title: Heterochromatin protein 1 homologue Swi6 acts in concert with Ers1 to regulate RNAi-directed heterochromatin assembly.
abstract: In fission yeast, the RNAi pathway is required for centromeric heterochromatin assembly. siRNAs derived from centromeric transcripts are incorporated into the RNA-induced transcriptional silencing (RITS) complex and direct it to nascent homologous transcripts. The RNA-induced transcriptional silencing-bound nascent transcripts further recruit the RNA-directed RNA polymerase complex (RDRC) to promote dsRNA synthesis and siRNA production. Heterochromatin coated with Swi6/Heterochromain Protein 1 is then formed following recruitment of chromatin modification machinery. Swi6 is also required for the upstream production of siRNA, although the mechanism for this has remained obscure. Here, we demonstrate that Swi6 recruits RDRC to heterochromatin through Ers1, an RNAi factor intermediate. An ers1(+) mutant allele (ers1-C62) was identified in a genetic screen for mutants that alleviate centromeric silencing, and this phenotype was suppressed by overexpression of either the Hrr1 RDRC subunit or Clr4 histone H3-K9 methyltransferase. Ers1 physically interacts with Hrr1, and loss of Ers1 impairs RDRC centromeric localization. Although Ers1 failed to bind Clr4, a direct interaction with Swi6 was detected, and centromeric localization of Swi6 was enhanced by Clr4 overexpression in ers1-C62 cells. Consistent with this, deletion of swi6(+) reduced centromeric localization of Ers1 and RDRC. Moreover, tethering of Ers1 or Hrr1 to centromeric heterochromatin partially bypassed Swi6 function. These findings demonstrate an alternative mechanism for RDRC recruitment and explain the essential role of Swi6/Heterochromain Protein 1 in RNAi-directed heterochromatin assembly.
authors.affiliation: Laboratory for Chromatin Dynamics,  RIKEN Center for Developmental Biology, Kobe, Hyogo 650-0047,  Japan.
authors.forename: Aki
authors.initials: A
authors.lastname: Hayashi
authors.forename: Mayumi
authors.initials: M
authors.lastname: Ishida
authors.forename: Rika
authors.initials: R
authors.lastname: Kawaguchi
authors.forename: Takeshi
authors.initials: T
authors.lastname: Urano
authors.forename: Yota
authors.initials: Y
authors.lastname: Murakami
authors.forename: Jun-ichi
authors.initials: J
authors.lastname: Nakayama
mesh_terms: D015153:Blotting, Western; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D002503:Centromere / Q000235:genetics / Q000378:metabolism; D047369:Chromatin Immunoprecipitation; D002868:Chromosomal Proteins, Non-Histone / Q000235:genetics / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006570:Heterochromatin / Q000235:genetics / Q000378:metabolism*; D011495:Histone-Lysine N-Methyltransferase; D008780:Methyltransferases / Q000235:genetics / Q000378:metabolism; D008856:Microscopy, Fluorescence; D009154:Mutation; D011485:Protein Binding; D034622:RNA Interference / Q000502:physiology*; D012331:RNA, Fungal / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D012324:RNA-Dependent RNA Polymerase / Q000378:metabolism; D034743:RNA-Induced Silencing Complex / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012568:Schizosaccharomyces / Q000235:genetics / Q000378:metabolism; D029702:Schizosaccharomyces pombe Proteins / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D018797:Cell Cycle Proteins; D002868:Chromosomal Proteins, Non-Histone; C532446:Ers1 protein, S pombe; D006570:Heterochromatin; D012331:RNA, Fungal; D034741:RNA, Small Interfering; D034743:RNA-Induced Silencing Complex; D029702:Schizosaccharomyces pombe Proteins; C465958:Swi6 protein, S pombe; D049452:Green Fluorescent Proteins; D008780:Methyltransferases; D011495:Histone-Lysine N-Methyltransferase; C112744:clr4 protein, S pombe; D012324:RNA-Dependent RNA Polymerase