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________________________________________________________________________________________________________ No Heterochromatin - Lose Chromosomes   Schizosaccharomyces pombe (aka fission yeast) cells lacking heterochromatin at centromeres have aberrant kinetochores and are defective in segregating chromosomes in mitosis and meiosis _________________________________________________________________________ Recent Publications: Stc1: A Critical Link between RNAi and Chromatin Modification Required for Heterochromatin Integrity The fission yeast LIM domain protein Stc1 is specifically required to establish H3K9 methylation via RNAi, and interacts both with the RNAi effector Ago1, and with the chromatin-modifying CLRC complex. Moreover, tethering Stc1 to a euchromatic locus is sufficient to induce silencing and heterochromatin formation independently of RNAi. We conclude that Stc1 associates with RITS on centromeric transcripts and recruits CLRC, thereby coupling RNAi to chromatin modification. See Bayne et al (2010) Cell 140: 666-678.
RDRP-dependent Induction of Secondary siRNA by Synthetic Hairpin RNA in Fission Yeast Expression of an exogenous double stranded GFP RNA is processed siRNA, these induce the RNA Dependent RNA Polymerase dependent production of ura4 homologous siRNA when targeted to a ura4-GFP fusion gene. For details see Simmer et al (2010) EMBO Reports 11: 112-118.  Detectionof ura4 secondary siRNAsby massively parallel sequencing. Sense (Top) and antisense (Bottom) siRNA werealigned to the ura4-GFP target gene. Secondary siRNAs complementary to the ura4 portion of ura4-GFPtarget are detected in wild-type but not rdp1Δ. Analysis of small RNA in fission yeast: centromeric siRNAs are potentially generated through a structured RNA. In this collaborative work we show that centromeric small RNAs from fission yeast are Dcr1 dependent, carry 5'-monophosphates and are associated with Ago1. Deep sequencing demonstrates that the majority of centromeric small RNAs originate from two remarkably well-conserved sequences that are present in all centromeres (see figure below). Centromeric siRNA persist in rdp1Δ cells. 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. For details see Djupedal et al (2009) EMBO J. 28:3832.
Common Ancestry of CENP-A Chaperones Scm3 and HJURP Analyses with Chris Ponting's lab of Scm3 and HJURP reconcile previous observations by demonstrating that fungal Scm3 proteins are distant counterparts of human HJURP. Thus, investigation of Scm3 and associated proteins is likely to be directly relevant to understanding the mechanism of HJURP-mediated CENP-A chromatin assembly at human centromeres. For details see Sanchez-Pulido et al (2009) Cell 137:1173
Synthetic Heterochromatin Bypass RNAi and Centromeric Repeats to Establish Functional Centromeres Artificially tethering the Clr4 H3 lysine 9 methyltransferase to Gal4 binding sites rewires centromeres so that outer repeats, their non-coding RNAs and RNAi rendered redundant in forming functional centromeres. For details see Kagansky et al (2009) Science 324: 1716-19  _______________________________________________________________________________ |
