| 1. |
- Bayne, Elizabeth H., et al.
(författare)
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Splicing factors facilitate RNAi-directed silencing in fission yeast
- 2008
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 322:5901, s. 602-606
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Tidskriftsartikel (refereegranskat)abstract
- Heterochromatin formation at fission yeast centromeres is directed by RNA interference (RNAi). Noncoding transcripts derived from centromeric repeats are processed into small interfering RNAs (siRNAs) that direct the RNA-induced transcriptional silencing (RITS) effector complex to engage centromer transcripts, resulting in recruitment of the histone H3 lysine 9 methyltransferase Clr4, and hence silencing. We have found that defects in specific splicing factors, but not splicing itself, affect the generation of centromeric siRNAs and consequently centromeric heterochromatin integrity. Moreover, splicing factors physically associate with Cid12, a component of the RNAi machinery, and with centromeric chromatin, consistent with a direct role in RNAi. We propose that spliceosomal complexes provide a platform for siRNA generation and hence facilitate effective centromere repeat silencing.
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| 2. |
- Bernard, Pascal, et al.
(författare)
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Cell-cycle regulation of cohesin stability along fission yeast chromosomes
- 2008
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Ingår i: EMBO Journal. - : Wiley. - 0261-4189 .- 1460-2075. ; 27:1, s. 111-121
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Tidskriftsartikel (refereegranskat)abstract
- Sister chromatid cohesion is mediated by cohesin, but the process of cohesion establishment during S-phase is still enigmatic. In mammalian cells, cohesin binding to chromatin is dynamic in G1, but becomes stabilized during S-phase. Whether the regulation of cohesin stability is integral to the process of cohesion establishment is unknown. Here, we provide evidence that fission yeast cohesin also displays dynamic behavior. Cohesin association with G1 chromosomes requires continued activity of the cohesin loader Mis4/Ssl3, suggesting that repeated loading cycles maintain cohesin binding. Cohesin instability in G1 depends on wpl1, the fission yeast ortholog of mammalian Wapl, suggestive of a conserved mechanism that controls cohesin stability on chromosomes. wpl1 is nonessential, indicating that a change in wpl1-dependent cohesin dynamics is dispensable for cohesion establishment. Instead, we find that cohesin stability increases at the time of S-phase in a reaction that can be uncoupled from DNA replication. Hence, cohesin stabilization might be a pre-requisite for cohesion establishment rather than its consequence.
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| 3. |
- Djupedal, Ingela, et al.
(författare)
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Analysis of small RNA in fission yeast; centromeric siRNAs are potentially generated through a structured RNA
- 2009
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Ingår i: EMBO Journal. - : Wiley. - 0261-4189 .- 1460-2075. ; 28:24, s. 3832-3844
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Tidskriftsartikel (refereegranskat)abstract
- The formation of heterochromatin at the centromeres in fission yeast depends on transcription of the outer repeats. These transcripts are processed into siRNAs that target homologous loci for heterochromatin formation. Here, high throughput sequencing of small RNA provides a comprehensive analysis of centromere-derived small RNAs. We found that the centromeric small RNAs are Dcr1 dependent, carry 50-monophosphates and are associated with Ago1. The majority of centromeric small RNAs originate from two remarkably well-conserved sequences that are present in all centromeres. The high degree of similarity suggests that this non-coding sequence in itself may be of importance. Consistent with this, secondary structure-probing experiments indicate that this centromeric RNA is partially double-stranded and is processed by Dicer in vitro. We further demonstrate the existence of small centromeric RNA in rdp1D cells. Our data suggest a pathway for siRNA generation that is distinct from the well-documented model involving RITS/RDRC. We propose that primary transcripts fold into hairpin-like structures that may be processed by Dcr1 into siRNAs, and that these siRNAs may initiate heterochromatin formation independent of RDRC activity. The EMBO Journal (2009) 28, 3832-3844. doi: 10.1038/emboj.2009.351; Published online 26 November 2009
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| 4. |
- Djupedal, Ingela
(författare)
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Characterization of RNA polymerase II subunit Rpb7 in silencing and transcription
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The DNA in eukaryotes is arranged in fibres of chromatin. The chromatin may be more or less compacted and the degree of condensation of the chromatin affects the accessibility of the DNA. The accessibility of the DNA, in turn, affects transcription and gene regulation. Genes within inaccessible DNA are commonly repressed whereasgenes within accessible DNA are active and expressed. This thesis concerns the interplay between chromatin and transcription with focus on the function of the RNA polymerase II (pol II) subunit Rpb7. We have demonstrated that processing of centromeric transcripts by the ribonuclease III family protein Dcr1 is required for heterochromatin formation at the centromeres of Schizosaccharomyces pombe. A point mutation in the pol II subunit Rpb7 caused a specific defect in centromeric heterochromatin formation. We have shown i) that the centromeric transcripts that accumulate in dcr1delta cells are products of pol II, ii) the rpbG150D mutation is deficient in recognition and/or initiation of transcription from the centromeric promoter. Transcription by pol II within the centromeres was surprising since insertion of marker genes within these loci normally results in repression of pol II transcription. Here, paradoxically, pol II transcription was required for the construction of the inaccessible heterochromatin structure. Our analysis of sRNA in S. pombe revealed that most centromeric siRNA are originating from two clusters, which are repeated several times within the centromeres. This lead us to propose a model in which centromeric transcripts fold into double stranded structures that are processed by Dcr1. The resulting siRNAs may contribute with the starting signal for the RNAi feedback loop required for heterochromatin formation at the centromeres. Finally, we demonstrate that the genome-wide association of Rpb7 is nearly identical to that of the core pol II subunit Rpb2, indicating a general role for Rpb7 in transcription. We further show that the occupancy pattern of Rpb4, a pol II subunit that forms a subcomplex together with Rpb7, differs from those of Rpb2 and Rpb7. Rpb4 may therefore have a less general function in transcription than Rpb7. Hence, transcription by pol II is required not only for gene expression but also for repression via formation of inaccessible heterochromatin.
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| 5. |
- Djupedal, Ingela, et al.
(författare)
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Epigenetics : heterochromatin meets RNAi
- 2009
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Ingår i: Cell Research. - : Springer Science and Business Media LLC. - 1001-0602 .- 1748-7838. ; 19:3, s. 282-295
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Tidskriftsartikel (refereegranskat)abstract
- The term epigenetics refers to heritable changes not encoded by DNA. The organization of DNA into chromatin fibers affects gene expression in a heritable manner and is therefore one mechanism of epigenetic inheritance. Large parts of eukaryotic genomes consist of constitutively highly condensed heterochromatin, important for maintaining genome integrity but also for silencing of genes within. Small RNA, together with factors typically associated with RNA interference (RNAi) targets homologous DNA sequences and recruits factors that modify the chromatin, commonly resulting in formation of heterochromatin and silencing of target genes. The scope of this review is to provide an overview of the roles of small RNA and the RNAi components, Dicer, Argonaute and RNA dependent polymerases in epigenetic inheritance via heterochromatin formation, exemplified with pathways from unicellular eukaryotes, plants and animals.
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| 6. |
- Djupedal, Ingela, et al.
(författare)
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Molecular biology - The paradox of silent heterochromatin
- 2008
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 320:5876, s. 624-625
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 7. |
- Djupedal, Ingela, et al.
(författare)
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RNA Pol II subunit Rpb7 promotes centromeric transcription and RNAi-directed chromatin silencing
- 2005
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Ingår i: Genes & Development. - : Cold Spring Harbor Laboratory. - 0890-9369 .- 1549-5477. ; 19:19, s. 2301-2306
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Tidskriftsartikel (refereegranskat)abstract
- Fission yeast centromeric repeats are transcribed into small interfering RNA (siRNA) precursors (pre-siRNAs), which are processed by Dicer to direct heterochromatin formation. Recently, Rpb1 and Rpb2 subunits of RNA polymerase II (RNA Pol II) were shown to mediate RNA interference (RNAi)-directed chromatin modification but did not affect pre-siRNA levels. Here we show that another Pol II subunit, Rpb7 has a specific role in presiRNA transcription. We define a centromeric presiRNA promoter from which initiation is exquisitely sensitive to the rpb7-G150D mutation. In contrast to other Pol II subunits, Rpb7 promotes pre-siRNA transcription required for RNAi-directed chromatin silencing.
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| 8. |
- Dunleavy, Elaine M., et al.
(författare)
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A NASP (N1/N2)-related protein, Sim3, binds CENP-A and is required for its deposition at fission yeast Centromeres
- 2007
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Ingår i: Molecular Cell. - : Elsevier BV. - 1097-2765 .- 1097-4164. ; 28:6, s. 1029-1044
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Tidskriftsartikel (refereegranskat)abstract
- A defining feature of centromeres is the presence of the histone H3 variant CENP-A(Cnp1). It is not known how CENP-A(Cnp1) is specifically delivered to, and assembled into, centromeric chromatin. Through a screen for factors involved in kinetochore integrity in fission yeast, we identified Sim3. Sim3 is homologous to known histone binding proteins NASP(Human) and N1/N2(Xenopus) and aligns with Hif1(S. cerevisiae), defining the SHNi-TPR family. Sim3 is distributed throughout the nucleoplasm, yet it associates with CENP-A(Cnp1) and also binds H3. Cells defective in Sim3 function have reduced levels of CENP-A(CnP1) at centromeres (and increased H3) and display chromosome segregation defects. Sim3 is required to allow newly synthesized CENP-A(Cnp1) to accumulate at centromeres in S and G2 phase-arrested cells in a replication-independent mechanism. We propose that one function of Sim3 is to act as an escort that hands off CENP-A(Cnp1) to chromatin assembly factors, allowing its incorporation into centromeric chromatin.
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| 9. |
- Durand-Dubief, Mickael, et al.
(författare)
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Heterochromatin tells CENP-A where to go
- 2008
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Ingår i: Bioessays. - : Wiley. - 0265-9247 .- 1521-1878. ; 30:6, s. 526-529
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Tidskriftsartikel (refereegranskat)abstract
- The centromere is the region of the chromosome where the kinetochore forms. Kinetochores are the attachment sites for spindle microtubules that separate duplicated chromosomes in mitosis and meiosis. Kinetochore formation depends on a special chromatin structure containing the histone H3 variant CENP-A. The epigenetic mechanisms that maintain CENP-A chromatin throughout the cell cycle have been studied extensively but little is known about the mechanism that targets CENP-A to naked centromeric DNA templates. In a recent report published in Science,((1)) such de novo centromere assembly of CENP-A is shown to be dependent on heterochromatin and the RNA interference pathway.
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| 10. |
- Durand-Dubief, Mickael, et al.
(författare)
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Specific functions for the fission yeast Sirtuins Hst2 and Hst4 in gene regulation and retrotransposon silencing
- 2007
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Ingår i: EMBO Journal. - : Wiley. - 0261-4189 .- 1460-2075. ; 26:10, s. 2477-2488
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Tidskriftsartikel (refereegranskat)abstract
- Expression profiling, ChiP-CHIP and phenotypic analysis were used to investigate the functional relationships of class III NAD(+)-dependent HDACs (Sirtuins) in fission yeast. We detected significant histone acetylation increases in Sirtuin mutants at their specific genomic binding targets and were thus able to identify an in vivo substrate preference for each Sirtuin. At heterochromatic loci, we demonstrate that although Hst2 is mainly cytoplasmic, a nuclear pool of Hst2 colocalizes with the other Sirtuins at silent regions (cen, mat, tel, rDNA), and that like the other Sirtuins, Hst2 is required for rDNA and centromeric silencing. Interestingly we found specific functions for the fission yeast Sirtuins Hst2 and Hst4 in gene regulation. Hst2 directly represses genes involved in transport and membrane function, whereas Hst4 represses amino-acid biosynthesis genes and Tf2 retrotransposons. A specific role for Hst4 in Tf2 50 mRNA processing was revealed. Thus, Sirtuins share functions at many genomic targets, but Hst2 and Hst4 have also evolved unique functions in gene regulation.
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