Reiterated WG/GW motifs form functionally and evolutionarily conserved ARGONAUTE-binding platforms in RNAi-related components

An effector of RNA-directed DNA methylation in arabidopsis is an ARGONAUTE 4- and RNA-binding protein

DNA methylation is a conserved epigenetic mark in plants and mammals. In Arabidopsis, DNA methylation can be triggered by small interfering RNAs (siRNAs) through an RNA-directed DNA methylation (RdDM) pathway. Here, we report the identification of an RdDM effector, KTF1. Loss-of-function mutations in KTF1 reduce DNA methylation and release the silencing of RdDM target loci without abolishing the siRNA triggers. KTF1 has similarity to the transcription elongation factor SPT5 and contains a C-terminal extension rich in GW/WG repeats. KTF1 colocalizes with ARGONAUTE 4 (AGO4) in punctate nuclear foci and binds AGO4 and RNA transcripts. Our results suggest KTF1 as an adaptor protein that binds scaffold transcripts generated by Pol V and recruits AGO4 and AGO4-bound siRNAs to form an RdDM effector complex. The dual interaction of an effector protein with AGO and small RNA target transcripts may be a general feature of RNA-silencing effector complexes.

The GW182 protein family in animal cells: new insights into domains required for miRNA-mediated gene silencing

GW182 family proteins interact directly with Argonaute proteins and are required for miRNA-mediated gene silencing in animal cells. The domains of the GW182 proteins have recently been studied to determine their role in silencing. These studies revealed that the middle and C-terminal regions function as an autonomous domain with a repressive function that is independent of both the interaction with Argonaute proteins and of P-body localization. Such findings reinforce the idea that GW182 proteins are key components of miRNA repressor complexes in metazoa.

Two GW repeat proteins interact with Tetrahymena thermophila argonaute and promote genome rearrangement

In conjugating Tetrahymena thermophila, massive DNA elimination occurs upon the development of the new somatic genome from the germ line genome. Small, approximately 28-nucleotide scan RNAs (scnRNAs) and Twi1p, an Argonaute family member, mediate H3K27me3 and H3K9me3 histone H3 modifications, which lead to heterochromatin formation and the excision of the heterochromatinized germ line-limited sequences. In our search for new factors involved in developmental DNA rearrangement, we identified two Twi1p-interacting proteins, Wag1p and CnjBp. Both proteins contain GW (glycine and tryptophan) repeats, which are characteristic of several Argonaute-interacting proteins in other organisms. Wag1p and CnjBp colocalize with Twi1p in the parental macronucleus early in conjugation and in the new developing macronucleus during later developmental stages. Around the time DNA elimination occurs, Wag1p forms multiple nuclear bodies in the developing macronuclei that do not colocalize with heterochromatic DNA elimination structures. Analyses of DeltaWAG1, DeltaCnjB, and double DeltaWAG1 DeltaCnjB knockout strains revealed that WAG1 and CnjB genes need to be deleted together to inhibit the downregulation of specific scnRNAs, the formation of DNA elimination structures, and DNA excision. Thus, Wag1p and CnjBp are two novel players with overlapping functions in RNA interference-mediated genome rearrangement in Tetrahymena.

Mammalian GW182 contains multiple Argonaute-binding sites and functions in microRNA-mediated translational repression

In mammalian cells, microRNAs (miRNAs) are incorporated into miRNA-induced silencing complexes (miRISCs), which regulate protein expression post-transcriptionally through binding to 3'-untranslated regions of target mRNAs. Argonaute2 (Ago2), a key component of the miRISC, recruits GW182, a component of the processing body (GW/P-body), to the target mRNAs. To elucidate the function of GW182 in an miRNA-mediated translational repression, we analyzed Argonaute-binding sites in GW182. We found that human GW182 contains three binding sites for Ago2, within the amino-terminal glycine tryptophan (GW/WG)-repeated region that is characteristic of the GW182 family proteins. We also found that the first and second Ago2-binding site is conserved within the amino-terminal half of TNRC6B, which is a paralog of GW182. Each of the Ago-binding sites is alone sufficient to bind Ago2. Furthermore, we demonstrated that multiple Argonaute proteins were connected via the GW182 protein. A GW182 fragment containing the Ago2-binding region partially relieved let-7-mediated repression of protein synthesis in a mammalian cell-free system. Coincidentally, let-7-directed target mRNA deadenylation was delayed. Together, these results strongly suggested that the interactions of GW182 with Argonautes may induce the formation of large complexes containing miRNA target mRNAs, and may be critical for miRNA-mediated translational repression.

The C-terminal domains of human TNRC6A, TNRC6B, and TNRC6C silence bound transcripts independently of Argonaute proteins

Proteins of the GW182 family are essential components of the miRNA pathway in animal cells. Vertebrate genomes encode three GW182 paralogs (TNRC6A, TNRC6B, and TNRC6C), which may be functionally redundant. Here, we show that the N-terminal GW-repeat-containing regions of all three TNRC6s interact with the four human Argonaute proteins (AGO1-AGO4). We also show that TNRC6A, TNRC6B, and TNRC6C silence the expression of bound mRNAs. This activity is mediated by their C-terminal silencing domains, and thus, is independent of the interaction with AGO1-AGO4. Silencing by TNRC6A, TNRC6B, and TNRC6C is effected by changes in protein expression and mRNA stability that can, in part, be attributed to deadenylation. Our findings indicate that TNRC6A, TNRC6B, and TNRC6C are recruited to miRNA targets through an interaction between their N-terminal domain and an Argonaute protein; the TNRC6s then promote translational repression and/or degradation of miRNA targets through a C-terminal silencing domain.

A C-terminal silencing domain in GW182 is essential for miRNA function

Proteins of the GW182 family are essential for miRNA-mediated gene silencing in animal cells; they interact with Argonaute proteins (AGOs) and are required for both the translational repression and mRNA degradation mediated by miRNAs. To gain insight into the role of the GW182-AGO1 interaction in silencing, we generated protein mutants that do not interact and tested them in complementation assays. We show that silencing of miRNA targets requires the N-terminal domain of GW182, which interacts with AGO1 through multiple glycine-tryptophan (GW)-repeats. Indeed, a GW182 mutant that does not interact with AGO1 cannot rescue silencing in cells depleted of endogenous GW182. Conversely, silencing is impaired by mutations in AGO1 that strongly reduce the interaction with GW182 but not with miRNAs. We further show that a GW182 mutant that does not localize to P-bodies but interacts with AGO1 rescues silencing in GW182-depleted cells, even though in these cells, AGO1 also fails to localize to P-bodies. Finally, we show that in addition to the N-terminal AGO1-binding domain, the middle and C-terminal regions of GW182 (referred to as the bipartite silencing domain) are essential for silencing. Together our results indicate that miRNA silencing in animal cells is mediated by AGO1 in complex with GW182, and that P-body localization is not required for silencing.

Functional dissection of the human TNRC6 (GW182-related) family of proteins

Argonaute (Ago) proteins through their association with small RNAs perform a critical function in the effector step of RNA interference. The TNRC6 (trinucleotide repeat containing 6) family of proteins have been shown to stably associate with Agos in mammalian cells. Here, we describe the isolation and functional characterization of TNRC6B- and TNRC6C-containing complexes. We show that TNRC6B and TNRC6C proteins associate with all four human Agos which are already loaded with microRNAs. Detailed domain analysis of TNRC6B protein indicated that distinct domains of the protein are required for Ago binding and P-body localization. Functional analysis using reporter constructs responsive to TNRC6B tethered through an MS2-binding domain indicates that neither the Ago-binding nor the P-body localization domains are required for translational silencing. In contrast, the C-terminal domain containing the RNA recognition motif plays a critical role in the silencing mediated by the TNRC6B protein.

The C-terminal half of human Ago2 binds to multiple GW-rich regions of GW182 and requires GW182 to mediate silencing

MicroRNA (miRNA)-mediated silencing is a post-transcriptional mechanism that regulates translation of mRNAs primarily via their 3'-UTR. Ago2 binds miRNA directly and is the core component of miRNA-induced silencing complex. GW182 is another important factor in miRNA-mediated silencing, and its interaction with Ago2 is evolutionarily conserved. However, the GW182-Ago2 interaction in humans has not been characterized thoroughly, and the role of GW182 in the mammalian miRNA pathway remains unclear. In the current study, we generated a set of GST-, green fluorescence protein (GFP)-, or 3xFlag-tagged deletion constructs of GW182 and Ago2 to further analyze GW182-Ago2 interactions. The C-terminal half of Ago2 interacted with four nonoverlapping GW-rich regions of GW182, and this interaction recruited Ago2 to GWB. Furthermore, the interaction with GW182 was observed in all four human Ago proteins. Most interestingly, tethering the C-terminal half of Ago2 to the 3'-UTR of reporter mRNA recapitulated translational repression comparable to that of tethered Ago2, and this repression was greatly impaired upon GW182 knockdown. In comparison, the N-terminal half of Ago2 did not bind GW182 and did not retain the repression function of Ago2. Our data strongly support a model in which Ago2 recruits GW182 to the 3'-UTR of mRNA to mediate silencing, and suggest that GW182 may contribute to enhancement in translational repression by interacting with multiple Ago proteins from multiple miRNA target sites in the same or adjacent 3'UTR.

RNA polymerase V transcription guides ARGONAUTE4 to chromatin

Retrotransposons and repetitive DNA elements in eukaryotes are silenced by small RNA-directed heterochromatin formation. In Arabidopsis, this process involves 24 nt siRNAs that bind to ARGONAUTE4 (AGO4) and facilitate the targeting of complementary loci1,2 via unknown mechanisms. Nuclear RNA Polymerase V is an RNA silencing enzyme recently shown to generate noncoding transcripts at loci silenced by 24nt siRNAs3. We show that AGO4 physically interacts with these Pol V transcripts and is thereby recruited to the corresponding chromatin. We further show that DEFECTIVE IN MERISTEM SILENCING3 (DMS3), a Structural Maintenance of Chromosomes (SMC) hinge-domain protein4, functions in the assembly of Pol V transcription initiation or elongation complexes. Collectively, our data suggest that AGO4 is guided to target loci through base-pairing of associated siRNAs with nascent Pol V transcripts.

RNA-directed DNA methylation requires an AGO4-interacting member of the SPT5 elongation factor family

Recent studies have identified a conserved WG/GW-containing motif, known as the Argonaute (AGO) hook, which is involved in the recruitment of AGOs to distinct components of the eukaryotic RNA silencing pathways. By using this motif as a model to detect new components in plant RNA silencing pathways, we identified SPT5-like, a plant-specific AGO4-interacting member of the nuclear SPT5 (Suppressor of Ty insertion 5) RNA polymerase (RNAP) elongation factor family that is characterized by the presence of a carboxy-terminal extension with more than 40 WG/GW motifs. Knockout SPT5-like mutants show a decrease in the accumulation of several 24-nt RNAs and hypomethylation at different loci revealing an implication in RNA-directed DNA methylation (RdDM). Here, we propose that SPT5-like emerged in plants as a facultative RNAP elongation factor. Its plant-specific origin and role in RdDM might reflect functional interactions with plant-specific RNA Pols required for RdDM.

RNA-directed DNA methylation and demethylation in plants

RNA-directed DNA methylation (RdDM) is a nuclear process in which small interfering RNAs (siRNAs) direct the cytosine methylation of DNA sequences that are complementary to the siRNAs. In plants, double stranded-RNAs (dsRNAs) generated by RNA-dependent RNA polymerase 2 (RDR2) serve as precursors for Dicer-like 3 dependent biogenesis of 24-nt siRNAs. Plant specific RNA polymerase IV (Pol IV) is presumed to generate the initial RNA transcripts that are substrates for RDR2. siRNAs are loaded onto an argonaute4-containing RISC (RNA-induced silencing complex) that targets the de novo DNA methyltransferase DRM2 to RdDM target loci. Nascent RNA transcripts from the target loci are generated by another plant-specific RNA polymerase, Pol V, and these transcripts help recruit complementary siRNAs and the associated RdDM effector complex to the target loci in a transcription-coupled DNA methylation process. Small RNA binding proteins such as ROS3 may direct target-specific DNA demethylation by the ROS1 family of DNA demethylases. Chromatin remodeling enzymes and histone modifying enzymes also participate in DNA methylation and possibly demethylation. One of the well studied functions of RdDM is transposon silencing and genome stability. In addition, RdDM is important for paramutation, imprinting, gene regulation, and plant development. Locus-specific DNA methylation and demethylation, and transposon activation under abiotic stresses suggest that RdDM is also important in stress responses of plants. Further studies will help illuminate the functions of RdDM in the dynamic control of epigenomes during development and environmental stress responses.

Identification of GW182 and its novel isoform TNGW1 as translational repressors in Ago2-mediated silencing

RNA interference is triggered by small interfering RNA and microRNA, and is a potent mechanism in post-transcriptional regulation for gene expression. GW182 (also known as TNRC6A), an 182-kDa protein encoded by TNRC6A, is important for this process, although details of its function remain unclear. Here, we report a novel 210-kDa isoform of human GW182, provisionally named trinucleotide GW1 (TNGW1) because it contains trinucleotide repeats in its mRNA sequence. TNGW1 was expressed independently of GW182 and was present in human testis and various human cancer cells. Using polyclonal and monoclonal antibodies, we detected TNGW1 in only approximately 30% of GW bodies. Expression of EGFP-tagged TNGW1 in HeLa cells was colocalized to cytoplasmic foci enriched in Ago2 (also known as EIF2C2) and RNA decay factors. Tethering TNGW1 or GW182 to the 3'-UTR of a luciferase-reporter mRNA led to strong repression activity independent of Ago2, whereas the tethered Ago2-mediated suppression was completely dependent on TNGW1 and/or GW182. Our data demonstrated that GW182 and, probably, TNGW1 acted as a repressor in Ago2-mediated translational silencing. Furthermore, TNGW1 might contribute to diversity in the formation and function of GW and/or P bodies.

Small silencing RNAs: an expanding universe

Since the discovery in 1993 of the first small silencing RNA, a dizzying number of small RNA classes have been identified, including microRNAs (miRNAs), small interfering RNAs (siRNAs) and Piwi-interacting RNAs (piRNAs). These classes differ in their biogenesis, their modes of target regulation and in the biological pathways they regulate. There is a growing realization that, despite their differences, these distinct small RNA pathways are interconnected, and that small RNA pathways compete and collaborate as they regulate genes and protect the genome from external and internal threats.

Small RNAs in transcriptional gene silencing and genome defence

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. 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.

RNA-mediated chromatin-based silencing in plants

Plants have evolved an elaborate transcriptional machinery dedicated to eliciting sequence-specific, chromatin-based gene silencing. Two Pol II-related, plant-specific RNA polymerases, named Pol IV and Pol V, collaborate with proteins of the RNA interference machinery to generate long and short noncoding RNAs involved in epigenetic regulation. As revealed by a variety of genetic, molecular, and genomic technologies, these RNAs are used extensively in plants to direct the establishment, spread, and removal of DNA cytosine methylation throughout their genomes. RNA-mediated chromatin-level silencing is increasingly implicated in development, stress responses, and natural epigenetic variation that may promote phenotypic diversity, physiological plasticity, and evolutionary change.

PolV(PolIVb) function in RNA-directed DNA methylation requires the conserved active site and an additional plant-specific subunit

Two forms of a plant-specific RNA polymerase (Pol), PolIV(PolIVa) and PolV(PolIVb), currently defined by their respective largest subunits [NRPD1(NRPD1a) and NRPE1(NRPD1b)], have been implicated in the production and activity of 24-nt small RNAs (sRNAs) in RNA-directed DNA methylation (RdDM). Prevailing models support the view that PolIV(PolIVa) plays an upstream role in RdDM by producing the 24-nt sRNAs, whereas PolV(PolIVb) would act downstream at a structural rather than an enzymatic level to reinforce sRNA production by PolIV(PolIVa) and mediate DNA methylation. However, the composition and mechanism of action of PolIV(PolIVa)/PolV(PolIVb) remain unclear. In this work, we have identified a plant-specific PolV(PolIVb) subunit, NRPE5a, homologous to NRPB5a, a common subunit shared by PolI-III and shown here to be present in PolIV(PolIVa). Our results confirm the combinatorial diversity of PolIV(PolIVa)/PolV(PolIVb) subunit composition and indicate that these plant-specific Pols are eukaryotic-type polymerases. Moreover, we show that nrpe5a-1 mutation differentially impacts sRNAs accumulation at various PolIV(PolIVa)/PolV(PolIVb)-dependent loci, indicating a target-specific requirement for NRPE5a in the process of PolV(PolIVb)-dependent gene silencing. We then describe that the triad aspartate motif present in the catalytic center of PolV(PolIVb) is required for recapitulation of all activities associated with this Pol complex in RdDM, suggesting that RNA polymerization is important for PolV(PolIVb) to perform its regulatory functions.

Deadenylation is a widespread effect of miRNA regulation

miRNAs silence gene expression by repressing translation and/or by promoting mRNA decay. In animal cells, degradation of partially complementary miRNA targets occurs via deadenylation by the CAF1-CCR4-NOT1 deadenylase complex, followed by decapping and subsequent exonucleolytic digestion. To determine how generally miRNAs trigger deadenylation, we compared mRNA expression profiles in D. melanogaster cells depleted of AGO1, CAF1, or NOT1. We show that approximately 60% of AGO1 targets are regulated by CAF1 and/or NOT1, indicating that deadenylation is a widespread effect of miRNA regulation. However, neither a poly(A) tail nor mRNA circularization are required for silencing, because mRNAs whose 3' ends are generated by a self-cleaving ribozyme are also silenced in vivo. We show further that miRNAs trigger mRNA degradation, even when binding by 40S ribosomal subunits is inhibited in cis. These results indicate that miRNAs promote mRNA decay by altering mRNP composition and/or conformation, rather than by directly interfering with the binding and function of ribosomal subunits.

Pol V transcribes to silence

In most cases, the functions of long noncoding RNAs remain uncertain. Working in the model plant Arabidopsis, Wierzbicki et al. (2008) provide evidence that transcription of intergenic noncoding regions by RNA polymerase V promotes heterochromatin formation and silencing of nearby genes.

Noncoding transcription by RNA polymerase Pol IVb/Pol V mediates transcriptional silencing of overlapping and adjacent genes

Nuclear transcription is not restricted to genes but occurs throughout the intergenic and noncoding space of eukaryotic genomes. The functional significance of this widespread noncoding transcription is mostly unknown. We show that Arabidopsis RNA polymerase IVb/Pol V, a multisubunit nuclear enzyme required for siRNA-mediated gene silencing of transposons and other repeats, transcribes intergenic and noncoding sequences, thereby facilitating heterochromatin formation and silencing of overlapping and adjacent genes. Pol IVb/Pol V transcription requires the chromatin-remodeling protein DRD1 but is independent of siRNA biogenesis. However, Pol IVb/Pol V transcription and siRNA production are both required to silence transposons, suggesting that Pol IVb/Pol V generates RNAs or chromatin structures that serve as scaffolds for siRNA-mediated heterochromatin-forming complexes. Pol IVb/Pol V function provides a solution to a paradox of epigenetic control: the need for transcription in order to transcriptionally silence the same region.

An atypical RNA polymerase involved in RNA silencing shares small subunits with RNA polymerase II

Genetic evidence indicates that plant-specific homologs of DNA-dependent RNA polymerase (Pol) II large subunits form Pol IV and Pol V complexes involved in small interfering RNA production and RNA-directed DNA methylation. Here we describe evidence that Pol V contains subunits shared with Pol II, but that RNA polymerase II subunit (RPB)-4 is missing from Pol V and that RPB5 is present as a Pol V-specific isomer, RPB5b. Pol V also has other proteins that are not present in Pol II, consistent with a role of this complex as an effector of silencing.

A stepwise pathway for biogenesis of 24-nt secondary siRNAs and spreading of DNA methylation

We used a transgene system to study spreading of RNA-directed DNA methylation (RdDM) during transcriptional gene silencing in Arabidopsis thaliana. Forward and reverse genetics approaches using this system delineated a stepwise pathway for the biogenesis of secondary siRNAs and unidirectional spreading of methylation from an upstream enhancer element into downstream sequences. Trans-acting, hairpin-derived primary siRNAs induce primary RdDM, independently of an enhancer-associated 'nascent' RNA, at the target enhancer region. Primary RdDM is a key step in the pathway because it attracts the secondary siRNA-generating machinery, including RNA polymerase IV, RNA-dependent RNA polymerase2 and Dicer-like3 (DCL3). These factors act in a turnover pathway involving a nascent RNA, which normally accumulates stably in non-silenced plants, to produce cis-acting secondary siRNAs that induce methylation in the downstream region. The identification of DCL3 in a forward genetic screen for silencing-defective mutants demonstrated a strict requirement for 24-nt siRNAs to direct methylation. A similar stepwise process for spreading of DNA methylation may occur in mammalian genomes, which are extensively transcribed in upstream regulatory regions.

Localization of double-stranded small interfering RNA to cytoplasmic processing bodies is Ago2 dependent and results in up-regulation of GW182 and Argonaute-2

Processing bodies (P-bodies) are cytoplasmic foci implicated in the regulation of mRNA translation, storage, and degradation. Key effectors of microRNA (miRNA)-mediated RNA interference (RNAi), such as Argonaute-2 (Ago2), miRNAs, and their cognate mRNAs, are localized to these structures; however, the precise role that P-bodies and their component proteins play in small interfering RNA (siRNA)-mediated RNAi remains unclear. Here, we investigate the relationship between siRNA-mediated RNAi, RNAi machinery proteins, and P-bodies. We show that upon transfection into cells, siRNAs rapidly localize to P-bodies in their native double-stranded conformation, as indicated by fluorescence resonance energy transfer imaging and that Ago2 is at least in part responsible for this siRNA localization pattern, indicating RISC involvement. Furthermore, siRNA transfection induces up-regulated expression of both GW182, a key P-body component, and Ago2, indicating that P-body localization and interaction with GW182 and Ago2 are important in siRNA-mediated RNAi. By virtue of being centers where these proteins and siRNAs aggregate, we propose that the P-body microenvironment, whether as microscopically visible foci or submicroscopic protein complexes, facilitates siRNA processing and siRNA-mediated silencing through the action of its component proteins.

DNA methylomes, histone codes and miRNAs: tying it all together

Our current knowledge of the deregulation that occurs during the onset and progression of cancer and other diseases leads us to recognize both genetic and epigenetic alterations as being at the core of the pathological state. The epigenetic landscape includes a variety of covalent modifications that affect the methylation status of DNA but also the post-translational modifications of histones, and determines the structural features of chromatin that ultimately control the transcriptional outcome of the cell to accommodate developmental, proliferative or environmental requirements. MicroRNAs are small non-coding RNAs that regulate the expression of complementary messenger RNAs and function as key controllers in a myriad of cellular processes, including proliferation, differentiation and apoptosis. In the last few years, increasing evidence has indicated that a substantial number of microRNA genes are subjected to epigenetic alterations, resulting in aberrant patterns of expression upon the occurrence of cancer. In this review we discuss microRNA genes that are epigenetically modified in cancer cells, and the role that microRNAs themselves can have as chromatin modifiers.

Small RNA-directed heterochromatin formation in the context of development: what flies might learn from fission yeast

A link between the RNAi system and heterochromatin formation has been established in several model organisms including Schizosaccharomyces pombe and Arabidopsis thaliana. However, the data to support a role for small RNAs and the associated machinery in transcriptional gene silencing in animal systems is more tenuous. Using the S. pombe system as a model, we analyze the role of small RNA pathway components and associated small RNAs in regulating transposable elements and potentially directing heterochromatin formation at these elements in Drosophila melanogaster.

Small RNA guides for de novo DNA methylation in mammalian germ cells

Germline genomic methylation is essential for gamete identity and integrity in mammals. The study by Kuramochi-Miyagawa and colleagues (908-917) in the previous issue of Genes & Development links the process of DNA methylation-dependent repression of retrotranspons with the presence of piwi-interacting RNAs (piRNAs) in fetal male germ cells undergoing de novo methylation.

Use, tolerance and avoidance of amplified RNA silencing by plants

In plants and several other organisms, the effects of RNA silencing can be amplified by the action of cellular RNA-DEPENDENT RNA POLYMERASES (RDRs). These enzymes were primarily studied for their role in antiviral defense in plants, but it is becoming increasingly apparent that they also have important endogenous functions, including the control of chromatin structure and the regulation of cellular gene expression. Recent evidence suggests that endogenous RDR activities intercept several RNA quality control pathways that normally prevent or restrain widespread amplification of silencing, which is likely to be detrimental. Plants appear, however, to have evolved sophisticated measures to tolerate or exploit amplified silencing under specific biological circumstances.

Small RNA metabolism in Arabidopsis

The Arabidopsis genome encodes two major classes of 20-24-nucleotide riboregulators: microRNAs and small interfering RNAs. These small RNAs act as sequence-specific repressors of target gene expression, either at the transcriptional level through DNA and/or histone methylation or at the post-transcriptional level through transcript cleavage or translational inhibition. Small RNAs are processed from precursor RNAs by one or more of the four DICER-LIKE RNase III enzymes, modified by HUA ENHANCER 1, a small RNA methyltransferase, and loaded onto an argonaute protein-containing RNA-induced silencing complex. Here, we review the biogenesis of small RNAs, and we discuss the major outstanding questions in small RNA metabolism and function.

Plant ARGONAUTES

ARGONAUTE (AGO) proteins are integral players in all known small RNA-directed regulatory pathways. Eukaryotes produce numerous types of small RNAs, such as microRNAs (miRNA), small interfering RNAs (siRNA), PIWI-interacting RNAs (piRNAs), scanRNAs and 21U-RNAs, and these RNA species associate with different types of AGO family members, such as AGO, PIWI and group 3 proteins. Small RNA-guided AGO proteins regulate gene expression at various levels, including internal genomic DNA sequence elimination (in ciliates), translational repression (animals), and RNA cleavage (all eukaryotes), which in some cases is followed by DNA methylation and chromatin remodeling. The plant model species Arabidopsis contains ten AGO proteins belonging to three phylogenetic clades. This review covers our current knowledge of plant AGO functions during miRNA- and siRNA-mediated regulation of development and stress responses, siRNA-mediated antiviral immune response, and siRNA-mediated regulation of chromatin structure and transposons.

Roles of RNA polymerase IV in gene silencing

Eukaryotes typically have three multi-subunit enzymes that decode the nuclear genome into RNA: DNA-dependent RNA polymerases I, II and III (Pol I, II and III). Remarkably, higher plants have five multi-subunit nuclear RNA polymerases: the ubiquitous Pol I, II and III, which are essential for viability; plus two non-essential polymerases, Pol IVa and Pol IVb, which specialize in small RNA-mediated gene silencing pathways. There are numerous examples of phenomena that require Pol IVa and/or Pol IVb, including RNA-directed DNA methylation of endogenous repetitive elements, silencing of transgenes, regulation of flowering-time genes, inducible regulation of adjacent gene pairs, and spreading of mobile silencing signals. Although biochemical details concerning Pol IV enzymatic activities are lacking, genetic evidence suggests several alternative models for how Pol IV might function.

TbRGG2, an essential RNA editing accessory factor in two Trypanosoma brucei life cycle stages

In the mitochondria of kinetoplastid protozoa, including Trypanosoma brucei, RNA editing inserts and/or deletes uridines from pre-mRNAs to produce mature, translatable mRNAs. RNA editing is carried out by several related multiprotein complexes known as editosomes, which contain all of the enzymatic components required for catalysis of editing. In addition, noneditosome accessory factors necessary for editing of specific RNAs have also been described. Here, we report the in vitro and in vivo characterization of the mitochondrial TbRGG2 protein (originally termed TbRGGm) and demonstrate that it acts as an editing accessory factor. TbRGG2 is an RNA-binding protein with a preference for poly(U). TbRGG2 protein levels are up-regulated 10-fold in procyclic form T. brucei compared with bloodstream forms. Nevertheless, the protein is essential for growth in both life cycle stages. TbRGG2 associates with RNase-sensitive and RNase-insensitive mitochondrial complexes, and a small fraction of the protein co-immunoprecipitates with editosomes. RNA interference-mediated depletion of TbRGG2 in both procyclic and bloodstream form T. brucei leads to a dramatic decrease in pan-edited RNAs and in some cases a corresponding increase in the pre-edited RNA. TbRGG2 down-regulation also results in moderate stabilization of never-edited and minimally edited RNAs. Thus, our data are consistent with a model in which TbRGG2 is multifunctional, strongly facilitating the editing of pan-edited RNAs and modestly destabilizing minimally edited and never-edited RNAs. This is the first example of an RNA editing accessory factor that functions in the mammalian infective T. brucei life cycle stage.

The epigenetic landscape of plants

In plants, DNA methylation, histone modifications, and RNA interference play critically important roles in regulating chromatin structure, thereby profoundly affecting transcription and other molecular events. Recent advances in microarray and high-throughput sequencing technologies have enabled genome-wide studies of these pathways in great detail. The vast amounts of "epigenomic" data generated so far have provided new insights into the mechanisms and functions of these pathways and have broadened our understanding of the structure and organization of plant chromatin as a whole.

GW182 interaction with Argonaute is essential for miRNA-mediated translational repression and mRNA decay

MicroRNAs (miRNAs) silence gene expression by binding 3' untranslated regions of target mRNAs. Recent studies suggested silencing is achieved through either recruitment of eIF6, which prevents ribosome assembly, or displacement of eIF4E from the mRNA 5' cap structure. Using Drosophila melanogaster cells, we show that eIF6 is not required for silencing. In contrast, silencing is abolished by mutating Argonaute 1 (AGO1) at two conserved phenylalanine residues predicted to mediate binding to the cap structure. Notably, we found these mutations also prevented AGO1 from interacting with GW182 and miRNAs, indicating that the essential role of these residues is unrelated to cap binding. Consistently, depleting GW182 or overexpressing its AGO1 binding domain relieved silencing of all reporters tested, including those lacking a poly(A) tail. Together, our findings show that miRNA function is effected by AGO1-GW182 complexes and the role of GW182 in silencing goes beyond promoting deadenylation.

Specificity of ARGONAUTE7-miR390 interaction and dual functionality in TAS3 trans-acting siRNA formation

Trans-acting siRNA form through a refined RNAi mechanism in plants. miRNA-guided cleavage triggers entry of precursor transcripts into an RNA-DEPENDENT RNA POLYMERASE6 pathway, and sets the register for phased tasiRNA formation by DICER-LIKE4. Here, we show that miR390-ARGONAUTE7 complexes function in distinct cleavage or noncleavage modes at two target sites in TAS3a transcripts. The AGO7 cleavage, but not the noncleavage, function could be provided by AGO1, the dominant miRNA-associated AGO, but only when AGO1 was guided to a modified target site through an alternate miRNA. AGO7 was highly selective for interaction with miR390, and miR390 in turn was excluded from association with AGO1 due entirely to an incompatible 5' adenosine. Analysis of AGO1, AGO2, and AGO7 revealed a potent 5' nucleotide discrimination function for some, although not all, ARGONAUTEs. miR390 and AGO7, therefore, evolved as a highly specific miRNA guide/effector protein pair to function at two distinct tasiRNA biogenesis steps.

Argonaute proteins: key players in RNA silencing

During the past decade, small non-coding RNAs have rapidly emerged as important contributors to gene regulation. To carry out their biological functions, these small RNAs require a unique class of proteins called Argonautes. The discovery and our comprehension of this highly conserved protein family is closely linked to the study of RNA-based gene silencing mechanisms. With their functional domains, Argonaute proteins can bind small non-coding RNAs and control protein synthesis, affect messenger RNA stability and even participate in the production of a new class of small RNAs, Piwi-interacting RNAs.