate autonomously in another variety of S. pombe, S. pombe var. malidevorans. These plasmids can be used for transformation of any wild-type S. pombe strain without the need for selection by induced auxotrophic mutations, or by selection by drug resistance markers, and should greatly assist genetic and molecular manipulations in these yeasts.

Protection of telomeres by the Ku protein in fission yeast

Schizosaccharomyces pombe cells survive loss of telomeres by a unique pathway of chromosome circularization. Factors potentially involved in this survival mechanism include the heterodimeric Ku protein and ligase IV, both of which are involved in the repair of DNA double-strand breaks in mammalian cells. Furthermore, Ku plays a role in telomere maintenance as well as in DNA double-strand break repair in Saccharomyces cerevisiae. We have identified Ku and ligase IV homologues in S. pombe and analyzed their functions during normal growth and in cells undergoing senescence. In the absence of either a Ku subunit (pku70(+)) or ligase IV (lig4(+)), nonhomologous DNA end-joining was severely reduced. Lack of functional Ku led to shorter but stable telomeres and caused striking rearrangements of telomere-associated sequences, indicating a function for Ku in inhibiting recombinational activities near chromosome ends. In contrast to S. cerevisiae, concurrent deletion of pku70(+) and the gene for the catalytic subunit of telomerase (trt1(+)) was not lethal, allowing for the first time the dissection of the roles of Ku during senescence. Our results support a model in which Ku protects chromosome termini from nucleolytic and recombinational activities but is not involved in the formation of chromosome end fusions during senescence. The conclusion that nonhomologous end-joining is not required for chromosome circularization was further supported by analysis of survivors in strains lacking the genes for both trt1(+) and lig4(+).

A family of multifunctional thiamine-repressible expression vectors for fission yeast

A series of thiamine-repressible shuttle vectors has been constructed to allow a more efficient DNA manipulation in Schizosaccharomyces pombe. These high-copy-number vectors with regulatable expression (pJR) are based on the backbone of the pREP-3X, pREP-41X and pREP-81X plasmids. The pJR vectors are all uniform in structure, containing: (a) sequences for replication (ori) and selection (AmpR) in Escherichia coli; (b) the f1 ori sequence of the phage f1 for packaging of ssDNA, making them suitable for site-directed mutagenesis; and (c) the ars1 sequence for replication in S. pombe. The pJR vectors differ among them in: (a) the selectable marker (Saccharomyces cerevisiae LEU 2 gene, which complements S. pombe leu1- gene and S. pombe ura4+ and his3+ genes); (b) the thiamine-repressible nmt1 promoter (3X, 41X and 81X with extremely high, moderate or low transcription efficiency, respectively); and (c) the multiple cloning site (two multiple cloning sites, with 12 restriction sites each). The expression level of the pJR vectors has been analysed using the beta-galactosidase gene as reporter. Three levels of expression for each nmt1 promoter version, with any selectable marker and for either repressed or induced conditions, have been found. The expression is dependent on the distance to the initiation codon, varying from 0.001 to 15 times the activity characterized for the pREP plasmids. Also, the gene expression has been found to be extremely sensitive to the nucleotide sequence prior to the initiation codon, being up to 50-fold higher with an A/T sequence than with a G/C sequence. Finally, the beta-galactosidase mRNA levels were found to be similar in each nmt1 series, suggesting a translational effect on gene expression. As a result, any of these 18 new vectors allow performing gene expression in fission yeast, as well as a more versatile cloning, sequencing and mutagenesis, directly in the plasmid without the need for subcloning into intermediary vectors.

Isolation of an essential Schizosaccharomyces pombe gene, prp31(+), that links splicing and meiosis

We carried out a screen for mutants that arrest prior to premeiotic S phase. One of the strains we isolated contains a temperature-sensitive allele mutation in the fission yeast prp31(+) gene. The prp31-E1 mutant is defective in vegetative cell growth and in meiotic progression. It is synthetically lethal with prp6 and displays a pre-mRNA splicing defect at the restrictive temperature. We cloned the wild-type gene by complementation of the temperature-sensitive mutant phenotype. Prp31p is closely related to human and budding yeast PRP31 homologs and is likely to function as a general splicing factor in both vegetative growth and sexual differentiation.

A 38 kb segment containing the cdc2 gene from the left arm of fission yeast chromosome II: sequence analysis and characterization of the genomic DNA and cDNAs encoded on the segment

A genomic 38 kbp segment on the c1750 cosmid clone containing the cdc2 gene, located in the left arm of chromosome II from Schizosaccharomyces pombe, was sequenced. The segment was found to have five previously known genes, pht1, cdc2, his3, act1 and mei4. Among 11 coding sequences (CDSs) predicted by the gene finding software INTRON.PLOT., four CDSs, pi007, pi010, pi014 and pi016, had considerable similarity to 40S ribosomal protein, glycosyltransferase, cdc2-related protein kinase and alpha-1, 2-mannosyltransferase, respectively. Another unusually huge open reading frame (ORF) (pi011), consisting of 2233 amino acids, existed, having significant homology to alpha-amylase, granule-bound glycogen synthase and the Sz. pombe YS 1110 clone product at the N-terminal, middle and C-terminal regions, respectively. All the predicted 11 CDSs were experimentally analysed by RACE PCR. The sequencing of the RACE products revealed that there were two small overlaps at the 3' untranslated regions (UTRs) between pi004 and pi005 (17 bp) and between pi007 and pi008 (2 bp). The distances between 5' end of the 5'UTR and the putative translation initiation codon varied from 10 to 302 nucleotides (nt) among the nine CDSs successfully analysed by 5'-RACE. The expression level of each CDS on this clone was determined. Among the 16 genes on this clone, the previously determined genes, pht1, cdc2, his3 and act1, were found to be most highly expressed. Finally, cDNAs of all the newly identified genes were detected by RACE, proving the actual expression of these genes. The nucleotide sequence has been submitted to the EMBL database under Accession No. AB004534.

Although calnexin is essential in S. pombe, its highly conserved central domain is dispensable for viability

In mammalian cells, the calnexin/calreticulin chaperones play a key role in glycoprotein folding and its control within the endoplasmic reticulum (ER), by interacting with folding intermediates via their monoglucosylated glycans. This lectin activity has been mapped in mammalian calnexin/calreticulin chaperones to the central region, which is a highly conserved feature of calnexin/calreticulin molecules across species. The central domain has also been implicated in Ca(2+) binding, and it has been proposed to be involved in the regulation of calcium homeostasis in the ER. Herein, we show that although the Schizosaccharomyces pombe calnexin is essential for viability, cells lacking its 317-amino-acid highly conserved central region are viable under normal growth conditions. However, the central region appears to be necessary for optimal growth under high ER-stress, suggesting that this region is important under extreme folding situations (such as DTT and temperature). The minimal length of calnexin required for viability spans the C-terminal 123 residues. Furthermore, cells with the central domain of the protein deleted were affected in their morphology at 37 degrees C, probably due to a defect in cell wall synthesis, although these mutant cells exhibited the same calcium tolerance as wild-type cells at 30 degrees C.

The role of nucleotide binding and hydrolysis in the function of the fission yeast cdc18(+) gene product

The fission yeast cdc18(+) gene is required for both initiation of DNA replication and the mitotic checkpoint that normally inhibits mitosis in the absence of DNA replication. The cdc18(+) gene product contains conserved Walker A and B box motifs. Studies of other ATPases have shown that these motifs are required for nucleotide binding and hydrolysis, respectively. We have observed that mutant strains in which either of these motifs is disrupted are inviable. The effects of these mutations were examined by determining the phenotypes of mutant strains following depletion of complementing wild-type Cdc18. In both synchronous and asynchronous cultures, the nucleotide-hydrolysis motif mutant (DE286AA) arrests with a 1C-2C DNA content, and thus exhibits no obvious defects in entry into S phase or in the mitotic checkpoint. In contrast, in cultures synchronized by hydroxyurea arrest and release, the nucleotide-binding motif mutant (K205A) exhibits the null phenotype, with 1C and <1C DNA content, indicating a block in entry into S phase and loss of checkpoint control. In asynchronous cultures this mutant exhibits a mixed phenotype: a percentage of the population displays the null phenotype, while the remaining fraction arrests with a 2C DNA content. Thus, the phenotype exhibited by the K205A mutant is dependent on the cell-cycle position at which wild-type Cdc18 is depleted. These data indicate that both nucleotide binding and hydrolysis are required for Cdc18 function. In addition, the difference in the phenotypes exhibited by the nucleotide-binding and hydrolysis motif mutants is consistent with a two-step model for Cdc18 function in which nucleotide binding and hydrolysis are required for distinct aspects of Cdc18 function that may be executed at different points in the cell cycle.

The msh2 gene of Schizosaccharomyces pombe is involved in mismatch repair, mating-type switching, and meiotic chromosome organization

We have identified in the fission yeast Schizosaccharomyces pombe a MutS homolog that shows highest homology to the Msh2 subgroup. msh2 disruption gives rise to increased mitotic mutation rates and increased levels of postmeiotic segregation of genetic markers. In bandshift assays performed with msh2Delta cell extracts, a general mismatch-binding activity is absent. By complementation assays, we showed that S. pombe msh2 is allelic with the previously identified swi8 and mut3 genes, which are involved in mating-type switching. The swi8-137 mutant has a mutation in the msh2 gene which causes a truncated Msh2 peptide lacking a putative DNA-binding domain. Cytological analysis revealed that during meiotic prophase of msh2-defective cells, chromosomal structures were frequently formed; such structures are rarely found in the wild type. Our data show that besides having a function in mismatch repair, S. pombe msh2 is required for correct termination of copy synthesis during mating-type switching as well as for proper organization of chromosomes during meiosis.

A Schizosaccharomyces pombe artificial chromosome large DNA cloning system

The feasibility of using the fission yeast, Schizosaccharomyces pombe , as a host for the propagation of cloned large fragments of human DNA has been investigated. Two acentric vector arms were utilized; these carry autonomously replicating sequences ( ars elements), selectable markers ( ura4(+) or LEU2 ) and 250 bp of S. pombe terminal telomeric repeats. All cloning was performed between the unique sites in both vector arms for the restriction endonuclease Not I. Initially the system was tested by converting six previously characterized cosmids from human chromosome 11p13 into a form that could be propagated in S.pombe as linear episomal elements of 50-60 kb in length. In all transformants analysed these cosmids were maintained intact. To test if larger fragments of human DNA could also be propagated total human DNA was digested with Not I and size fractionated by pulsed field gel electrophoresis (PFGE). Fractions of 100-1000 kb were ligated to Not I-digested vector arms and transformed into S.pombe protoplasts in the presence of lipofectin. Prototrophic ura+leu+transformants were obtained which upon examination by PFGE were found to contain additional linear chromosomes migrating at between 100 and 500 kb with a copy number of 5-10 copies/cell. Hybridization analyses revealed that these additional bands contained human DNA. Fluorescent in situ hybridization (FISH) analyses of several independent clones indicated that the inserts were derived from single loci within the human genome. These analyses clearly demonstrate that it is possible to clone large fragments of heterologous DNA in fission yeast using this S.p ombe artificial chromosome system which we have called SPARC. This vector-host system will complement the various other systems for cloning large DNA fragments.

The high mobility group domain protein Cmb1 of Schizosaccharomyces pombe binds to cytosines in base mismatches and opposite chemically altered guanines

The mismatch-binding activity Cmb1 of Schizosaccharomyces pombe was enriched from wild type cells, and N-terminal sequencing enabled cloning of the respective gene. The deduced amino acid sequence of cmb1(+) contains a high mobility group domain, a motif that is common to a heterogeneous family of DNA-binding proteins. In crude protein extracts of a cmb1 gene-disruption strain, specific binding to C/T, C/A, and C/Delta was abolished. Weak binding to C/C revealed the presence of a second mismatch-binding activity, Cmb2. Cmb1, enriched from S. pombe and purified from Escherichia coli, bound specifically to C/C, C/T, C/A, T/T, and C/Delta but showed little or no affinity to other mismatches and small loops. Cmb1 recognizes 1,2 GpG intrastrand cross-links, produced by the chemotherapeutic drug cisplatin, when two cytosines are opposite the cross-linked guanines but not when other bases are present. Consistently, O6-methylguanine:C but not O6-methylguanine/T lesions were bound. Thus, cytosines in mismatches and opposite chemically modified guanines are the preferred target of Cmb1 recognition. cmb1 mutant cells are more sensitive to cisplatin than wild type cells, indicating a role of Cmb1 in repair of cisplatin-induced DNA damage.

Mutational effect of fission yeast polalpha on cell cycle events

Polalpha is the principal DNA polymerase for initiation of DNA replication and also functions in postinitiation DNA synthesis. In this study, we investigated the cell cycle responses induced by mutations in polalpha+. Germinating spores carrying either a deletion of polalpha+ (polalphaDelta) or a structurally intact but catalytically dead polalpha mutation proceed to inappropriate mitosis with no DNA synthesis. This suggests that the catalytic function, and not the physical presence of Polalpha, is required to generate the signal that prevents the cells from entering mitosis prematurely. Cells with a polalphats allele arrest the cell cycle near the hydroxyurea arrest point, but, surprisingly, polalphats in cdc20 (polepsilon mutant) background arrested with a cdc phenoytpe, not a polalphats-like phenotype. At 25 degrees C, replication perturbation caused by polalphats alleles induces Cds1 kinase activity and requires the checkpoint Rads, Cds1, and Rqh1, but not Chk1, to maintain cell viability. At 36 degrees C, replication disruption caused by polalphats alleles induces the phosphorylation of Chk1; however, mutant cells arrest with heterogeneous cell sizes with a population of the cells entering aberrant mitosis. Together, our results indicate that the initiation DNA structure synthesized by Polalpha is required to bring about the S phase to mitosis checkpoint, whereas replication defects of different severity caused by polalphats mutations induce differential downstream kinase responses.

sud1(+) targets cyclin-dependent kinase-phosphorylated Cdc18 and Rum1 proteins for degradation and stops unwanted diploidization in fission yeast

In the fission yeast Schizosaccharomyces pombe, S phase is limited to a single round per cell cycle through cyclin-dependent kinase phosphorylation of critical replication factors, including the Cdc18 replication initiator protein. Because defects in Cdc18 phosphorylation lead to a hyperstable and hyperactive form of Cdc18 that promotes high levels of overreplication in vivo, we wished to identify the components of the Cdc18 proteolysis pathway in fission yeast. In this paper we describe one such component, encoded by the sud1(+) gene. sud1(+) shares homology with the budding yeast CDC4 gene and is required to prevent spontaneous re-replication in fission yeast. Cells lacking sud1(+) accumulate high levels of Cdc18 and the CDK inhibitor Rum1, because they cannot degrade these two key cell cycle regulators. Through genetic analysis we show that hyperaccumulation of Rum1 contributes to re-replication in Deltasud1 cells, but is not the cause of the defect in Cdc18 proteolysis. Rather, Sud1 itself is associated with the ubiquitin pathway in fission yeast and binds to Cdc18 in vivo. Most importantly, Sud1-Cdc18 binding requires prior phosphorylation of the Cdc18 polypeptide at CDK consensus sites. These results provide a biochemical mechanism for the phosphorylation-dependent degradation of Cdc18 and other cell cycle regulators, including Rum1. Evolutionary conservation of the Sud1/CDC4 pathway suggests that phosphorylation-coupled proteolysis may be a general feature of nearly all eukaryotic cell cycles.

A series of vectors to construct lacZ fusions for the study of gene expression in Schizosaccharomyces pombe

We have constructed a series of plasmids to facilitate the fusion of promoters with or without coding regions of genes of Schizosaccharomyces pombe to the lacZ gene of Escherichia coli. These vectors carry a multiple cloning region in which fission yeast DNA may be inserted in three different reading frames with respect to the coding region of lacZ. The plasmids were constructed with the ura4+ or the his3+ marker of S. pombe. Functionality of the plasmids was tested measuring in parallel the expression of fructose 1,6-bisphosphatase and beta-galactosidase under the control of the fbp1+ promoter in different conditions.

Mutational analysis of the gene for Schizosaccharomyces pombe RNase MRP RNA, mrp1, using plasmid shuffle by counterselection on canavanine

Reverse genetics in fission yeast is hindered by the lack of a versatile established plasmid shuffle system. In order to screen efficiently and accurately through plasmid-borne mutations in the essential gene for the RNA component of RNase MRP, mrp1, we have developed a system for plasmid shuffling in fission yeast using counterselection on canavanine. The system takes advantage of the ability of the Saccharomyces cerevisiae CAN1 gene to complement a Schizosaccharomyces pombe can1-1 mutation. Two general use plasmids were constructed that allow directional cloning and initial selection for histidine before counterselection by canavanine. The strain constructed for plasmid shuffling carries auxotrophic markers for ade6, leul, ura4 and his3 along with the can1-1 mutation. Using this system we examined several partial deletions and point mutations in conserved nucleotides of Schizosaccharomyces pombe RNase MRP RNA for their ability to complement a chromosomal deletion of the mrp1 gene. The degree of background canavanine resistance as well as plasmid-plasmid recombination encountered in these experiments was sufficiently low to suggest that the system we have set up for counterselection by canavanine in fission yeast using multicopy plasmids will be widely useful.

Construction of vectors and a genomic library for use with his3-deficient strains of Schizosaccharomyces pombe

The construction of vectors for use in Schizosaccharomyces pombe using the his3+ gene as a selectable marker is described. In addition, we report the construction of a genomic library in a his3(+)-containing shuttle vector to facilitate the cloning of genes by complementation of mutant function in strains defective for His3 activity.

Novel alleles of cdc13 and cdc2 isolated as suppressors of mitotic catastrophe in Schizosaccharomyces pombe

Cell cycle control in the fission yeast Schizosaccharomyces pombe involves interplay amongst a number of regulatory molecules, including the cdc2, cdc13, cdc25, wee1, and mik1 gene products. Cdc2, Cdc13, and Cdc25 act as positive regulators of cell cycle progression at the G2/M boundary, while Wee1 and Miky1 play a negative regulatory role. Here, we have screened for suppressors of the lethal premature entry into mitosis, termed mitotic catastrophe, which results from simultaneous loss of function of both Wee1 and Mik1. Through such a screen, we hoped to identify additional components of the cell cycle regulatory network, and/or G2/M-specific substrates of Cdc2. Although we did not identify such molecules, we isolated a number of alleles of both cdc2 and cdc13, including a novel wee allele of cdc2, cdc2-5w. Here, we characterize cdc2-5w and two alleles of cdc13, which have implications for the understanding of details of the interactions amongst Cdc2, Cdc13, and Wee1.

Phylogenesis of fission yeasts. Contradictions surrounding the origin of a century old genus

The phylogenesis of fungi is controversial due to their simple morphology and poor fossilization. Traditional classification supported by morphological studies and physiological traits placed the fission yeasts in one group with ascomycetous yeasts. The rRNA sequence comparisons, however, revealed an enormous evolutionary gap between Saccharomyces and Schizosaccharomyces. As shown in this review, the protein sequences also show a large gap which is almost as large as that separating Schizosaccharomyces from higher animals. Since the two yeasts share features (both cytological and molecular) in common which are also characteristic of ascomycetous fungi, their separation must have taken place later than the sequence differences may suggest. Possible reasons for the paradox are discussed. The sequence data also suggest a slower evolutionary rate in the Schizosaccharomyces lineage than in the Saccharomyces branch. In the fission yeast lineage two ramifications can be supposed. First S. japonicus (Hasegawaea japonica) branched off, then S. octosporus (Octosporomyces octosporus) separated from S. pombe.

Schizosaccharomyces pombe: a model for molecular studies of eukaryotic genes

Several features of the fission yeast Schizosaccharomyces pombe make it exceptionally well suited for the study of eukaryotic genes. It is a relatively simple eukaryote that can be readily grown and manipulated in the laboratory, using a variety of highly developed and sophisticated methodologies. Schizosaccharomyces pombe cells share many molecular, genetic, and biochemical features with cells from multicellular organisms, making it a particularly useful model to study the structure, function, and regulation of genes from more complex species. For examples, this yeast divides by binary fission, has many genes that contain introns, is capable of using mammalian gene promoters and polyadenylation signals, and has been used to clone mammalian genes by functional complementation of mutants. We present a summary of the biology of S. pombe, useful features that make it amenable to laboratory studies, and molecular techniques available to manipulate the genome of this organism as well as other eukaryotic genes within the fission yeast cellular environment.