Genetic engineering of Schizosaccharomyces pombe: a system for gene disruption and replacement using the ura4 gene as a selectable marker

Synchronized meiosis and recombination in fission yeast: observations with pat1-114 diploid cells

The mutation pat1-114 has been used to synchronize meiosis in the fission yeast Schizosaccharomyces pombe. We have investigated several aspects of such synchronized meiotic cultures. In both pat1-114 and pat1+ diploids, meiotic landmark events are initiated at the same time after meiosis induction, but synchrony is much more pronounced in the pat1-114-driven meiosis. Commitment to recombination and to meiosis have been timed at 2 h after meiotic induction. Due to a seven-fold reduction of intragenic recombination frequency in the ade6 region of pat1-114 diploids, physical analysis of recombination has not been possible. We have distinguished three factors that influence intragenic recombination frequencies: temperature, azygotic versus zygotic meiosis, and the nature of the pat1 allele. Differences and similarities in the timing of meiotic landmarks in S. cerevisiae and S. pombe are discussed.

A fission-yeast gene encoding a protein with features of protein-tyrosine-phosphatases

Degenerate oligonucleotide probes encoding sequences conserved among mammalian protein-tyrosine-phosphatases (PTPases) were used to amplify DNA fragments from a Schizosaccharomyces pombe cDNA library by polymerase chain reaction (PCR) methods. A cloned PCR product predicted peptide sequences similar to those found in PTPases but not identical to any published sequences. A S. pombe gene, designated pyp1+, was identified in a cDNA library with this PCR probe, cloned, and sequenced. The sequence of the gene predicted a 550-amino acid protein with Mr 61,586, which includes amino acid sequences that are highly conserved in mammalian PTPases. Disruption of the pyp1+ gene resulted in viable cells. Overexpression of the pyp1+ gene in S. pombe permitted detection of a protein of apparent Mr 63,000.

prp4 from Schizosaccharomyces pombe, a mutant deficient in pre-mRNA splicing isolated using genes containing artificial introns

We have generated a bank of temperature-sensitive (ts) Schizosaccharomyces pombe mutant strains. About 150 of these mutants were transformed with a ura4 gene containing an artificial intron. We screened these ts mutants for mutants deficient in splicing of the ura4 intron. With this approach three mutants were isolated which have a general defect in the splicing process. Two of these mutants fall into the prp1 complementation group and one defines a new complementation group, prp4.

mik1 and wee1 cooperate in the inhibitory tyrosine phosphorylation of cdc2

wee1 acts antagonistically to cdc25 in the tyrosine dephosphorylation and activation of cdc2, yet biochemical evidence suggests that wee1 is not required for tyrosine phosphorylation and its role is obscure. We show here that a related 66 kd kinase, called mik1, acts redundantly with wee1 in the negative regulation of cdc2 in S. pombe. A null allele of mik1 has no discernible phenotype, but a mik1 wee1 double mutant is hypermitotically lethal: all normal M phase checkpoints are bypassed, including the requirement for initiation of cell cycle "start," completion of S phase, and function of the cdc25+ mitotic activator. In the absence of mik1 and wee1 activity, cdc2 rapidly loses phosphate on tyrosine, both in strains undergoing mitotic lethality and in those that are viable owing to a compensating mutation within cdc2. The data suggest that mik1 and wee1 act cooperatively on cdc2, either directly as the inhibitory tyrosine kinase or as essential activators of that kinase.

Identification of a gene encoding the predicted ribosomal protein L7b divergently transcribed from POL1 in fission yeast Schizosaccharomyces pombe

A 0.85 Kb RNA molecule is transcribed in the region upstream from the 5'-end of the S. pombe POL1 gene encoding the catalytic subunit of DNA polymerase alpha. The nucleotide sequence of the DNA region hybridizing with the 0.85 Kb transcript allowed us to identify an open reading frame coding for a predicted peptide which shows 50% identity with the rat ribosomal protein L7 and which is transcribed divergently from POL1. We have named this gene RPL7b because of the existence in S. pombe of a different sequence, named RPL7, which also codes for a putative protein showing homology with the rat ribosomal protein L7. The RPL7b gene includes a 291 bp-long intron containing the sequences necessary for intron excision and RNA splicing in S. pombe. The precise location of the intron was established by amplification and sequencing of a partial cDNA copy of the mRNA, whereas the initiation site of transcription was determined by reverse transcription of the 5' region of the mRNA. The 320 bp separating the starting methionine codons of RPL7b and POL1 genes should contain the signals necessary for their divergent transcription and regulation. The sequence 5'-AAGACAGTCACA-3', whose primary structure is homologous to a conserved block present in the 5'-untranscribed regions of other S. pombe genes of ribosomal proteins, is located about 50 bp upstream the transcription initiation site of RPL7b.

The Schizosaccharomyces pombe homolog of Saccharomyces cerevisiae HAP2 reveals selective and stringent conservation of the small essential core protein domain

The fission yeast Schizosaccharomyces pombe is immensely diverged from budding yeast (Saccharomyces cerevisiae) on an evolutionary time scale. We have used a fission yeast library to clone a homolog of S. cerevisiae HAP2, which along with HAP3 and HAP4 forms a transcriptional activation complex that binds to the CCAAT box. The S. pombe homolog php2 (S. pombe HAP2) was obtained by functional complementation in an S. cerevisiae hap2 mutant and retains the ability to associate with HAP3 and HAP4. We have previously demonstrated that the HAP2 subunit of the CCAAT-binding transcriptional activation complex from S. cerevisiae contains a 65-amino-acid "essential core" structure that is divisible into subunit association and DNA recognition domains. Here we show that Php2 contains a 60-amino-acid block that is 82% identical to this core. The remainder of the 334-amino-acid protein is completely without homology to HAP2. The function of php2 in S. pombe was investigated by disrupting the gene. Strikingly, like HAP2 in S. cerevisiae, the S. pombe gene is specifically involved in mitochondrial function. This contrasts to the situation in mammals, in which the homologous CCAAT-binding complex is a global transcriptional activator.

New vectors in fission yeast: application for cloning the his2 gene

We describe a new Escherichia coli vector (pON5) that allows positive selection for recombinant clones. In this plasmid, the bla gene from pBR322 is permanently active, whereas the neo gene from transposon Tn5 is repressed by the cI-encoded lambda repressor. When DNA is inserted into the Bc/I or HindIII restriction sites situated within the cI gene, the neo gene becomes transcribed from the lambda pR promoter. We have also made a Schizosaccharomyces pombe derivative of pON5 (= pON163) by introducing the fission yeast ars1 and ura4+ sequences. We show that this plasmid is capable of transforming Sc. pombe ura4 strains, as well as ura 3 strains of the distantly related budding yeast Saccharomyces cerevisiae. We have used pON163 for the construction of two fission yeast genomic libraries. From these gene banks clones were isolated that were able to complement fission yeast his2 mutants. Such plasmids could also rescue his4C mutants of Sa. cerevisiae, defective in the histidinol dehydrogenase activity of the multifunctional HIS4 gene product. Finally, we describe the plasmid pDW232 which is useful for functional analysis of fission yeast genes. It is a pGEM3 derivative adapted to fission yeast, carrying multiple cloning sites between the T7 and SP6 promoters, together with ars1 and ura4+ from Sc. pombe.

The strong ADH1 promoter stimulates mitotic and meiotic recombination at the ADE6 gene of Schizosaccharomyces pombe

The effect of the strong promoter from the alcohol dehydrogenase gene on mitotic and meiotic intragenic recombination has been studied at the ade6 locus of the fission yeast Schizosaccharomyces pombe. A 700-bp fragment containing the functional adh1 promoter was used to replace the weak wild-type promoter of the ade6 gene. Analysis of mRNA showed that strains with this ade6::adh1 fusion construct had strongly elevated ade6-specific mRNA levels during vegetative growth as well as in meiosis. These increased levels of mRNA correlated with a 20- to 25-fold stimulation of intragenic recombination in meiosis and a 7-fold increased prototroph formation during vegetative growth. Analysis of flanking marker configurations of prototrophic recombinants indicated that simple conversions as well as conversions associated with crossing over were stimulated in meiosis. The strongest stimulation of recombination was observed when the adh1 promoter was homozygous. Studies with heterologous promoter configurations revealed that the highly transcribed allele was the preferred acceptor of genetic information. The effect of the recombinational hot spot mutation ade6-M26 was also investigated in this system. Its effect was only partly additive to the elevated recombination rate generated by the ade6::adh1 fusion construct.

The Schizosaccharomyces pombe homolog of Saccharomyces cerevisiae HAP2 reveals selective and stringent conservation of the small essential core protein domain

The fission yeast Schizosaccharomyces pombe is immensely diverged from budding yeast (Saccharomyces cerevisiae) on an evolutionary time scale. We have used a fission yeast library to clone a homolog of S. cerevisiae HAP2, which along with HAP3 and HAP4 forms a transcriptional activation complex that binds to the CCAAT box. The S. pombe homolog php2 (S. pombe HAP2) was obtained by functional complementation in an S. cerevisiae hap2 mutant and retains the ability to associate with HAP3 and HAP4. We have previously demonstrated that the HAP2 subunit of the CCAAT-binding transcriptional activation complex from S. cerevisiae contains a 65-amino-acid "essential core" structure that is divisible into subunit association and DNA recognition domains. Here we show that Php2 contains a 60-amino-acid block that is 82% identical to this core. The remainder of the 334-amino-acid protein is completely without homology to HAP2. The function of php2 in S. pombe was investigated by disrupting the gene. Strikingly, like HAP2 in S. cerevisiae, the S. pombe gene is specifically involved in mitochondrial function. This contrasts to the situation in mammals, in which the homologous CCAAT-binding complex is a global transcriptional activator.

Two related families of retrotransposons from Schizosaccharomyces pombe

Two related families of transposons were isolated from schizosaccharomyces pombe, an organism which has been the object of extensive genetic studies which had previously produced no evidence for the existence of such elements. These two classes of repeated DNAs, dubbed Tf1 (transposon of fission yeast 1) and Tf2 have many properties of retrotransposons. Tf1 and Tf2 both possess long terminal repeats and predicted protein sequences that resemble the protease, reverse transcriptase, and integrase domains of retroviruses. The chromosomal locations and total numbers of Tf1 and Tf2 differ greatly in various isolates of S. pombe. The Tf elements are expressed in the form of 4.5-kb mRNAs. The complete sequence of Tf1 was determined and suggests that a novel mechanism for regulating its gene expression may be used.

Fission yeast genes that confer resistance to staurosporine encode an AP-1-like transcription factor and a protein kinase related to the mammalian ERK1/MAP2 and budding yeast FUS3 and KSS1 kinases

Staurosporine, a potent inhibitor of protein kinase C, arrests fission yeast cell elongation specifically at a stage immediately after cell division. We isolated two genes, which, when carried on multicopy plasmids, confer drug resistance in fission yeast. One, spk1+, encodes a protein kinase highly similar (54% identity) to those encoded by the mammalian ERK1/MAP2 kinase and the budding yeast KSS1 and FUS3 genes. It is not essential for vegetative growth of Schizosaccharomyces pombe cells but is required for conjugation. The spk1+ gene product is a 45-kD protein enriched in the nucleus, and its level increases 10-fold after addition of staurosporine. The other gene pap1+ encodes an AP-1-like transcription factor that contains a region rich in basic amino acids followed by a "leucine zipper" motif. The pap1+ gene is required for spk1(+)-conferred staurosporine resistance. These two genes appear to function as a part of the fission yeast growth control pathway.

The strong ADH1 promoter stimulates mitotic and meiotic recombination at the ADE6 gene of Schizosaccharomyces pombe

The effect of the strong promoter from the alcohol dehydrogenase gene on mitotic and meiotic intragenic recombination has been studied at the ade6 locus of the fission yeast Schizosaccharomyces pombe. A 700-bp fragment containing the functional adh1 promoter was used to replace the weak wild-type promoter of the ade6 gene. Analysis of mRNA showed that strains with this ade6::adh1 fusion construct had strongly elevated ade6-specific mRNA levels during vegetative growth as well as in meiosis. These increased levels of mRNA correlated with a 20- to 25-fold stimulation of intragenic recombination in meiosis and a 7-fold increased prototroph formation during vegetative growth. Analysis of flanking marker configurations of prototrophic recombinants indicated that simple conversions as well as conversions associated with crossing over were stimulated in meiosis. The strongest stimulation of recombination was observed when the adh1 promoter was homozygous. Studies with heterologous promoter configurations revealed that the highly transcribed allele was the preferred acceptor of genetic information. The effect of the recombinational hot spot mutation ade6-M26 was also investigated in this system. Its effect was only partly additive to the elevated recombination rate generated by the ade6::adh1 fusion construct.

Two related families of retrotransposons from Schizosaccharomyces pombe

Two related families of transposons were isolated from schizosaccharomyces pombe, an organism which has been the object of extensive genetic studies which had previously produced no evidence for the existence of such elements. These two classes of repeated DNAs, dubbed Tf1 (transposon of fission yeast 1) and Tf2 have many properties of retrotransposons. Tf1 and Tf2 both possess long terminal repeats and predicted protein sequences that resemble the protease, reverse transcriptase, and integrase domains of retroviruses. The chromosomal locations and total numbers of Tf1 and Tf2 differ greatly in various isolates of S. pombe. The Tf elements are expressed in the form of 4.5-kb mRNAs. The complete sequence of Tf1 was determined and suggests that a novel mechanism for regulating its gene expression may be used.

Isolation and molecular analysis of the orotidine-5'-phosphate decarboxylase gene (pyrG) of Phycomyces blakesleeanus

The pyrG gene of Phycomyces was isolated from a Phycomyces genomic library, constructed in the cosmid pHS255, by hybridization with a 170 bp fragment of the pyrG gene of Aspergillus niger. This fragment includes a consensus sequence found in almost all species in which the orotidine-5'-phosphate decarboxylase (OMPdecase) gene has been sequenced. The complete nucleotide sequence of the cloned pyrG gene from Phycomyces was determined and the transcription start sites mapped. In the predicted amino acid sequence there are regions of strong homology to the equivalent genes of Saccharomyces cerevisiae, A. niger, Schizophyllum commune and Homo sapiens. Analysis of the sequence revealed the presence of two introns. The precise length and location of these introns was determined by sequencing the pyrG cDNA and comparing it with the genomic clone. Non-coding flanking regions showed obvious homology to the consensus TATA and CAAT boxes, and the polyadenylation signal "AATAAA". The pyrG gene is the second Phycomyces gene that has been cloned and analysed. This is the first time that introns have been reported in Phycomyces.

Distinct, essential roles of type 1 and 2A protein phosphatases in the control of the fission yeast cell division cycle

The activities of type 1 protein phosphatase (PP1) and 2A (PP2A) have distinct, essential roles in cell cycle control. Two previously identified PP1 genes (dis2+ and sds21+) and two PP2A genes (ppa1+ and ppa2+), highly homologous to mammalian PP2A, have been isolated from fission yeast. Only double gene disruption of both PP2A genes results in lethality, as is the case for PP1 genes. By fractionating and assaying PPases in wild-type, various deletion, and point mutant strains, the decrease of PP1 or PP2A activity is shown to cause mitotic defects, exhibiting strikingly different cell cycle phenotypes: cold-sensitive mutations in the same amino acid lesion of PP1 and PP2A produce chromosome nondisjunction and premature mitosis, respectively. Consistently, PP1 and PP2A genes cannot be functionally substituted. Although the overall levels of PP1 and PP2A activities do not fluctuate during the cell cycle, subpopulations might be regulated.

The recombinational hot spot mutation ade6-M26 of Schizosaccharomyces pombe stimulates recombination at sites in a nearby interval

With the help of in vitro constructed intragenic double mutants, we investigated the influence of the recombinational hot spot mutation ade6-M26 on meiotic recombination between two additional ade6 mutations proximal to it. Recombination was stimulated four-fold when M26 was present in a heterozygous condition and ten-fold when homozygous. M26 itself remained unaffected in a substantial number of these events. This indicates that the stimulation can not only be due to a preferred conversion of M26 to wild-type with co-conversion of the second mutation in cis. A model is proposed in which M26 acts as an "entry site" for recombinational enzymes.

Adenylyl cyclase is dispensable for vegetative cell growth in the fission yeast Schizosaccharomyces pombe

Disruption of the cyr1 gene of Schizosaccharomyces pombe, which encodes adenylyl cyclase, did not confer lethality to fission yeast cells, although they grew 40% slower than wild-type strains in complete medium. These cells contained no measurable amount of cAMP and no adenylyl cyclase activity. When h+ and h- cyr1 disruptants were mixed, they underwent mating even in rich medium. Propagation of homothallic cyr1 disruptants was difficult, probably because such cells readily mate and produce asci and thus stop growing. A greater than 10-fold increase in the amount of cyr1 mRNA was observed when cloned cyr1+ was introduced into Sch. pombe cells on a multicopy plasmid. The total adenylyl cyclase activity was similarly high in these transformants. However, the level of intracellular cAMP was hardly affected. Evidence suggests that this was not due to increased phosphodiesterase activity. Thus, cAMP level in growing fission yeast cells appears to be regulated not by the amount of adenylyl cyclase protein but by a feedback mechanism at the enzyme level. The cAMP level fell by approximately 50% under nitrogen starvation, which triggers sexual development in Sch. pombe. We suggest that fission yeast controls the level of intracellular cAMP primarily to regulate sexual development rather than to drive or arrest the cell cycle.

Expression of the beta-glucuronidase gene under the control of the CaMV 35s promoter in Schizosaccharomyces pombe

We have transformed Schizosaccharomyces pombe with the beta-glucuronidase (GUS) gene from Escherichia coli under the control of the plant cauliflower mosaic virus (CaMV) 35S promoter element. Efficient expression of GUS enzyme was observed. Moreover, transcription initiated at a unique site identical to that used in plant cells.

Cauliflower mosaic virus promoters direct efficient expression of a bacterial G418 resistance gene in Schizosaccharomyces pombe

A system is presented for transformation of the fission yeast Schizosaccharomyces pombe to resistance against the antibiotic G418. The bacterial resistance gene of the transposon Tn5 is expressed under the control of promoters and transcription terminators from cauliflower mosaic virus (CaMV). The promoter of the S. pombe alcohol dehydrogenase gene has also been used. Transformants can be selected directly on medium containing G418 (up to 1 mg/ml) due to inactivation of G418 by the Tn5 gene product, the aminoglycoside 3'-phosphotransferase (II). The plant viral promoter 35S confers higher resistance to G418 than the 19S promoter. This corresponds to the relative strengths of these promoters in plant cells. The strong plant promoter 35S yields resistance comparable to that obtained with the strong S. pombe promoter from the alcohol dehydrogenase gene. The constructions with the two plant promoters have been used on multicopy shuttle plasmids that replicate autonomously in S. pombe and Escherichia coli. In addition the 35S and the 19S constructions have been inserted into the S. pombe genome where they confer G418 resistance as single copy genes. Since vector sequences are excluded in this case, all the necessary signals for expression of G418 resistance are contained within the DNA fragments containing the plant promoters, the resistance gene and the plant terminators. This transformation system is independent of S. pombe mutants. It may be useful for the transformation of other lower eukaryotes. The activity of the CaMV promoters in S. pombe may be exploited for the expression of plant genes in fission yeast.

Sequence and transcript analysis of the C. albicans URA3 gene encoding orotidine-5'-phosphate decarboxylase

The human pathogen Candida albicans grows either as a yeast or in filamentous form. We have determined the sequence of a 1.365 kb genomic C. albicans fragment that complements Saccharomyces cerevisiae ura3 and Escherichia coli pyrF mutations. An open reading frame within this fragment corresponds to a protein of 270 amino acids that shows homology to orotidine-5'-phosphate decarboxylases (ODCases) of other fungal species. The C. albicans ODCase is most closely related to the ODCases of the budding yeasts Kluyveromyces lactis and S. cerevisiae (74% and 71% homology, respectively). Most 5' ends of URA3 transcripts in the authentic host and in the heterologous host S. cerevisiae were found to be identical. These results demonstrate a close taxonomic relationship between non-pathogenic budding yeasts and C. albicans.

Schizosaccharomyces pombe U4 small nuclear RNA closely resembles vertebrate U4 and is required for growth

The single-copy gene snu4, which encodes the small nuclear RNA (snRNA) U4, has been cloned and sequenced. Schizosaccharomyces pombe U4 is 128 nucleotides in length, similar in size to vertebrate U4 and shows substantial primary and secondary structure homology. The gene lacks sequences closely resembling vertebrate snRNA transcription signals, but has a TATA box at -33 to -30; TATA sequences flanked by several additional conserved nucleotides are found in the same position in the 5' regions of other snRNA genes from Schiz. pombe. The cloned snu4 gene was disrupted by transposon mutagenesis and used to replace one chromosomal copy of snu4 in a diploid strain. On sporulation snu4- haploid strains could not be recovered, demonstrating that U4 is required, at least for spore germination. Haploid snu4- strains are viable if they also carry snu4+ on a replicating plasmid but are unable to loose the plasmid under non-selective growth, demonstrating a continuous requirement for U4 for viability.

Molecular cloning and sequence analysis of mutant alleles of the fission yeast cdc2 protein kinase gene: implications for cdc2+ protein structure and function

The cdc2+ gene function plays a central role in the control of the mitotic cell cycle of the fission yeast Schizosaccharomyces pombe. Recessive temperature-sensitive mutations in the cdc2 gene cause cell cycle arrest when shifted to the restrictive temperature, while a second class of mutations within the cdc2 gene causes a premature advancement into mitosis. Previously the cdc2+ gene has been cloned and has been shown to encode a 34 kDa phosphoprotein with in vitro protein kinase activity. Here we describe the cloning of 11 mutant alleles of the cdc2 gene using two simple methods, one of which is presented here for the first time. We have sequenced these alleles and find a variety of single amino acid substitutions mapping throughout the cdc2 protein. Analysis of these mutations has identified a number of regions within the cdc2 protein that are important for cdc2+ activity and regulation. These include regions which may be involved in the interaction of the cdc2+ gene product with the proteins encoded by the wee1+, cdc13+ and suc1+ genes.

Observations on integrative transformation in Schizosaccharomyces pombe

Three different Schizosaccharomyces pombe strains have been transformed with a circular or linearized non-ars plasmid carrying the ura4+ gene as a selectable marker. The first strain shows full homology between the genomic ura4-294 gene (point mutation) and the marker gene on the plasmid. The second strain carries a 600 bp deletion (ura4-D6) that decreases homology between plasmid and chromosome. No homology remains in the third strain which has a complete deletion of the ura4 gene on the chromosome (ura4-D18). When sequence homology exists between transforming DNA and the chromosomal ura4 region, gene conversion is strongly preferred over integration of the circular plasmid. Reduction of the length of homology leads to a decrease of transformation frequencies, and homology dependent as well as a minority of homology independent integrations are observed. In the complete absence of homology two rare types of transformants are encountered: either the circular plasmid replicates autonomously, although it is devoid of an ars sequence, or alternatively the plasmid integrates into the genome at various positions. Transformation with plasmid cut within the coding region of ura4 can lead to tandemly arranged multiple integrations, when no homology exists between the free ends and the chromosome. The integrations occur at the ura4 locus, when homology is retained between plasmid and chromosome, and at various sites in the genome of the strain with a complete deletion of the ura4 gene. The results suggest that homology dependent events (conversion, integration) are strongly preferred in transformation of S. pombe with non-ars plasmids. In addition low frequency integration by illegitimate recombination is observed.(ABSTRACT TRUNCATED AT 250 WORDS)