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Review
. 2009 Jan 22;457(7228):413-20.
doi: 10.1038/nature07756.

Small RNAs in transcriptional gene silencing and genome defence

Danesh Moazed  1
Affiliations
Review

Small RNAs in transcriptional gene silencing and genome defence

Danesh Moazed. Nature. .

Abstract (VSports最新版本)

Small RNA molecules of about 20-30 nucleotides have emerged as powerful regulators of gene expression and genome stability. Studies in fission yeast and multicellular organisms suggest that effector complexes, directed by small RNAs, target nascent chromatin-bound non-coding RNAs and recruit chromatin-modifying complexes VSports手机版. Interactions between small RNAs and nascent non-coding transcripts thus reveal a new mechanism for targeting chromatin-modifying complexes to specific chromosome regions and suggest possibilities for how the resultant chromatin states may be inherited during the process of chromosome duplication. .

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Figures

Figure 1
Figure 1. Pathways of RNA processing and biogenesis of small RNAs
a, Generation of endogenous siRNAs from dsRNA resulting from convergent transcription (sense–antisense RNA base-pairing; top) or transcription through inverted repeat sequences (hairpin RNA formation; bottom). TER, transcription termination signal. b, Processing of non-coding and aberrant RNAs by the RDRC and TRAMP complexes, containing the Cid12 and Cid14 non-canonical polyadenylation polymerases, respectively; the RDRC/Dicer pathway produces duplex siRNAs, whereas the TRAMP/exosome pathway produces single-stranded degradation products. c, Generation of a free 3′ end by the slicer activity of an Argonaute protein, which can be processed into dsRNA by RdRP or targeted for degradation by the exosome (not shown). d, Pathway for the generation of piRNAs by the PIWI clade of Argonaute proteins: PIWI, AUB and AGO3.
Figure 2
Figure 2. Chromosome organization and the nascent transcript model for heterochromatic gene-silencing assembly in Schizosaccharomyces pombe
a, The structure of S. pombe centromeric repeat regions, highlighting the presence of non-coding centromeric transcripts (cenRNA) and association with histone H3 that is dimethylated and trimethylated on lysine 9 (red lollipops) as opposed to histone H3 that is methylated on lysine 4 (green lollipops) in euchromatic regions. b, The nascent transcript model for heterochromatin assembly. The RITS is tethered to chromatin through base-paring interactions between siRNAs and nascent non-coding transcripts and interactions with H3K9-methylated nucleosomes, resulting in the recruitment of RDRC–Dicer, dsRNA synthesis and siRNA amplification. This RNAi positive-feedback loop then recruits the CLRC H3K9 methyltransferase. Efficient silencing also requires two HP1 proteins (Swi6 and Chp2), which promote the association of RITS with the non-coding RNA or mediate TGS through recruitment of the SHREC2 deacetylase complex, respectively. Another tier of regulation, involving the degradation of heterochromatic transcripts by the TRAMP/exosome pathway, further ensures full gene silencing. Blue arrows (bottom) highlight convergent transcription resulting in synthesis of sense and antisense RNAs, which may contribute to the production of trigger siRNAs.
Figure 3
Figure 3. Argonaute complexes that link RNA silencing to chromatin modifications
Argonaute proteins in different silencing pathways, including miRNA- and siRNA-mediated PTGS, are associated with conserved GW-motif-containing adaptor proteins, which help direct them to different targets. a, In many organisms, GW182 (a GW-motif-containing protein) or one of its homologues associates with the AGO1 and AGO2 proteins and directs them to P bodies. b, In S. pombe, Ago1 in the RITS is linked to heterochromatin through its association with the GW protein Tas3, which also binds to Chp1. Chp1 in turn associates with H3K9 methylated nucleosomes (H3K9me) through its chromodomain (CD). Swi6 (a homologue of HP1) acts as an accessory factor that helps tether the non-coding RNA to heterochromatin. The chromoshadow domain (CSD) is involved in protein–protein interactions. cenDNA, centromeric repeat DNA. c, In D. melanogaster, PIWI is targeted to heterochromatin through direct interactions with HP1; the association of PIWI with HP1 is mediated through the PXVXL motif, present in many HP1-binding proteins, rather than through a GW motif. d, In D. melanogaster and possibly other organisms, AGO1 and AGO2 have been implicated in mediating chromatin modifications, but the putative chromatin adaptor (CAD) protein has not been identified. e, In A. thaliana, AGO4 is linked to Pol IVB, which contains a GW motif at its carboxyl terminus and is specifically required for DNA methylation and silencing of heterochromatic repeats.
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