Fission yeast Cid14, a component of the TRAMP (Cid14/Trf4-Air1-Mtr4 polyadenylation) complex, polyadenylates nuclear RNA and stimulates degradation by the exosome for RNA quality control. Here, we analyze patterns of global gene expression in cells lacking the Cid14 or the Dis3/Rpr44 subunit of the nuclear exosome. We found that transcripts from many genes induced during meiosis, including key regulators, accumulated in the absence of Cid14 or Dis3. Moreover, our data suggest that additional substrates include transcripts involved in heterochromatin assembly. Mutant cells lacking Cid14 and/or Dis3 accumulate transcripts corresponding to naturally silenced repeat elements within heterochromatic domains, reflecting defects in centromeric gene silencing and derepression of subtelomeric gene expression. We also uncover roles for Cid14 and Dis3 in maintaining the genomic integrity of ribosomal DNA. Our data indicate that polyadenylation-assisted nuclear RNA turnover functions in eliminating a variety of RNA targets to control diverse processes, such as heterochromatic gene silencing, meiotic differentiation, and maintenance of genomic integrity.Polyadenylation is important for the maturation of mRNAs (26), while our recent work has also uncovered unexpected links between polyadenylation and chromosome replication and segregation (33,35,37). In addition to its function in the maturation of mRNAs, nuclear polyadenylation is important for diverse cellular activities, such as RNA interference (RNAi)-mediated heterochromatin assembly and quality control of noncoding RNAs (33,35,37). These functions involve Cid12 and Cid14, members of a widespread family of noncanonical poly(A) polymerases found in eukaryotes from yeasts to humans (30). Besides its function in checkpoint control, Cid12 is required for faithful chromosome segregation and RNAi-mediated heterochromatin assembly at centromeres (19,37). RNAi silencing is triggered by double-stranded RNA, which is processed by the RNase III-like RNase Dicer into small interfering RNA (siRNA) molecules of around 21 nucleotides. These siRNAs become incorporated into the RNAinduced transcriptional silencing (RITS) complex, directing the complex to homologous RNA targets (6). In worms, plants, and fungi, RNAi also requires RNA-directed RNA polymerases (RDRs), which are involved in the siRNA-and template-directed production of double-stranded RNA (1, 17, 29). Motamedi et al. (19) identified Cid12 in an RDR complex (RDRC) which also contains Rdp1 (the fission yeast RDR homolog) and Hrr1 (an RNA helicase). RDRC physically interacts with the RITS complex in a manner that requires Dicer and the histone methyltransferase Clr4. In cells lacking Cid12, RITS complexes are devoid of siRNA and fail to localize to centromeric DNA repeats to initiate heterochromatin assembly.In Saccharomyces cerevisiae, the Cid1-like proteins Trf4 and Trf5 share an essential function that involves the polyadenylation of nuclear RNAs as part of a pathway of exosomemediated RNA turnover (10,12,31,38). We have ide...

Polyadenylation is an essential processing step for most eukaryotic mRNAs. In the nucleus, poly(A) polymerase adds poly(A) tails to mRNA 3 ends, contributing to their export, stability and translatability. Recently, a novel class of non-canonical poly(A) polymerases was discovered in yeast, worms and vertebrates. Different members of the Cid1 family, named after its founding member in the fission yeast Schizosaccharomyces pombe, are localized in the nucleus and the cytoplasm and are thought to target specific RNAs for polyadenylation. Polyadenylation of a target RNA by a Cid1-like poly(A) polymerase can lead to its degradation or stabilization, depending on the enzyme involved. Cid1-like proteins have important roles in diverse biological processes, including RNA surveillance pathways, DNA integrity checkpoint responses and RNAi-dependent heterochromatin formation.

Multiple KH-domain proteins, collectively known as vigilins, are evolutionarily highly conserved proteins that are present in eukaryotic organisms from yeast to metazoa. Proposed roles for vigilins include chromosome segregation, messenger RNA (mRNA) metabolism, translation and tRNA transport. As a step toward understanding its biological function, we have identified the fission yeast vigilin, designated Vgl1, and have investigated its role in cellular response to environmental stress. Unlike its counterpart in Saccharomyces cerevisiae, we found no indication that Vgl1 is required for the maintenance of cell ploidy in Schizosaccharomyces pombe. Instead, Vgl1 is required for cell survival under thermal stress, and vgl1Δ mutants lose their viability more rapidly than wild-type cells when incubated at high temperature. As for Scp160 in S. cerevisiae, Vgl1 bound polysomes accumulated at endoplasmic reticulum (ER) but in a microtubule-independent manner. Under thermal stress, Vgl1 is rapidly relocalized from the ER to cytoplasmic foci that are distinct from P-bodies but contain stress granule markers such as poly(A)-binding protein and components of the translation initiation factor eIF3. Together, these observations demonstrated in S. pombe the presence of RNA granules with similar composition as mammalian stress granules and identified Vgl1 as a novel component that required for cell survival under thermal stress.

Fission yeast Cid12 is a member of the Cid1 family of specialized poly(A) polymerases. Like cells lacking cid1, cid12⌬ mutants were shown to have checkpoint defects when DNA replication was inhibited. Here, we show that Cid12 is also required for faithful chromosome segregation and that mutation of amino acid residues predicted to be essential for poly(A) polymerase activity resulted in loss of Cid12 function in vivo. Cells lacking Cid12 had an increased chromosome segregation failure rate due to precocious loss of sister chromatid cohesion at the centromere but not along the chromosome arms. In keeping with a recently described function for Cid12 in RNA interference (RNAi)-mediated heterochromatin assembly, this was accompanied by an accumulation of polyadenylated transcripts corresponding to naturally silenced repeat elements within heterochromatic domains, with consequent defects in centromeric gene silencing. These cells also suffered increased meiotic defects, and their viability was dependent on the spindle checkpoint protein Bub1. To account for the effects of Cid12 on various aspects of DNA metabolism, including chromosome segregation and the checkpoint control, we suggest that Cid12 has dual functions in RNAi silencing and regulating mRNA stability.The accurate transmission of genetic information during cell proliferation depends on chromosome duplication and the subsequent segregation of the sister chromatids to the opposite poles of the cell during mitosis. In eukaryotic cells, duplicated DNA molecules remain physically connected by cohesion from the time of their synthesis in S phase until they are separated in anaphase. Cohesion is a prerequisite for the bipolar attachment of chromatid pairs to the spindle apparatus in mitosis that enables the equal segregation of the duplicated genome to daughter cells long after DNA replication has occurred (15).Sister chromatid cohesion is mediated by a conserved multiprotein complex called cohesin, which consists of a heterodimer of SMC proteins (Smc1 and Smc3) and two additional proteins, Scc1/Rad21 and Scc3 (15). At the metaphaseto-anaphase transition, dissolution of cohesion is brought about by the cleavage of the Scc1/Rad21 subunit of cohesin, allowing sister chromatid separation (44). In most organisms, cohesin protein complexes are enriched at centromeric repeats (23,41,42,47,53). The presence of a specialized heterochromatin structure at centromeres is vital for tight physical cohesion between sister centromeres to ensure accurate chromosome segregation (6, 31). Mutations that affect the formation of heterochromatin within centromeres adversely affect chromosome segregation. In the fission yeast Schizosaccharomyces pombe, silencing factors such as the heterochromatin protein 1 homolog Swi6 and the histone H3 Lys-9 methyltransferase Clr4 are required for centromere function and chromosome segregation (1, 29). The RNA interference (RNAi) pathway has also been implicated in heterochromatin assembly and is critical for accurate chromosome segregation (11,12,4...

Schizosaccharomyces pombe Rqh1 is a member of the RecQ DNA helicase family. Members of this protein family are mutated in cancer predisposition diseases, causing Bloom's, Werner, and Rothmund-Thomson syndromes. Rqh1 forms a complex with topoisomerase III and is proposed to process or disrupt aberrant recombination structures that arise during S phase to allow proper chromosome segregation during mitosis. Intriguingly, in the absence of Rqh1, processing of these structures appears to be dependent on Rad3 (human ATR) in a manner that is distinct from its role in checkpoint control. Here, we show that rad3 rqh1 mutants are normally committed to a lethal pathway of DNA repair requiring homologous recombination, but blocking this pathway by Rhp51 inactivation restores viability. Remarkably, viability is also restored by overexpression of Cut8, a nuclear envelope protein involved in tethering and proper function of the proteasome. In keeping with a recently described function of the proteasome in the repair of DNA double-strand breaks, we found that Cut8 is also required for DNA double-strand break repair and is essential for proper chromosome segregation in the absence of Rqh1, suggesting that these proteins might function in a common pathway in homologous recombination repair to ensure accurate nuclear division in S. pombe.Eukaryotic cells engage a variety of protective responses following DNA damage. These include enzymes which repair specific DNA lesions after DNA damage as well as regulators that coordinate DNA repair and cell cycle progression. Mutations abolishing many of these processes result in genome instability and cause cancer-prone syndromes in humans. One important class of these syndromes is caused by defects in RecQ helicases (1). These enzymes unwind DNA in a 3Ј to 5Ј direction and are proposed to function in DNA recombination repair. Another example of this is the phosphatidylinositol 3-kinase-like protein kinases ATM (mutated in ataxia telangiectasia) (12) and ATR (ATM and Rad3 related). These proteins are associated with DNA damage surveillance and control of cell cycle checkpoints. Three checkpoint protein complexes have been suggested to play roles in sensing DNA damage. The components of these complexes in Schizosaccharomyces pombe are the checkpoint Rad proteins, including the Rad3-Rad26 complex (human ATR and ATRIP, respectively), the PCNA-like Rad9-Rad1-Hus1 (9-1-1) complex, and the Rad17-replication factor C clamp loader complex (4). They are involved in the activation of two downstream effector kinases, Chk1 and Cds1 (19,29). The Cds1-dependent S-phase checkpoint is required for arresting the cell cycle, stabilizing replication forks, and preventing late origin firing. The Chk1-dependent DNA damage checkpoint prevents entry into mitosis until the completion of DNA repair.DNA double-strand breaks (DSBs) are the most severe damage to DNA caused by environmental factors and occur spontaneously during normal cellular metabolism. In eukaryotes, DSBs are repaired by homologous recombination (HR)...