RNA Polymerase I Activators Count and Adjust Ribosomal RNA Gene Copy Number.

Ribosome is the most abundant RNA-protein complex in a cell, and many copies of the ribosomal RNA gene (rDNA) have to be maintained. However, arrays of tandemly repeated rDNA genes can lose the copies by intra-repeat recombination. Loss of the rDNA copies of Saccharomyces cerevisiae is counteracted by gene amplification whereby the number of rDNA repeats stabilizes around 150 copies, suggesting the presence of a monitoring mechanism that counts and adjusts the number. Here, we report that, in response to rDNA copy loss, the upstream activating factor (UAF) for RNA polymerase I that transcribes the rDNA is released and directly binds to a RNA polymerase II-transcribed gene, SIR2, whose gene products silence rDNA recombination, to repress. We show that the amount of UAF determines the rDNA copy number that is stably maintained. UAF ensures rDNA production not only by rDNA transcription activation but also by its copy-number maintenance.

Cellular responses to compound stress induced by atmospheric-pressure plasma in fission yeast.

The stress response is one of the most fundamental cellular processes. Although the molecular mechanisms underlying responses to a single stressor have been extensively studied, cellular responses to multiple stresses remain largely unknown. Here, we characterized fission yeast cellular responses to a novel stress inducer, non-thermal atmospheric-pressure plasma. Plasma irradiation generates ultraviolet radiation, electromagnetic fields and a variety of chemically reactive species simultaneously, and thus can impose multiple stresses on cells. We applied direct plasma irradiation to fission yeast and showed that strong plasma irradiation inhibited fission yeast growth. We demonstrated that mutants lacking sep1 and ace2, both of which encode transcription factors required for proper cell separation, were resistant to plasma irradiation. Sep1-target transcripts were downregulated by mild plasma irradiation. We also demonstrated that plasma irradiation inhibited the target of rapamycin kinase complex 1 (TORC1). These observations indicate that two pathways, namely the Sep1-Ace2 cell separation pathway and TORC1 pathway, operate when fission yeast cope with multiple stresses induced by plasma irradiation.

Evaluation of repair activity by quantification of ribonucleotides in the genome.

Ribonucleotides incorporated in the genome are a source of endogenous DNA damage and also serve as signals for repair. Although recent advances of ribonucleotide detection by sequencing, the balance between incorporation and repair of ribonucleotides has not been elucidated. Here, we describe a competitive sequencing method, Ribonucleotide Scanning Quantification sequencing (RiSQ-seq), which enables absolute quantification of misincorporated ribonucleotides throughout the genome by background normalization and standard adjustment within a single sample. RiSQ-seq analysis of cells harboring wild-type DNA polymerases revealed that ribonucleotides were incorporated nonuniformly in the genome with a 3'-shifted distribution and preference for GC sequences. Although ribonucleotide profiles in wild-type and repair-deficient mutant strains showed a similar pattern, direct comparison of distinct ribonucleotide levels in the strains by RiSQ-seq enabled evaluation of ribonucleotide excision repair activity at base resolution and revealed the strand bias of repair. The distinct preferences of ribonucleotide incorporation and repair create vulnerable regions associated with indel hotspots, suggesting that repair at sites of ribonucleotide misincorporation serves to maintain genome integrity and that RiSQ-seq can provide an estimate of indel risk.

How do cells count multi-copy genes?: "Musical Chair" model for preserving the number of rDNA copies.

To supply abundant ribosomes, multiple copies of ribosomal RNA genes (rDNA) are conserved from bacterial to human cells. In eukaryotic genomes, clusters of tandemly repeated rDNA units are present, and their number is stably maintained. Due to high level of transcription of rRNA genes, the repetitive structure is prone to rearrangement. In budding yeast, rDNA homeostasis can compensate for this by the regulation of recombination events that will change the copy number. The histone deacetylase Sir2 plays a key role in rDNA copy maintenance and its expression level determines a state of "maintenance" or "amplification" of rDNA copy number. We recently showed that Upstream Activating Factors (UAF) for RNA polymerase I act as a RNA polymerase II repressor of SIR2 transcription in response to rDNA copy loss. Furthermore, the amount of UAF, which is limited in the cell, determines the stable copy number of rDNA and is a molecular switch for rDNA recovery. In this mini-review, we propose a "Musical Chair" model for rDNA copy counting as mediated by UAF and Sir2. The model describes how a straightforward molecular mechanism can account for the "cellular memory" of the proper rDNA copy number.

Mutation in fission yeast phosphatidylinositol 4-kinase Pik1 is synthetically lethal with defect in telomere protection protein Pot1.

Fission yeast Pik1p is one of three phosphatidylinositol 4-kinases associated with the Golgi complex, but its function is not fully understood. Deletion of pot1+ causes telomere degradation and chromosome circularization. We searched for the gene which becomes synthetically lethal with pot1Δ. We obtained a novel pik1 mutant, pik1-1, which is synthetically lethal with pot1Δ. We found phosphoinositol 4-phosphate in the Golgi was reduced in pik1-1. To investigate the mechanism of the lethality of the pot1Δ pik1-1 double mutant, we constructed the nmt-pot1-aid pik1-1 strain, where Pot1 function becomes low by drugs, which leads to telomere loss and chromosome circularization, and found pik1-1 mutation does not affect telomere resection and chromosome circularization. Thus, our results suggest that pik1+ is required for the maintenance of circular chromosomes.

siRNA-mediated heterochromatin establishment requires HP1 and is associated with antisense transcription.

Heterochromatic gene silencing at the pericentromeric DNA repeats in fission yeast requires the RNA interference (RNAi) machinery. The RNA-induced transcriptional silencing (RITS) complex mediates histone H3 lysine 9 (H3K9) methylation and recruits the RNA-dependent RNA polymerase complex (RDRC) to promote double-stranded RNA (dsRNA) synthesis and siRNA generation. Here we show that ectopic expression of a long hairpin RNA bypasses the requirement for chromatin-dependent steps in siRNA generation. The ability of hairpin-produced siRNAs to silence homologous sequences in trans is subject to local chromatin structure, requires HP1, and correlates with antisense transcription at the target locus. Furthermore, although hairpin siRNAs can be produced in the absence of RDRC, trans-silencing of reporter genes by hairpin-produced siRNAs is completely dependent on the dsRNA synthesis activity of RDRC. These results provide insights into the regulation of siRNA action and reveal roles for cis-dsRNA synthesis and HP1 in siRNA-mediated heterochromatin assembly.

Mudi, a web tool for identifying mutations by bioinformatics analysis of whole-genome sequence.

In forward genetics, identification of mutations is a time-consuming and laborious process. Modern whole-genome sequencing, coupled with bioinformatics analysis, has enabled fast and cost-effective mutation identification. However, for many experimental researchers, bioinformatics analysis is still a difficult aspect of whole-genome sequencing. To address this issue, we developed a browser-accessible and easy-to-use bioinformatics tool called Mutation discovery (Mudi; http://naoii.nig.ac.jp/mudi_top.html), which enables 'one-click' identification of causative mutations from whole-genome sequence data. In this study, we optimized Mudi for pooled-linkage analysis aimed at identifying mutants in yeast model systems. After raw sequencing data are uploaded, Mudi performs sequential analysis, including mapping, detection of variant alleles, filtering and removal of background polymorphisms, prioritization, and annotation. In an example study of suppressor mutants of ptr1-1 in the fission yeast Schizosaccharomyces pombe, pooled-linkage analysis with Mudi identified mip1(+) , a component of Target of Rapamycin Complex 1 (TORC1), as a novel component involved in RNA interference (RNAi)-related cell-cycle control. The accessibility of Mudi will accelerate systematic mutation analysis in forward genetics.

iAID: an improved auxin-inducible degron system for the construction of a 'tight' conditional mutant in the budding yeast Saccharomyces cerevisiae.

Isolation of a 'tight' conditional mutant of a gene of interest is an effective way of studying the functions of essential genes. Strategies that use ubiquitin-mediated protein degradation to eliminate the product of a gene of interest, such as heat-inducible degron (td) and auxin-inducible degron (AID), are powerful methods for constructing conditional mutants. However, these methods do not work with some genes. Here, we describe an improved AID system (iAID) for isolating tight conditional mutants in the budding yeast Saccharomyces cerevisiae. In this method, transcriptional repression by the 'Tet-OFF' promoter is combined with proteolytic elimination of the target protein by the AID system. To provide examples, we describe the construction of tight mutants of the replication factors Dpb11 and Mcm10, dpb11-iAID, and mcm10-iAID. Because Dpb11 and Mcm10 are required for the initiation of DNA replication, their tight mutants are unable to enter S phase. This is the case for dpb11-iAID and mcm10-iAID cells after the addition of tetracycline and auxin. Both the 'Tet-OFF' promoter and the AID system have been shown to work in model eukaryotes other than budding yeast. Therefore, the iAID system is not only useful in budding yeast, but also can be applied to other model systems to isolate tight conditional mutants.

RNA interference regulates the cell cycle checkpoint through the RNA export factor, Ptr1, in fission yeast.

Ago1, an effector protein of RNA interference (RNAi), regulates heterochromatin silencing and cell cycle arrest in fission yeast. However, the mechanism by which Ago1 controls cell cycle checkpoint following hydroxyurea (HU) treatment has not been elucidated. In this study, we show that Ago1 and other RNAi factors control cell cycle checkpoint following HU treatment via a mechanism independent of silencing. While silencing requires dcr1(+), the overexpression of ago1(+) alleviated the cell cycle defect in dcr1Δ. Ago1 interacted with the mRNA export factor, Ptr1. The ptr1-1 mutation impaired cell cycle checkpoint but gene silencing was unaffected. Genetic analysis revealed that the regulation of cell cycle checkpoint by ago1(+) is dependent on ptr1(+). Nuclear accumulation of poly(A)(+) RNAs was detected in mutants of ago1(+) and ptr1(+), suggesting there is a functional link between the cell cycle checkpoint and RNAi-mediated RNA quality control.

Two different Argonaute complexes are required for siRNA generation and heterochromatin assembly in fission yeast.

The RNA-induced transcriptional silencing (RITS) complex, containing Ago1, Chp1, Tas3 and centromeric small interfering RNAs (siRNAs), is required for heterochromatic gene silencing at centromeres. Here, we identify a second fission yeast Argonaute complex (Argonaute siRNA chaperone, ARC), which contains, in addition to Ago1, two previously uncharacterized proteins, Arb1 and Arb2, both of which are required for histone H3 Lys9 (H3-K9) methylation, heterochromatin assembly and siRNA generation. Furthermore, whereas siRNAs in the RITS complex are mostly single-stranded, siRNAs associated with ARC are mostly double-stranded, indicating that Arb1 and Arb2 inhibit the release of the siRNA passenger strand from Ago1. Consistent with this observation, purified Arb1 inhibits the slicer activity of Ago1 in vitro, and purified catalytically inactive Ago1 contains only double-stranded siRNA. Finally, we show that slicer activity is required for the siRNA-dependent association of Ago1 with chromatin and for the spreading of histone H3-K9 methylation.

RPS12 and UBC4 Are Related to Senescence Signal Production in the Ribosomal RNA Gene Cluster.

Genome instability causes cellular senescence in many organisms. The rRNA gene cluster (rDNA) is one of the most unstable regions in the genome and this instability might convey a signal that induces senescence in the budding yeast. The instability of rDNA mostly depends on replication fork blocking (RFB) activity which induces recombination and gene amplification. By overexpression of Fob1, responsible for the RFB activity, we found that unstable rDNA induces cell cycle arrest and restricts replicative life span. We isolated yeast mutants that grew normally while Fob1 was overexpressed, expecting that some of the mutated genes would be related to the production of a "senescence signal" that elongates cell cycle, stops cell division and finally restricts replicative life span. Our screen identified three suppressor genes, RPS12, UBC4, and CCR4. Replicative life spans of the rps12 and ubc4 mutants were longer than that of wild-type cells. An increase in the levels of extrachromosomal rDNA circles and noncoding transcripts, known to shorten replicative life span, was observed in ubc4 and rps12 respectively, while DNA double strand-breaks at the RFB that are triggers of rDNA instability were reduced in the rps12 mutant. Overall, our observations indicate that Rps12 and Ubc4 contribute to the connection between rDNA instability and replicative life span.

Establishment of an "in saccharo" experimental system.

Many proteins form complexes that function in reaction pathways. Therefore, to understand protein function, it is necessary to reconstitute complexes and pathways in vitro. However, it is not straightforward to achieve full activity in reconstituted systems. To address this problem, we present a yeast system named "in saccharo" analysis, which uses budding yeast for simultaneous expression and analysis of many kinds of non-host proteins, such as human proteins. For this purpose, vectors that can accommodate many genes are required. Here, we describe the construction of a chromosome vector by insertion of unique barcode sequences (BCs) into the ribosomal RNA gene repeat (rDNA). Each unit of the rDNA has a BC that is used as the target for integration of an external gene. Because rDNA is naturally capable of maintaining many repetitive copies, the vector is expected to retain the numerous external genes that may be required for reconstitution of functional protein complexes and reaction pathways. Consistent with this prediction, we were able to clone three human genes that form the RNA silencing pathway, which has no functional equivalent in budding yeast, and to demonstrate functionality in this in saccharo analysis system.

Conserved ribonuclease, Eri1, negatively regulates heterochromatin assembly in fission yeast.

RNA interference (RNAi) is a conserved silencing mechanism that has widespread roles in RNA degradation, translational repression, and the epigenetic control of chromatin structure [1]. In fission yeast, heterochromatin assembly requires RNAi machinery and is initiated by small interference RNAs (siRNAs) derived from heterochromatic regions and by the RNA-induced transcriptional silencing (RITS) complex [2-7]. Although recent studies have been successful in uncovering the functions of effector complexes in the RNAi pathway [4, 5, 8-10], exactly how heterochromatic siRNAs are processed and function in assembling heterochromatin remains unclear. In this study we focused on a conserved ribonuclease, Eri1, which was originally identified as a negative regulator of RNAi in C. elegans [11], and show the importance of the Eri1 protein in RNAi-mediated heterochromatin assembly in fission yeast. Eri1 specifically degrades double-stranded siRNAs through two functional domains and represses the accumulation of cellular siRNAs in vivo. Deletion of eri1(+) causes an increase in siRNAs associated with the RITS complex and enhances heterochromatic silencing, which is accompanied by increased levels of histone H3-K9 methylation and the Swi6 protein. Our findings suggest that the fission yeast Eri1 controls the accumulation of heterochromatic siRNAs and negatively regulates the RNAi-mediated heterochromatin assembly.

RNAi-dependent heterochromatin assembly in fission yeast Schizosaccharomyces pombe requires heat-shock molecular chaperones Hsp90 and Mas5.

BACKGROUND: Heat-shock molecular chaperone proteins (Hsps) promote the loading of small interfering RNA (siRNA) onto RNA interference (RNAi) effector complexes. While the RNAi process is coupled with heterochromatin assembly in several model organisms, it remains unclear whether the Hsps contribute to epigenetic gene regulation. In this study, we used the fission yeast Schizosaccharomyces pombe as a model organism and investigated the roles of Hsp90 and Mas5 (a nucleocytoplasmic type-I Hsp40 protein) in RNAi-dependent heterochromatin assembly. RESULTS: Using a genetic screen and biochemical analyses, we identified Hsp90 and Mas5 as novel silencing factors. Mutations in the genes encoding these factors caused derepression of silencing at the pericentromere, where heterochromatin is assembled in an RNAi-dependent manner, but not at the subtelomere, where RNAi is dispensable. The mutations also caused a substantial reduction in the level of dimethylation of histone H3 at Lys9 at the pericentromere, where association of the Argonaute protein Ago1 was also abrogated. Consistently, siRNA corresponding to the pericentromeric repeats was undetectable in these mutant cells. In addition, levels of Tas3, which is a protein in the RNA-induced transcriptional silencing complex along with Ago1, were reduced in the absence of Mas5. CONCLUSIONS: Our results suggest that the Hsps Hsp90 and Mas5 contribute to RNAi-dependent heterochromatin assembly. In particular, Mas5 appears to be required to stabilize Tas3 in vivo. We infer that impairment of Hsp90 and Hsp40 also may affect the integrity of the epigenome in other organisms.

Noncompetitive counteractions of DNA polymerase epsilon and ISW2/yCHRAC for epigenetic inheritance of telomere position effect in Saccharomyces cerevisiae.

Relocation of euchromatic genes near the heterochromatin region often results in mosaic gene silencing. In Saccharomyces cerevisiae, cells with the genes inserted at telomeric heterochromatin-like regions show a phenotypic variegation known as the telomere-position effect, and the epigenetic states are stably passed on to following generations. Here we show that the epigenetic states of the telomere gene are not stably inherited in cells either bearing a mutation in a catalytic subunit (Pol2) of replicative DNA polymerase epsilon (Pol epsilon) or lacking one of the nonessential and histone fold motif-containing subunits of Pol epsilon, Dpb3 and Dpb4. We also report a novel and putative chromatin-remodeling complex, ISW2/yCHRAC, that contains Isw2, Itc1, Dpb3-like subunit (Dls1), and Dpb4. Using the single-cell method developed in this study, we demonstrate that without Pol epsilon and ISW2/yCHRAC, the epigenetic states of the telomere are frequently switched. Furthermore, we reveal that Pol epsilon and ISW2/yCHRAC function independently: Pol epsilon operates for the stable inheritance of a silent state, while ISW2/yCHRAC works for that of an expressed state. We therefore propose that inheritance of specific epigenetic states of a telomere requires at least two counteracting regulators.

Population genomics of the fission yeast Schizosaccharomyces pombe.

The fission yeast Schizosaccharomyces pombe has been widely used as a model eukaryote to study a diverse range of biological processes. However, population genetic studies of this species have been limited to date, and we know very little about the evolutionary processes and selective pressures that are shaping its genome. Here, we sequenced the genomes of 32 worldwide S. pombe strains and examined the pattern of polymorphisms across their genomes. In addition to introns and untranslated regions (UTRs), intergenic regions also exhibited lower levels of nucleotide diversity than synonymous sites, suggesting that a considerable amount of noncoding DNA is under selective constraint and thus likely to be functional. A number of genomic regions showed a reduction of nucleotide diversity probably caused by selective sweeps. We also identified a region close to the end of chromosome 3 where an extremely high level of divergence was observed between 5 of the 32 strains and the remain 27, possibly due to introgression, strong positive selection, or that region being responsible for reproductive isolation. Our study should serve as an important starting point in using a population genomics approach to further elucidate the biology of this important model organism.

Spt6 prevents transcription-coupled loss of posttranslationally modified histone H3.

The tail of histone H3 is an ideal medium for storing epigenetic information because displacement of histone H3 is heavily restricted during transcription. To maintain the locus-specific modifications of histone H3, histone molecules should be retained locally at the original position through multiple rounds of transcription. Here, we found that fission yeast Spt6, a highly conserved RNA polymerase II-interacting histone H3-H4 chaperone, is essential for the maintenance of Lys-4 and Lys-9 methylation of histone H3 in euchromatin and heterochromatin, respectively. In euchromatin, loss of Lys-4 methylated histone H3 and deposition of newly synthesized Lys-56 acetylated histone H3 induced by Spt6 inactivation were coupled with transcription. While in heterochromatin, Spt6 prevents histone turnover and cryptic transcription in parallel with Clr3 histone deacetylase. We propose that Spt6 retains posttranslationally modified histone H3 during transcription to maintain epigenome integrity.

A chromodomain protein, Chp1, is required for the establishment of heterochromatin in fission yeast.

The chromodomain is a conserved motif that functions in the epigenetic control of gene expression. Here, we report the functional characterization of a chromodomain protein, Chp1, in the heterochromatin assembly in fission yeast. We show that Chp1 is a structural component of three heterochromatic regions-centromeres, the mating-type region, and telomeres-and that its localization in these regions is dependent on the histone methyltransferase Clr4. Although deletion of the chp1(+) gene causes centromere-specific decreases in Swi6 localization and histone H3-K9 methylation, we show that the role of Chp1 is not exclusive to the centromeres. We found that some methylation persists in native centromeric regions in the absence of Chp1, which is also true for the mating-type region and telomeres, and determined that Swi6 and Chp2 are critical to maintaining this residual methylation. We also show that Chp1 participates in the establishment of repressive chromatin in all three chromosomal regions. These results suggest that different heterochromatic regions share common structural properties, and that centromeric heterochromatin requires Chp1-mediated establishment steps differently than do other heterochromatic regions.