Genetics of the fission yeast Schizosaccharomyces pombe

The Pheromone and Pheromone Receptor Mating-Type Locus Is Involved in Controlling Uniparental Mitochondrial Inheritance in Cryptococcus

Mitochondria are inherited uniparentally during sexual reproduction in the majority of eukaryotic species studied, including humans, mice, and nematodes, as well as many fungal species. Mitochondrial uniparental inheritance (mito-UPI) could be beneficial in that it avoids possible genetic conflicts between organelles with different genetic backgrounds, as recently shown in mice, and it could prevent the spread of selfish genetic elements in the mitochondrial genome. Despite the prevalence of observed mito-UPI, the underlying mechanisms and the genes involved in controlling this non-Mendelian inheritance are poorly understood in many species. In Cryptococcus neoformans, a human pathogenic basidiomyceteous fungus, mating types (MATα and MATa) are defined by alternate alleles at the single MAT locus that evolved from fusion of the two MAT loci (P/R encoding pheromones and pheromone receptors, and HD encoding homeodomain transcription factors) that are the ancestral state in the basidiomycota. Mitochondria are inherited uniparentally from the MATa parent in C. neoformans, and this requires the SXI1α and SXI2a HD factors encoded by MAT However, there is evidence that additional genes contribute to the control of mito-UPI in Cryptococcus Here, we show that in C. amylolentus, a sibling species of C. neoformans with unlinked P/R and HD MAT loci, mito-UPI is controlled by the P/R locus and is independent of the HD locus. Consistently, by replacing the MATα alleles of the pheromones (MF) and pheromone receptor (STE3) with the MATa alleles, we show that these P/R locus-defining genes indeed affect mito-UPI in C. neoformans during sexual reproduction. Additionally, we show that during early stages of C. neoformans sexual reproduction, conjugation tubes are always produced by the MATα cells, resulting in unidirectional migration of the MATα nucleus into the MATa cell during zygote formation. This process is controlled by the P/R locus and could serve to physically restrict movement of MATα mitochondria in the zygotes, and thereby contribute to mito-UPI. We propose a model in which both physical and genetic mechanisms function in concert to prevent the coexistence of mitochondria from the two parents in the zygote, and subsequently in the meiotic progeny, thus ensuring mito-UPI in pathogenic Cryptococcus, as well as in closely related nonpathogenic species. The implications of these findings are discussed in the context of the evolution of mito-UPI in fungi and other more diverse eukaryotes.

Studying anti-oxidative properties of inclusion complexes of α-lipoic acid with γ-cyclodextrin in single living fission yeast by confocal Raman microspectroscopy

α-lipoic acid (ALA) is an essential cofactor for many enzyme complexes in aerobic metabolism, especially in mitochondria of eukaryotic cells where respiration takes place. It also has excellent anti-oxidative properties. The acid has two stereo-isomers, R- and S- lipoic acid (R-LA and S-LA), but only the R-LA has biological significance and is exclusively produced in our body. A mutant strain of fission yeast, Δdps1, cannot synthesize coenzyme Q10, which is essential during yeast respiration, leading to oxidative stress. Therefore, it shows growth delay in the minimal medium. We studied anti-oxidant properties of ALA in its free form and their inclusion complexes with γ-cyclodextrin using this mutant yeast model. Both free forms R- and S-LA as well as 1:1 inclusion complexes with γ-cyclodextrin recovered growth of Δdps1 depending on the concentration and form. However, it has no effect on the growth of wild type fission yeast strain at all. Raman microspectroscopy was employed to understand the anti-oxidant property at the molecular level. A sensitive Raman band at 1602cm^-1 was monitored with and without addition of ALAs. It was found that 0.5mM and 1.0mM concentrations of ALAs had similar effect in both free and inclusion forms. At 2.5mM ALAs, free forms inhibited the growth while inclusion complexes helped in recovered. 5.0mM ALA showed inhibitory effect irrespective of form. Our results suggest that the Raman band at 1602cm^-1 is a good measure of oxidative stress in fission yeast.

Yeast for virus research

Budding yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) are two popular model organisms for virus research. They are natural hosts for viruses as they carry their own indigenous viruses. Both yeasts have been used for studies of plant, animal and human viruses. Many positive sense (+) RNA viruses and some DNA viruses replicate with various levels in yeasts, thus allowing study of those viral activities during viral life cycle. Yeasts are single cell eukaryotic organisms. Hence, many of the fundamental cellular functions such as cell cycle regulation or programed cell death are highly conserved from yeasts to higher eukaryotes. Therefore, they are particularly suited to study the impact of those viral activities on related cellular activities during virus-host interactions. Yeasts present many unique advantages in virus research over high eukaryotes. Yeast cells are easy to maintain in the laboratory with relative short doubling time. They are non-biohazardous, genetically amendable with small genomes that permit genome-wide analysis of virologic and cellular functions. In this review, similarities and differences of these two yeasts are described. Studies of virologic activities such as viral translation, viral replication and genome-wide study of virus-cell interactions in yeasts are highlighted. Impacts of viral proteins on basic cellular functions such as cell cycle regulation and programed cell death are discussed. Potential applications of using yeasts as hosts to carry out functional analysis of small viral genome and to develop high throughput drug screening platform for the discovery of antiviral drugs are presented.

The modified base isopentenyladenosine and its derivatives in tRNA

Base 37 in tRNA, 3′-adjacent to the anticodon, is occupied by a purine base that is thought to stabilize codon recognition by stacking interactions on the first Watson-Crick base pair. If the first codon position forms an A.U or U.A base pair, the purine is likely further modified in all domains of life. One of the first base modifications found in tRNA is N⁶-isopentenyl adenosine (i⁶A) present in a fraction of tRNAs in bacteria and eukaryotes, which can be further modified to 2-methyl-thio-N⁶-isopentenyladenosine (ms²i⁶A) in a subset of tRNAs. Homologous tRNA isopentenyl transferase enzymes have been identified in bacteria (MiaA), yeast (Mod5, Tit1), roundworm (GRO-1), and mammals (TRIT1). In eukaryotes, isopentenylation of cytoplasmic and mitochondrial tRNAs is mediated by products of the same gene. Accordingly, a patient with homozygous mutations in TRIT1 has mitochondrial disease. The role of i⁶A in a subset of tRNAs in gene expression has been linked with translational fidelity, speed of translation, skewed gene expression, and non-sense suppression. This review will not cover the action of i⁶A as a cytokinin in plants or the potential function of Mod5 as a prion in yeast.

A revised chromosome map of the fission yeast Schizosaccharomyces pombe

The genetic map of the nuclear genome of the fission yeast Schizosaccharomyces pombe has been extended by mitotic and meiotic mapping data. A total of 158 markers are now assigned to the three linkage groups known in this organism, and 118 of them have been located on the corresponding chromosome map. Chromosome II and III each consist of one linkage group. There is some indication that the two large fragments which define chromosome I are meiotically linked, but the linkage observed is significant at the P = 0.05 level only. The length of the map is at least 1,700 map units, corresponding to an average of about 8 kilobases per map unit. The latter figure is comparable to the one obtained for intragenic recombination in the sup3 gene (Hofer et al. 1979). The basic frequency of gene conversion as measured for 21 genes varies according to a distribution of Poisson (with a modal value of 0.6% conversion per meiosis and per gene), in sharp contrast with Saccharomyces cerevisiae (Fogel et al. 1980) and Ascobolus immersus (Nicolas 1979). This may reflect the rarity of gene or region-specific rec alleles in S. pombe and may be related to the homothallism of this organism.

Plasticity and diversity of tRNA anticodon determinants of substrate recognition by eukaryotic A37 isopentenyltransferases

The N(6)-(isopentenyl)adenosine (i(6)A) modification of some tRNAs at position A37 is found in all kingdoms and facilitates codon-specific mRNA decoding, but occurs in different subsets of tRNAs in different species. Here we examine yeasts' tRNA isopentenyltransferases (i.e., dimethylallyltransferase, DMATase, members of the Δ(2)-isopentenylpyrophosphate transferase, IPPT superfamily) encoded by tit1(+) in Schizosaccharomyces pombe and MOD5 in Saccharomyces cerevisiae, whose homologs are Escherichia coli miaA, the human tumor suppressor TRIT1, and the Caenorhabditis elegans life-span gene product GRO-1. A major determinant of miaA activity is known to be the single-stranded tRNA sequence, A36A37A38, in a stem-loop. tRNA(Trp)(CCA) from either yeast is a Tit1p substrate, but neither is a Mod5p substrate despite the presence of A36A37A38. We show that Tit1p accommodates a broader range of substrates than Mod5p. tRNA(Trp)(CCA) is distinct from Mod5p substrates, which we sort into two classes based on the presence of G at position 34 and other elements. A single substitution of C34 to G converts tRNA(Trp)(CCA) to a Mod5p substrate in vitro and in vivo, consistent with amino acid contacts to G34 in existing Mod5p-tRNA(Cys)(GCA) crystal structures. Mutation of Mod5p in its G34 recognition loop region debilitates it differentially for its G34 (class I) substrates. Multiple alignments reveal that the G34 recognition loop sequence of Mod5p differs significantly from Tit1p, which more resembles human TRIT1 and other DMATases. We show that TRIT1 can also modify tRNA(Trp)(CCA) consistent with broad recognition similar to Tit1p. This study illustrates previously unappreciated molecular plasticity and biological diversity of the tRNA-isopentenyltransferase system of eukaryotes.

Sex in fungi

Sexual reproduction enables genetic exchange in eukaryotic organisms as diverse as fungi, animals, plants, and ciliates. Given its ubiquity, sex is thought to have evolved once, possibly concomitant with or shortly after the origin of eukaryotic organisms themselves. The basic principles of sex are conserved, including ploidy changes, the formation of gametes via meiosis, mate recognition, and cell-cell fusion leading to the production of a zygote. Although the basic tenants are shared, sex determination and sexual reproduction occur in myriad forms throughout nature, including outbreeding systems with more than two mating types or sexes, unisexual selfing, and even examples in which organisms switch mating type. As robust and diverse genetic models, fungi provide insights into the molecular nature of sex, sexual specification, and evolution to advance our understanding of sexual reproduction and its impact throughout the eukaryotic tree of life.

Mitochondrial inheritance in fungi

Faithful inheritance of mitochondria is essential for growth and development. Uniparental inheritance of mitochondria is a common phenomenon in sexual eukaryotes and has been reported for numerous fungal species. Uniparental inheritance is a genetically regulated process, aimed to gain a homoplasmic state within cells, and this is often associated with selective elimination of one parental mitochondria population. This review will focus on recent developments in our understanding of common and specified regulatory circuits of selective mitochondrial inheritance during sexual development. It further refers to the influence of mitochondrial fusion on generation of recombinant mitochondrial DNA molecules. The latter aspect appears rather exciting in the context of intron homing and could bring a new twist to the debate on the significance of uniparental inheritance. The emergence of genome-wide studies offers new perspectives to address potential relationships between uniparental inheritance, vegetative inheritance and last but not least cellular scavenging systems to dispose of disintegrated organelles.

Biochemical interactions between proteins and mat1 cis-acting sequences required for imprinting in fission yeast

DNA recombination required for mating type (mat1) switching in Schizosaccharomyces pombe is initiated by mat1 imprinting. The imprinting event is regulated by mat1 cis-acting elements and by several trans-acting factors, including swi1 (for switch), swi3, swi7, and sap1. swi1 and swi3 were previously shown to function in dictating unidirectional mat1 DNA replication by controlling replication fork movement around the mat1 region and, second, by pausing fork progression around the imprint site. With biochemical studies, we investigated whether the trans-acting factors function indirectly or directly by binding to the mat1 cis-acting sequences. First, we report the identification and DNA sequence of the swi3 gene. swi3 is not essential for viability, and, like the other factors, it exerts a stimulatory effect on imprinting. Second, we showed that only Swi1p and Swi3p interact to form a multiprotein complex and that complex formation did not require their binding to a DNA region defined by the smt-0 mutation. Third, we found that the Swi1p-Swi3p complex physically binds to a region around the imprint site where pausing of replication occurs. Fourth, the protein complex also interacted with the mat1-proximal polar terminator of replication (RTS1). These results suggest that the stimulatory effect of swi1 and swi3 on switching and imprinting occurs through interaction of the Swi1p-Swi3p complex with the mat1 regions.

Characterization of vps33+, a gene required for vacuolar biogenesis and protein sorting in Schizosaccharomyces pombe

From the fission yeast Schizosaccharomyces pombe we have identified and deleted vps33, a gene encoding a homologue of VPS33, which is required for vacuolar biogenesis in S. cerevisiae cells. When the vps33(+) gene is disrupted, Sz. pombe strains are temperature-sensitive for growth and contain numerous small vesicular structures stained with FM4-64 in the cells. Deletion of the Sz. pombe vps33(+) gene results in pleiotropic phenotypes consistent with the absence of normal vacuoles, including missorting of vacuolar carboxypeptidase Y, various ion- and drug-sensitivities, and sporulation defects. These results are consistent with Vps33p being necessary for the morphogenesis of vacuoles and subsequent expression of vacuolar functions in Sz. pombe cells.

Schizosaccharomyces pombe Pmf1p is structurally and functionally related to Mmf1p of Saccharomyces cerevisiae

A novel family of small proteins, termed p14.5 or YERO57c/YJGFc, has been identified. Independent studies indicate that p14.5 family members are multifunctional proteins involved in several pathways, e.g. regulation of translation or activation of the protease mu-calpain. We have previously shown that Mmf1p, a p14.5 of the budding yeast Saccharomyces cerevisiae, is localized in the mitochondria and influences mitochondrial DNA stability. In addition, we have demonstrated that Mmf1p is functionally related to p14.5 of mammalian cells. To explore further the evolutionary conservation of the mitochondrial function(s) of the p14.5s we have extended our study to the fission yeast, Schizosaccharomyces pombe. In this organism two p14.5 homologous proteins are present: Pmf1p (pombe mitochondrial factor 1) and Hpm1p (homologous Pmf1p factor 1). We have generated a specific Pmf1p antibody, which recognizes a single band of approximately 15 kDa in total cellular extracts. Cellular fractionation experiments indicate that Pmf1p localizes in the mitochondria as well as in the cytoplasm. We also show that Pmf1p shares several properties of S. cerevisiae Mmf1p. Indeed, Pmf1p restores the wild-type phenotype when expressed in delta mmf1 S. cerevisiae cells. Deletion of the leader sequence of Pmf1p abrogates its ability to localize in mitochondria and to functionally replace Mmf1p. Thus, these data together with our previous study show that the mitochondrial function(s) of the p14.5 family members are highly conserved in eukaryotic cells.

Vacuolar protein sorting in fission yeast: cloning, biosynthesis, transport, and processing of carboxypeptidase Y from Schizosaccharomyces pombe

PCR was used to isolate a carboxypeptidase Y (CPY) homolog gene from the fission yeast Schizosaccharomyces pombe. The cloned S. pombe cpy1+ gene has a single open reading frame, which encodes 950 amino acids with one potential N-glycosylation site. It appears to be synthesized as an inactive pre-pro protein that likely undergoes processing following translocation into appropriate intracellular organelles. The C-terminal mature region is highly conserved in other serine carboxypeptidases. In contrast, the N-terminal pro region containing the vacuolar sorting signal in CPY from Saccharomyces cerevisiae shows fewer identical residues. The pro region contains two unusual repeating sequences; repeating sequence I consists of seven contiguous repeating segments of 13 amino acids each, and repeating sequence II consists of seven contiguous repeating segments of 9 amino acids each. Pulse-chase radiolabeling analysis revealed that Cpy1p was initially synthesized in a 110-kDa pro-precursor form and via the 51-kDa single-polypeptide-chain intermediate form which has had its pro segment removed is finally converted to a heterodimer, the mature form, which is detected as a 32-kDa protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. Like S. cerevisiae CPY, S. pombe Cpy1p does not require the N-linked oligosaccharide moiety for vacuolar delivery. To investigate the vacuolar sorting signal of S. pombe Cpy1p, we have constructed cpy1+-SUC2 gene fusions that direct the synthesis of hybrid proteins consisting of N-terminal segments of various lengths of S. pombe Cpy1p fused to the secreted enzyme S. cerevisiae invertase. The N-terminal 478 amino acids of Cpy1 are sufficient to direct delivery of a Cpy1-Inv hybrid protein to the vacuole. These results showed that the pro peptide of Cpy1 contains the putative vacuolar sorting signal.

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.

High-level expression of human lipocortin I in the fission yeast Schizosaccharomyces pombe using a novel expression vector

We have developed a novel expression system that allows the fission yeast, Schizosaccharomyces pombe, to be used for the efficient overproduction of heterologous proteins. As an example of the utility of this system, human lipocortin I was expressed to 50 percent of soluble protein, and 150 mg of highly purified material was obtained from 10 grams of wet cell paste. Expression of lipocortin I was driven by the human cytomegalovirus (hCMV) promoter in a vector that also contains a neomycin resistance gene (neo) under the control of the SV40 early promoter, permitting selection for increasing copy-number with increasing concentrations of the antibiotic G418. The purified protein was equivalent to its native counterpart with respect to antigenicity and biochemical properties such as phospholipase A2 inhibition, actin binding and N-terminal acetylation. We have also used this system to produce comparable amounts of other proteins including rat arginase, rat NDP-kinase and human interleukin-6.

Mechanisms of gene conversion in Saccharomyces cerevisiae

In red-white sectored colonies of Saccharomyces cerevisiae, derived from mitotic cells grown to stationary phase and irradiated with a light dose of x-rays, all of the segregational products of gene conversion and crossing over can be ascertained. Approximately 80% of convertants are induced in G1, the remaining 20% in G2. Crossing over, in the amount of 20%, is found among G1 convertants but most of the crossovers are delayed until G2. About 20% of all sectored colonies had more than one genotype in one or the other sector, thus confirming the hypothesis that conversion also occurs in G2. The principal primary event in G2 conversion is a single DNA heteroduplex. It is suggested that the close contact that this implies carries over to G2 when crossing over and a second round of conversion occurs.

New expression vectors for the fission yeast Schizosaccharomyces pombe

A general expression vector (pMB332) for the fission yeast Schizosaccharomyces pombe was constructed. The heterologous gene expression is driven by the S. pombe alcohol dehydrogenase (adh) promoter. Transcription termination signals were isolated from the S. pombe actin gene. The vectors carry the Saccharomyces cerevisiae Ura3 gene, which complements the S. pombe ura4 mutation. The plasmid stability is conferred by the S. pombe ars and stb elements isolated from pFL120 [(1983) Cell 32, 371-377]. An 'ATG' vector (pMB340) was created, which allows the expression of protein fragments fused to a translational start codon downstream of the adh promoter. The function of this vector system is shown by the production of the human blood coagulation protein factor XIIIa.

Circadian control of heat tolerance in stationary phase cultures of Schizosaccharomyces pombe

The capacity of stationary phase cultures of Schizosaccharomyces pombe to survive a heat treatment at 55 degrees C is controlled by a circadian rhythm. In a synchronizing light-dark-cycle this rhythm shows a stable phase relationship to the onset of light. In continuous darkness it persists for several cycles without marked damping. The free-running period of about 27 h at 30 degrees C is only slightly longer at 20 degrees C, hence temperature-compensated. These results indicate that S. pombe is a suitable experimental organism for further research into both heat tolerance and circadian rhythms.

Two distinct mechanisms for deletion in mitochondrial DNA of Schizosaccharomyces pombe mutator strains. Slipped mispairing mediated by direct repeats and erroneous intron splicing

Mutator strains of the fission yeast Schizosaccharomyces pombe produce mitochondrial respiratory deficient mutants at a high rate, and roughly 20% of these mutants carry deletions in the range of 50 to 1500 base-pairs. To elucidate the mechanism of deletion we have sequenced ten deletion mutants in the mosaic gene encoding apocytochrome b (cob) and three in the split gene coding for the first subunit of cytochrome c oxidase (cox1). Of 13 deletions, ten are correlated with the presence of direct repeats, which could promote deletions by slipped mispairing during DNA replication. In some of these mutants, the termini are located in possible DNA secondary structures. In three independently isolated mutants with identical deletions in the cob gene, the 5' deletion endpoint coincides with the 3' splice point of the intron, whereas the 3' endpoint of the deletion exhibits pronounced homology with the 5' splice point of the intron. This result suggests that these deletions might be initiated by erroneous RNA splicing.

Cloning and expression of the OMP decarboxylase gene URA4 from Schizosaccharomyces pombe

URA4, the gene coding for orotidine monophosphate decarboxylase (OMPdecase), has been cloned from the fission yeast by homologous complementation and restricted in an Escherichia coli-Schizosaccharomyces pombe (E. coli-S. pombe) replicative plasmid to a 1.76 kb HindIII fragment. This plasmid is maintained at a high copy number in S. pombe and allows OMPdecase expression in Saccharomyces cerevisiae (S. cerevisiae) as well as in E. coli. After characterisation by restriction mapping and Southern hybridisation, the cloned gene was used as a probe to measure URA4 transcription and to examine its regulation. Messenger RNA levels were measured by DNA/RNA filter-hybridisation with pulse labelled RNAs during 6-azauridine (6-AUR) inhibited growth in wild type and 6-AUR sensitive strains. We found that in S. pombe the OMP analogue 6-AUR does not regulate the level of OMPdecase formation as it does in S. cerevisiae but rather modifies the ratio of total polyA+ to polyA- RNAs in the cell. Based on these results and on corresponding enzyme activities this study demonstrates divergent pyrimidine pathway regulation in the two yeasts S. cerevisiae and S. pombe. Finally, we propose the use of the URA4 gene as a convenient selective marker for genetic engineering in S. pombe.

Resolution of DNA molecules greater than 5 megabases by contour-clamped homogeneous electric fields

Excellent resolution of chromosomal DNA molecules from Saccharomyces cerevisiae, Candida albicans and Schizosaccharomyces pombe has been obtained using alternating contour-clamped homogeneous electric field (CHEF) gel electrophoresis. The largest of these molecules is greater than 5 Mb in size and is resolved after 130 hours in a 0.6% agarose gel at a field strength of 1.3 V/cm and a switching interval of 1 hour. Separation of concatamers of phage lambda DNA reveals four regions of resolution in alternating CHEF gel electrophoresis. There are two regions of good resolution in which mobility approximates a linear function of molecular weight. These are separated by a region of lower resolution and bounded at high molecular weights by a region of little or no resolution. The four regions are of practical and possibly theoretical importance.

Distribution of mitochondrial introns in the species Schizosaccharomyces pombe and the origin of the group II intron in the gene encoding apocytochrome b

The mitochondrial genome size of 26 different Schizosaccharomyces pombe strains varies between 17.6 and 24.6 kilobase pairs due to the presence or absence of introns. One of these is the group II intron in the gene encoding apocytochrome b (cob: intron cobI1). Partial DNA sequences of continuous cob genes from six strains (including strain EF1: Trinkl et al. 1985) revealed identical nucleotide sequence in the region where the group II intron is inserted in the mosaic form of the gene. In contrast, analysis of the mosaic cob gene in strain UCD-FstI revealed several base pair changes in the exon regions flanking the splice point, compared with the continuous genes and with the mosaic cob gene in strain 50 (Lang et al. 1985). The base pair differences between the exons of the two mosaic cob genes and the identity of exons in all continuous cob genes argue in favour of the two cob introns in strains 50 and UCD-FstI as independent later acquisitions of the genes, rather than loss of the intron from a common mosaic ancestor of all strains. Other introns present in some but not all strain include two group I introns without open reading frame in the gene encoding subunit 1 of cytochrome c oxidase (cox1: introns cox1I2a and cox1I3), and two group I introns with open reading frames in the same gene (introns cox1I1 and cox1I2b).

Initiation of meiotic recombination by double-strand DNA breaks in S. pombe

Mitotic gene conversion and reciprocal recombination have recently been shown to be efficiently initiated by double-strand DNA breaks (DSBs) in both Saccharomyces cerevisiae and Schizosaccharomyces pombe. We tested whether DSBs could also initiate meiotic recombination at the mat1 locus in S. pombe. The mat1 switching-mechanism-generated DSB found in mitotically growing cells can be repaired without mat1 switching, since strains deleted for both donor loci (mat2-P and mat3-M) have the break but do not produce inviable cells. A (mat1-P X mat1-M) cross produced a high frequency (20%) of 3:1 gene conversions of mat1 in meiotic tetrads. Gene conversion events were associated with the recombination of flanking markers. Strains lacking the DSB failed to convert. Thus, the DSB at mat1 promotes efficient meiotic recombination in fission yeast.

Inactivation of nonsense suppressor transfer RNA genes in Schizosaccharomyces pombe. Intergenic conversion and hot spots of mutation

Intergenic conversion is a mechanism for the concerted evolution of repeated DNA sequences. A new approach for the isolation of intergenic convertants of serine tRNA genes in the yeast Schizosaccharomyces pombe is described. Contrary to a previous scheme, the intergenic conversion events studied in this case need not result in functional tRNA genes. The procedure utilizes crosses of strains that are homozygous for an active UGA suppressor tRNA gene, and the resulting progeny spores are screened for loss of suppressor activity. In this way, intergenic convertants of a tRNA gene are identified that inherit varying stretches of DNA sequence from either of two other tRNA genes. The information transferred between genes includes anticodon and intron sequences. Two of the three tRNA genes involved in these information transfers are located on different chromosomes. The results indicate that intergenic conversion is a conservative process. No infidelity is observed in the nucleotide sequence transfers. This provides further evidence for the hypothesis that intergenic conversion and allelic conversion are the result of the same molecular mechanism. The screening procedure for intergenic revertants also yields spontaneous mutations that inactivate the suppressor tRNA gene. Point mutations and insertions of A occur at various sites at low frequency. In contrast, A insertions at one specific site occur with high frequency in each of the three tRNA genes. This new type of mutation hot spot is found also in vegetative cells.

Yeast promoters URA1 and URA3. Examples of positive control

Transcription of the two unlinked structural genes URA1 and URA3 of Saccharomyces cerevisiae is positively regulated by the gene product PPR1. We have used S1 digestion and primer extension mapping to investigate the RNAs produced in different genetic backgrounds: wild-type, ppr1 deletion mutants, constitutively induced and non-inducible ppr1 mutants. Results show that each structural gene specifies multiple messenger RNA classes with different 5'-terminal sequences. The basal level of these transcripts does not require a functional PPR1 gene. Induction of URA1 results from an even increase of the level of synthesis of all the transcripts in contrast to that of URA3 which is effected by selectively increasing the levels of synthesis of one subset of transcripts. The PPR1-mediated control was also studied in the foreign genetic background of Schizosaccharomyces pombe using autonomously replicating hybrid plasmids carrying the gene URA1 or URA3 along with the regulatory gene PPR1, either in a constitutive or non-inducible allelic form. The 5' ends of the transcripts URA1 and URA3 made in S. pombe map upstream from the initiation sites used in S. cerevisiae. In contrast to S. cerevisiae, in S. pombe the URA3 but not URA1 transcripts respond to the PPR1-induction. We have identified a minimal control region for the PPR1-specific induction of URA1, that includes sequences located between the T-A-T-A box and the translation start codon. This region contains sequence features in common with URA3. There is an extensive alternating Pu:Py region including the T-A-T-A box of both promoters and an eight base-pair exact homology; further downstream, there is another 11 base-pair highly conserved sequence which either overlaps or lies in close proximity to the unregulated start sites of URA1 in S. pombe and of URA3 in S. cerevisiae. A positive regulatory model taking into accounts all these observations is presented.

First identification of an amber nonsense mutation in Schizosaccharomyces pombe: major differences in the efficiency of homologous versus heterologous yeast suppressor tRNA genes

In Saccharomyces cerevisiae ochre and opal, as well as amber mutations are known, whereas in the fission yeast Schizosaccharomyces pombe no amber alleles have been described. We have characterized trp1-566, an amber allele in the trp1 locus of S. pombe. The identification of trp1-566 as an amber allele is based on the following results: (a) The nonsense allele can be converted to an ochre allele by nitrosoguanidine mutagenesis. (b) trp1-566 is suppressed by a bona fide S. pombe amber suppressor tRNA, supSI. The supSI gene was obtained by primer-directed in vitro mutagenesis of a tRNASer from S. pombe. Unexpectedly, an S. cerevisiae amber suppressor tRNASer, supR21, transformed into S. pombe, failed to suppress trp1-566. Northern analysis of S. pombe transformants, containing supRL1 or S. cerevisiae tRNALeu or tRNATyr genes reveals that these genes are not transcribed in the fission yeast. As an additional tool for the analysis of nonsense mutations in S. pombe, we obtained by nitrosoguanidine mutagenesis two unlinked amber suppressor alleles, sup13 and sup14, which act on trp1-566.

Mutations preventing expression of sup3 tRNASer nonsense suppressors of Schizosaccharomyces pombe

Suppression of nonsense codons in Schizosaccharomyces pombe by sup3-e tRNASerUGA or sup3-i tRNASerUAA is reduced or abolished by mutations within the suppressor locus. Twenty-five suppressor-inactive sup3-e genes and thirteen mutant sup3-i genes were isolated from S. pombe genomic clone banks by colony hybridization. Sequence analysis of these revertant alleles corroborates genetic evidence for mutational hotspots within the sup3 tRNA gene. Fifteen types of point mutations or insertions were found. Many of these replace bases which are highly or completely conserved in eucaryotic tRNA genes. Transcription of the altered sup3 genes in a Saccharomyces cerevisiae extract enabled the identification of mutations which affect the rate of 5'-end maturation or splicing of the tRNA precursors or both. A total of seven mutations were found which alter transcriptional efficiencies. Of these, five are located outside the internal transcription control regions.

Direct selection of mutants influencing gene conversion in the yeast Schizosaccharomyces pombe

In Schizosaccharomyces pombe, a suppressor-active mutation at the anticodon site of the tRNASerUCA gene sup3 leads to opal (UGA)-specific suppression. Second-site mutations (rX) in sup3 inactivate the suppressor. The sup3-UGA, rX double mutants are genetically unstable in meiotic selfings, due to the intergenic transfer of information between sup3 and the unlinked genes sup9 and sup12 (Hofer et al. 1979; Munz and Leupold 1981; Munz et al. 1982). These three genes have considerable sequence homology over about 200 base pairs (Hottinger et al. 1982). Mutants showing a decrease or an increase of the meiotic instability at sup3 have been selected. One mutation (rec3-8) increases both the genetic instability and the frequency of intragenic recombination in sup3 by one order of magnitude. It has no effect on the stability of the nonsense alleles arg1-230 (UAA), ade6-704 and ural1-61 (UGA) or on the frequency of crossing-over between sup3 and the closely linked gene cdc8. The existence of a common genetic control over intragenic recombination and genetic instability at sup3 provides a direct way of selecting for rec mutants in homothallic haploid strains of S. pombe carrying a suppressor-inactive allele of sup3. It also supports the hypothesis that the instability of mutant alleles of this gene is due to chromosome mispairing at meiosis allowing sup3 to pair with sup9 or sup12 and then to undergo recombination by gene conversion restoring the suppressor-active allele sup3-UGA from the suppressor-inactive allele sup3-UGA, rX.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutations preventing expression of sup3 tRNASer nonsense suppressors of Schizosaccharomyces pombe

Suppression of nonsense codons in Schizosaccharomyces pombe by sup3-e tRNASerUGA or sup3-i tRNASerUAA is reduced or abolished by mutations within the suppressor locus. Twenty-five suppressor-inactive sup3-e genes and thirteen mutant sup3-i genes were isolated from S. pombe genomic clone banks by colony hybridization. Sequence analysis of these revertant alleles corroborates genetic evidence for mutational hotspots within the sup3 tRNA gene. Fifteen types of point mutations or insertions were found. Many of these replace bases which are highly or completely conserved in eucaryotic tRNA genes. Transcription of the altered sup3 genes in a Saccharomyces cerevisiae extract enabled the identification of mutations which affect the rate of 5'-end maturation or splicing of the tRNA precursors or both. A total of seven mutations were found which alter transcriptional efficiencies. Of these, five are located outside the internal transcription control regions.

The linear extrachromosomal DNA of Physarum polycephalum replicates and is maintained under non-selective conditions in two different lower eukaryotes

The slime mould Physarum polycephalum contains 100 to 200 molecules of extrachromosomal linear DNA (PeDNA). Two sets of the 19S and 26S ribosomal genes are located on each molecule of PeDNA. In the nonmitotic phase of the cell cycle PeDNA is localised in the nucleolus. The molecules are maintained throughout vegetative growth. In order to study the signals responsible for its maintenance, PeDNA was purified and introduced into the two distantly related yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Surprisingly, intact PeDNA transforms both yeasts with high frequency and PeDNA sequences are maintained in the absence of selective pressure.

Cloning and characterization of two genes restoring acid phosphatase activity in pho1- mutants of Schizosaccharomyces pombe

Schizosaccharomyces pombe acid phosphatase (APh) is a secreted cell surface glycoprotein which is deficient in pho1 mutants. By screening an S. pombe gene bank for sequences which can functionally rescue the pho1-44 mutation, we have isolated two genomic clones carried in plasmids pSp4B and pSp4C/2. These two sequences map of different genetic loci and show no cross hybridization by Southern blotting. pSp4C/2 was found to contain the PHO1 gene, and cells transformed with this plasmid produce a protein which cross-reacts with antibodies raised against the protein moiety of APh. Data from Northern blotting experiments show that pSp4C/2 encodes a 1.6-kb transcript, and that mRNA levels are increased when cells are grown in low concentrations of inorganic phosphate. The results indicate that pSp4C/2 contains the structural gene for APh, PHO1, whereas pSp4B appears to carry a gene coding for a minor species of APh, PHO4 which is not regulated by extracellular phosphate.

The sup8 tRNALeu gene of Schizosaccharomyces pombe has an unusual intervening sequence and reduced pairing in the anticodon stem

We have cloned and sequenced the wild-type and suppressor alleles of the S. pombe sup8 tRNA gene. The wild-type allele has a leucine UAA anticodon and the suppressor (sup8-e) carries the opal suppressor anticodon UCA. The gene has a 16 base pair intervening sequence that, in the RNA, is predicted to form a secondary structure which involves base pairing to the 5', rather than the usual 3' side of the 5' splice site. When incubated in Saccharomyces cerevisiae cell-free extracts both alleles are efficiently transcribed, the 5' leader and 3' trailer sequences are removed and CCA is added to the 3' processed end; however, the intervening sequence is not excised. This finding implies that the structural requirements of the splicing endonucleases in the two yeasts have diverged. No other tRNA genes with related sequences were detected in S. pombe DNA by hybridization, suggesting that other UUA isoacceptors may be structurally dissimilar to sup8 or that the UUA codon may be decoded by a UUG leucine isoacceptor.

The mitochondrial genome of the fission yeast Schizosaccharomyces pombe. 3. Gene mapping in strain EF1 (CBS 356) and analysis of hybrids between the strains EF1 and ade7-50h-

The Schizosaccharomyces pombe strain EF1 (CBS 356) is haploid, prototrophic, respiratory competent, and of homothallic mating type. From restriction enzyme analysis the length of the mitochondrial genome is 17.3 kilobase pairs, which is in good agreement with the value of 17.1 kilobase pairs determined by electron microscopy. The mitochondrial genome of strain EF1 is thus about 2.3 kilobase pairs shorter than that of strain ade7-50h- (about 19.4 kilobase pairs). A restriction map was constructed for 11 enzymes: For most, but not all of them, the pattern is nearly identical to that of strain ade7-50h-. The genes for the large ribosomal RNA, the subunits 1, 2, and 3 of cytochrome c oxidase, subunits 6 and 9 of ATP synthetase, and cytochrome b were localized by hybridization with mitochondrial DNA probes from Saccharomyces cerevisiae. The gene order was found to be the same in both yeast strains. From Southern hybridization of strain ade7-50h- with nick-translated mitochondrial DNA from strain EF1 it is evident that strain EF1 does not possess the intron, which is present in the cytochrome b gene of Schizosaccharomyces pombe strain ade7-50h-. Crosses between strain ade7-50h- and EF1 demonstrate that both the nuclear and the mitochondrial genomes are able to recombine. The mitochondrial genomes of 2 out of 30 independently isolated hybrids between the two strains are described as the result of recombination between the two parental mitochondrial genomes.