Continuous requirement for the Clr4 complex but not RNAi for centromeric heterochromatin assembly in fission yeast harboring a disrupted RITS complex.
Formation of centromeric heterochromatin in fission yeast requires the combined action of chromatin modifying enzymes and small RNAs derived from centromeric transcripts. Positive feedback mechanisms that link the RNAi pathway and the Clr4/Suv39h1 histone H3K9 methyltransferase complex (Clr-C) result in requirements for H3K9 methylation for full siRNA production and for siRNA production to achieve full histone methylation. Nonetheless, it has been proposed that the Argonaute protein, Ago1, is the key initial trigger for heterochromatin assembly via its association with Dicer-independent "priRNAs." The RITS complex physically links Ago1 and the H3-K9me binding protein Chp1. Here we exploit an assay for heterochromatin assembly in which loss of silencing by deletion of RNAi or Clr-C components can be reversed by re-introduction of the deleted gene. We showed previously that a mutant version of the RITS complex (Tas3(WG)) that biochemically separates Ago1 from Chp1 and Tas3 proteins permits maintenance of heterochromatin, but prevents its formation when Clr4 is removed and re-introduced. Here we show that the block occurs with mutants in Clr-C, but not mutants in the RNAi pathway. Thus, Clr-C components, but not RNAi factors, play a more critical role in assembly when the integrity of RITS is disrupted. Consistent with previous reports, cells lacking Clr-C components completely lack H3K9me2 on centromeric DNA repeats, whereas RNAi pathway mutants accumulate low levels of H3K9me2. Further supporting the existence of RNAi-independent mechanisms for establishment of centromeric heterochromatin, overexpression of clr4(+) in clr4Δago1Δ cells results in some de novo H3K9me2 accumulation at centromeres. These findings and our observation that ago1Δ and dcr1Δ mutants display indistinguishable low levels of H3K9me2 (in contrast to a previous report) challenge the model that priRNAs trigger heterochromatin formation. Instead, our results indicate that RNAi cooperates with RNAi-independent factors in the assembly of heterochromatin.
Department of Biochemistry, St Jude Children's Research Hospital, Memphis, Tennessee, United States of America.
Sreenath
S
Shanker
Godwin
G
Job
Olivia L
OL
George
Kevin M
KM
Creamer
Alaa
A
Shaban
Janet F
JF
Partridge
D060565:Argonaute Proteins; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000235:genetics* / Q000378:metabolism; D002503:Centromere / Q000235:genetics; D047369:Chromatin Immunoprecipitation; D002868:Chromosomal Proteins, Non-Histone / Q000235:genetics / Q000378:metabolism; D044842:Cullin Proteins / Q000235:genetics / Q000378:metabolism; D006570:Heterochromatin / Q000235:genetics* / Q000378:metabolism; D011495:Histone-Lysine N-Methyltransferase; D006657:Histones / Q000378:metabolism; D008239:Lysine / Q000378:metabolism; D008745:Methylation; D008780:Methyltransferases / Q000235:genetics* / Q000378:metabolism; D009154:Mutation; D034622:RNA Interference; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D012568:Schizosaccharomyces / Q000235:genetics* / Q000378:metabolism; D029702:Schizosaccharomyces pombe Proteins / Q000235:genetics* / Q000378:metabolism
C481643:Ago1 protein, S pombe; D060565:Argonaute Proteins; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C115018:Chp1 protein, S pombe; D002868:Chromosomal Proteins, Non-Histone; D044842:Cullin Proteins; D006570:Heterochromatin; D006657:Histones; C513900:Pcu4 protein, S pombe; D016601:RNA-Binding Proteins; C489651:Rik1 protein, S pombe; D029702:Schizosaccharomyces pombe Proteins; C499720:Tas3 protein, S pombe; D008780:Methyltransferases; D011495:Histone-Lysine N-Methyltransferase; C112744:clr4 protein, S pombe; D008239:Lysine