Regulation of heterochromatic silencing and histone H3 lysine-9 methylation by RNAi.
13 Sep 2002-Science (American Association for the Advancement of Science)-Vol. 297, Iss: 5588, pp 1833-1837
TL;DR: It is proposed that double-stranded RNA arising from centromeric repeats targets formation and maintenance of heterochromatin through RNAi.
Abstract: Eukaryotic heterochromatin is characterized by a high density of repeats and transposons, as well as by modified histones, and influences both gene expression and chromosome segregation. In the fission yeast Schizosaccharomyces pombe, we deleted the argonaute, dicer, and RNA-dependent RNA polymerase gene homologs, which encode part of the machinery responsible for RNA interference (RNAi). Deletion results in the aberrant accumulation of complementary transcripts from centromeric heterochromatic repeats. This is accompanied by transcriptional de-repression of transgenes integrated at the centromere, loss of histone H3 lysine-9 methylation, and impairment of centromere function. We propose that double-stranded RNA arising from centromeric repeats targets formation and maintenance of heterochromatin through RNAi.
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TL;DR: Although they escaped notice until relatively recently, miRNAs comprise one of the more abundant classes of gene regulatory molecules in multicellular organisms and likely influence the output of many protein-coding genes.
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TL;DR: Two founding members of the microRNA family were originally identified in Caenorhabditis elegans as genes that were required for the timed regulation of developmental events and indicate the existence of multiple RISCs that carry out related but specific biological functions.
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TL;DR: Great potential lies in the development of ‘epigenetic therapies’ — several inhibitors of enzymes controlling epigenetic modifications, specifically DNA methyltransferases and histone deacetylases, have shown promising anti-tumorigenic effects for some malignancies.
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3,051 citations
TL;DR: The evidence supports a model in which Argonaute contributes “Slicer” activity to RISC, providing the catalytic engine for RNAi.
Abstract: Gene silencing through RNA interference (RNAi) is carried out by RISC, the RNA-induced silencing complex. RISC contains two signature components, small interfering RNAs (siRNAs) and Argonaute family proteins. Here, we show that the multiple Argonaute proteins present in mammals are both biologically and biochemically distinct, with a single mammalian family member, Argonaute2, being responsible for messenger RNA cleavage activity. This protein is essential for mouse development, and cells lacking Argonaute2 are unable to mount an experimental response to siRNAs. Mutations within a cryptic ribonuclease H domain within Argonaute2, as identified by comparison with the structure of an archeal Argonaute protein, inactivate RISC. Thus, our evidence supports a model in which Argonaute contributes "Slicer" activity to RISC, providing the catalytic engine for RNAi.
2,704 citations
TL;DR: The importance of miRNA-directed gene regulation during plant development is now particularly clear and typically at the cores of gene regulatory networks, targeting genes that are themselves regulators, such as those encoding transcription factors and F-box proteins.
Abstract: MicroRNAs (miRNAs) are small, endogenous RNAs that regulate gene expression in plants and animals. In plants, these approximately 21-nucleotide RNAs are processed from stem-loop regions of long primary transcripts by a Dicer-like enzyme and are loaded into silencing complexes, where they generally direct cleavage of complementary mRNAs. Although plant miRNAs have some conserved functions extending beyond development, the importance of miRNA-directed gene regulation during plant development is now particularly clear. Identified in plants less than four years ago, miRNAs are already known to play numerous crucial roles at each major stage of development-typically at the cores of gene regulatory networks, targeting genes that are themselves regulators, such as those encoding transcription factors and F-box proteins.
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Cites background from "Regulation of heterochromatic silen..."
...In fission yeast, Dicer produces small RNAs corresponding to heterochromatic repeats (113), and deletion of Dicer or Argonaute disrupts heterochromatin silencing (135)....
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TL;DR: Dicer is a member of the RNase III family of nucleases that specifically cleave double-stranded RNAs, and is evolutionarily conserved in worms, flies, plants, fungi and mammals, and has a distinctive structure, which includes a helicase domain and dualRNase III motifs.
Abstract: RNA interference (RNAi) is the mechanism through which double-stranded RNAs silence cognate genes. In plants, this can occur at both the transcriptional and the post-transcriptional levels; however, in animals, only post-transcriptional RNAi has been reported to date. In both plants and animals, RNAi is characterized by the presence of RNAs of about 22 nucleotides in length that are homologous to the gene that is being suppressed. These 22-nucleotide sequences serve as guide sequences that instruct a multicomponent nuclease, RISC, to destroy specific messenger RNAs. Here we identify an enzyme, Dicer, which can produce putative guide RNAs. Dicer is a member of the RNase III family of nucleases that specifically cleave double-stranded RNAs, and is evolutionarily conserved in worms, flies, plants, fungi and mammals. The enzyme has a distinctive structure, which includes a helicase domain and dual RNase III motifs. Dicer also contains a region of homology to the RDE1/QDE2/ARGONAUTE family that has been genetically linked to RNAi.
5,229 citations
TL;DR: It is shown that ‘loss-of-function’ phenotypes can be created in cultured Drosophila cells by transfection with specific double-stranded RNAs, which coincides with a marked reduction in the level of cognate cellular messenger RNAs.
Abstract: In a diverse group of organisms that includes Caenorhabditis elegans, Drosophila, planaria, hydra, trypanosomes, fungi and plants, the introduction of double-stranded RNAs inhibits gene expression in a sequence-specific manner. These responses, called RNA interference or post-transcriptional gene silencing, may provide anti-viral defence, modulate transposition or regulate gene expression. We have taken a biochemical approach towards elucidating the mechanisms underlying this genetic phenomenon. Here we show that 'loss-of-function' phenotypes can be created in cultured Drosophila cells by transfection with specific double-stranded RNAs. This coincides with a marked reduction in the level of cognate cellular messenger RNAs. Extracts of transfected cells contain a nuclease activity that specifically degrades exogenous transcripts homologous to transfected double-stranded RNA. This enzyme contains an essential RNA component. After partial purification, the sequence-specific nuclease co-fractionates with a discrete, approximately 25-nucleotide RNA species which may confer specificity to the enzyme through homology to the substrate mRNAs.
3,208 citations
TL;DR: The 25-nucleotide antisense RNA detected in transgene-induced PTGS is likely synthesized from an RNA template and may represent the specificity determinant of PTGS.
Abstract: Posttranscriptional gene silencing (PTGS) is a nucleotide sequence-specific defense mechanism that can target both cellular and viral mRNAs. Here, three types of transgene-induced PTGS and one example of virus-induced PTGS were analyzed in plants. In each case, antisense RNA complementary to the targeted mRNA was detected. These RNA molecules were of a uniform length, estimated at 25 nucleotides, and their accumulation required either transgene sense transcription or RNA virus replication. Thus, the 25-nucleotide antisense RNA is likely synthesized from an RNA template and may represent the specificity determinant of PTGS.
3,202 citations
TL;DR: It is shown that mammalian methyltransferases that selectively methylate histone H3 on lysine 9 (Suv39h HMTases) generate a binding site for HP1 proteins—a family of heterochromatic adaptor molecules implicated in both gene silencing and supra-nucleosomal chromatin structure.
Abstract: Distinct modifications of histone amino termini, such as acetylation, phosphorylation and methylation, have been proposed to underlie a chromatin-based regulatory mechanism that modulates the accessibility of genetic information. In addition to histone modifications that facilitate gene activity, it is of similar importance to restrict inappropriate gene expression if cellular and developmental programmes are to proceed unperturbed. Here we show that mammalian methyltransferases that selectively methylate histone H3 on lysine 9 (Suv39h HMTases) generate a binding site for HP1 proteins--a family of heterochromatic adaptor molecules implicated in both gene silencing and supra-nucleosomal chromatin structure. High-affinity in vitro recognition of a methylated histone H3 peptide by HP1 requires a functional chromo domain; thus, the HP1 chromo domain is a specific interaction motif for the methyl epitope on lysine9 of histone H3. In vivo, heterochromatin association of HP1 proteins is lost in Suv39h double-null primary mouse fibroblasts but is restored after the re-introduction of a catalytically active SWUV39H1 HMTase. Our data define a molecular mechanism through which the SUV39H-HP1 methylation system can contribute to the propagation of heterochromatic subdomains in native chromatin.
2,820 citations
TL;DR: A stepwise model for the formation of a transcriptionally silent heterochromatin is provided: SUV39H1 places a ‘methyl marker’ on histone H3, which is then recognized by HP1 through its chromo domain, which may also explain the stable inheritance of theheterochromatic state.
Abstract: Heterochromatin protein 1 (HP1) is localized at heterochromatin sites where it mediates gene silencing. The chromo domain of HP1 is necessary for both targeting and transcriptional repression. In the fission yeast Schizosaccharomyces pombe, the correct localization of Swi6 (the HP1 equivalent) depends on Clr4, a homologue of the mammalian SUV39H1 histone methylase. Both Clr4 and SUV39H1 methylate specifically lysine 9 of histone H3 (ref. 6). Here we show that HP1 can bind with high affinity to histone H3 methylated at lysine 9 but not at lysine 4. The chromo domain of HP1 is identified as its methyl-lysine-binding domain. A point mutation in the chromo domain, which destroys the gene silencing activity of HP1 in Drosophila, abolishes methyl-lysine-binding activity. Genetic and biochemical analysis in S. pombe shows that the methylase activity of Clr4 is necessary for the correct localization of Swi6 at centromeric heterochromatin and for gene silencing. These results provide a stepwise model for the formation of a transcriptionally silent heterochromatin: SUV39H1 places a 'methyl marker' on histone H3, which is then recognized by HP1 through its chromo domain. This model may also explain the stable inheritance of the heterochromatic state.
2,811 citations