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The discovery of the genetic code and tRNAs as decoders of the code transformed life science. However, after establishing the role of tRNAs in protein synthesis, the field moved to other parts of the RNA world. Now, tRNA research is blooming again, with demonstration of the involvement of tRNAs in various other pathways beyond translation and in adapting translation to environmental cues. These roles are linked to the presence of tRNA sequence variants known as isoacceptors and isodecoders, various tRNA base modifications, the versatility of protein binding partners and tRNA fragmentation events, all of which collectively create an incalculable complexity. This complexity provides a vast repertoire of tRNA species that can serve various functions in cellular homeostasis and in adaptation of cellular functions to changing environments, and it likely arose from the fundamental role of RNAs in early evolution.

RNA silencing is a well-established antiviral immunity system in plants, in which small RNAs guide Argonaute proteins to targets in viral RNA or DNA, resulting in virus repression. Virus-encoded suppressors of silencing counteract this defence system. In this Review, we discuss recent findings about antiviral RNA silencing, including the movement of RNA through plasmodesmata and the differentiation between plant self and viral RNAs. We also discuss the emerging role of RNA silencing in plant immunity against non-viral pathogens. This immunity is mediated by transkingdom movement of RNA into and out of the infected plant cells in vesicles or as extracellular nucleoproteins and, like antiviral immunity, is influenced by the silencing suppressors encoded in the pathogens’ genomes. Another effect of RNA silencing on general immunity involves host-encoded small RNAs, including microRNAs, that regulate NOD-like receptors and defence signalling pathways in the innate immunity system of plants. These RNA silencing pathways form a network of processes with both positive and negative effects on the immune systems of plants.RNA silencing through small RNAs is a major antiviral immunity system in plants. Recent findings are uncovering the roles of RNA silencing in immunity against non-viral pathogens, which is mediated by trans-kingdom RNA movements in vesicles or as extracellular nucleoproteins. RNA silencing also enables the crosstalk between other plant immunity systems.

Discoveries over the past decade portend a paradigm shift in molecular biology; evidence suggests that RNA is not only functional as a messenger between DNA and protein but also involved in the regulation of genome organization and gene expression. This Timeline article surveys the emergence of the previously unsuspected world of regulatory RNA from a historical perspective. Discoveries over the past decade portend a paradigm shift in molecular biology. Evidence suggests that RNA is not only functional as a messenger between DNA and protein but also involved in the regulation of genome organization and gene expression, which is increasingly elaborate in complex organisms. Regulatory RNA seems to operate at many levels; in particular, it plays an important part in the epigenetic processes that control differentiation and development. These discoveries suggest a central role for RNA in human evolution and ontogeny. Here, we review the emergence of the previously unsuspected world of regulatory RNA from a historical perspective.

RNAi therapy has undergone two stages of development, direct injection of synthetic siRNAs and delivery with artificial vehicles or conjugated ligands; both have not solved the problem of efficient in vivo siRNA delivery. Here, we present a proof-of-principle strategy that reprogrammes host liver with genetic circuits to direct the synthesis and self-assembly of siRNAs into secretory exosomes and facilitate the in vivo delivery of siRNAs through circulating exosomes. By combination of different genetic circuit modules, in vivo assembled siRNAs are systematically distributed to multiple tissues or targeted to specific tissues (e.g., brain), inducing potent target gene silencing in these tissues. The therapeutic value of our strategy is demonstrated by programmed silencing of critical targets associated with various diseases, including EGFR/KRAS in lung cancer, EGFR/TNC in glioblastoma and PTP1B in obesity. Overall, our strategy represents a next generation RNAi therapeutics, which makes RNAi therapy feasible.

Key Points Functional diversification and expansion of silencing pathways in plants relies on duplication of DICER-LIKE proteins (DCLs) and ARGONAUTE proteins (AGOs). The main small-RNA classes in plants are microRNAs (miRNAs), 21–22-nucleotide secondary siRNAs and 24-nucleotide heterochromatic siRNAs (hetsiRNAs). All small RNAs in plants are modified at their 3′-end by 2′- O -methylation, including miRNAs, which lack this modification in animals. This modification confers stability and protection from degradation. Plant miRNAs are mainly involved in post-transcriptional gene silencing (PTGS) by transcript cleavage or translational repression, and also trigger secondary siRNA production from RNA polymerase II (Pol II) transcripts. Secondary small RNAs of 21 and 22 nucleotides in length are involved in cleavage or translational repression of target transcripts in cis and in trans . They are also able to initiate TGS by establishing DNA methylation at particular loci. The majority of siRNAs in plants are 24-nucleotide hetsiRNAs and are involved in silencing repeats and transposable elements by RNA-directed DNA methylation (RdDM). Small RNAs in plants are involved in reproductive transitions, including meiosis and gametogenesis, and regulate important epigenetic mechanisms such as genomic imprinting and paramutation. Plant genomes encode diverse small RNAs, such as microRNAs, secondary siRNAs, heterochromatic siRNAs and various RNA-dependent RNA polymerases, DICER proteins and ARGONAUTE proteins. Together, these constitute several genetic and epigenetic silencing pathways with diverse cellular and developmental functions, in processes including reproductive transitions, genomic imprinting and paramutation. Plant genomes encode various small RNAs that function in distinct, yet overlapping, genetic and epigenetic silencing pathways. However, the abundance and diversity of small-RNA classes varies among plant species, suggesting coevolution between environmental adaptations and gene-silencing mechanisms. Biogenesis of small RNAs in plants is well understood, but we are just beginning to uncover their intricate regulation and activity. Here, we discuss the biogenesis of plant small RNAs, such as microRNAs, secondary siRNAs and heterochromatic siRNAs, and their diverse cellular and developmental functions, including in reproductive transitions, genomic imprinting and paramutation. We also discuss the diversification of small-RNA-directed silencing pathways through the expansion of RNA-dependent RNA polymerases, DICER proteins and ARGONAUTE proteins.

Key Points Several Phase I trials evaluating the use of siRNA for the treatment of solid cancers have now been completed. All trials to date have used nanoparticle-based delivery systems to transport therapeutic siRNA to tumours following systemic administration. Despite concerns about potential overwhelming immunostimulation following systemic siRNA administration in humans, the data reported to date for clinically evaluated siRNA therapeutics (both naked and chemically modified siRNA) have shown these therapeutics to be well tolerated, with only modest and treatable immunostimulatory reactions. Successful delivery of functional siRNA to human tumours has been demonstrated in multiple trials, providing proof-of-principle for RNAi- based therapeutics in humans. Results from these trials show that safe target gene inhibition can be achieved in patients, when systemically treated twice per week with either lipid- or polymer-based nanoparticle formulations containing siRNAs at doses in the 0.5–1.0 mg siRNA per kg range. Several Phase I trials evaluating systemically administered siRNA-based therapeutics for cancer have recently been completed. Here, Zuckerman and Davis critically assess these studies and discuss key lessons learnt and implications for the future development of siRNA-based therapeutics and clinical trial design. Small interfering RNA (siRNA)-based therapies are emerging as a promising new anticancer approach, and a small number of Phase I clinical trials involving patients with solid tumours have now been completed. Encouraging results from these pioneering clinical studies show that these new therapeutics can successfully and safely inhibit targeted gene products in patients with cancer, and have taught us important lessons regarding appropriate dosages and schedules. In this Review, we critically assess these Phase I studies and discuss their implications for future clinical trial design. Key challenges and future directions in the development of siRNA-containing anticancer therapeutics are also considered.

Key Points RNAi has been used for genome-wide screening and other studies that aim to uncover the function of genes and gene networks. Sequence-specific RNAi off-target effects (OTEs) must be taken into consideration when interpreting RNAi data. New experimental and computational strategies such as the detection of microRNA-like seed sequence matches and the use of C911 RNAi controls help to address OTEs and improve data quality. Innovations in RNAi screening include new applications for high-content imaging, screens for synthetic interactions using sensitized cell backgrounds, screening in three-dimensional tissue cultures, parallel screening in different species or using different approaches followed by result integration, and new strategies for in vivo RNAi screening. RNAi and the genome-editing CRISPR (clustered regularly interspaced short palindromic repeats)–Cas9 system are complementary technologies, and using these two techniques together should result in improved assay development, screening and validation of screen results. With careful attention to reagent and assay design, data analysis and experimental follow-ups, improved genome-wide RNAi screens are uncovering gene function in all areas of biology. RNAi is used for genome-wide functional screens in cultured cells and animals. New experimental and bioinformatics approaches, including the combination of RNAi with genome-editing strategies, has improved the efficacy of RNAi screening and follow-up experiments, and enhanced our understanding of gene function and regulatory networks. Gene silencing through sequence-specific targeting of mRNAs by RNAi has enabled genome-wide functional screens in cultured cells and in vivo in model organisms. These screens have resulted in the identification of new cellular pathways and potential drug targets. Considerable progress has been made to improve the quality of RNAi screen data through the development of new experimental and bioinformatics approaches. The recent availability of genome-editing strategies, such as the CRISPR (clustered regularly interspaced short palindromic repeats)–Cas9 system, when combined with RNAi, could lead to further improvements in screen data quality and follow-up experiments, thus promoting our understanding of gene function and gene regulatory networks.

A single miRNA can target hundreds of distinct transcripts, but some miRNAs such as C. elegans lsy-6 have very low target proficiency. The reasons behind this are now identified as weak seed-pairing stability and high target-site abundance. These findings have implications for understanding off-target effects of siRNAs and improving miRNA target predictions. Most metazoan microRNAs (miRNAs) target many genes for repression, but the nematode lsy-6 miRNA is much less proficient. Here we show that the low proficiency of lsy-6 can be recapitulated in HeLa cells and that miR-23, a mammalian miRNA, also has low proficiency in these cells. Reporter results and array data indicate two properties of these miRNAs that impart low proficiency: their weak predicted seed-pairing stability (SPS) and their high target-site abundance (TA). These two properties also explain differential propensities of small interfering RNAs (siRNAs) to repress unintended targets. Using these insights, we expand the TargetScan tool for quantitatively predicting miRNA regulation (and siRNA off-targeting) to model differential miRNA (and siRNA) proficiencies, thereby improving prediction performance. We propose that siRNAs designed to have both weaker SPS and higher TA will have fewer off-targets without compromised on-target activity.

Dicer has a key role in small RNA biogenesis, processing double-stranded RNAs (dsRNAs) 1 , 2 . Human DICER (hDICER, also known as DICER1) is specialized for cleaving small hairpin structures such as precursor microRNAs (pre-miRNAs) and has limited activity towards long dsRNAs—unlike its homologues in lower eukaryotes and plants, which cleave long dsRNAs. Although the mechanism by which long dsRNAs are cleaved has been well documented, our understanding of pre-miRNA processing is incomplete because structures of hDICER in a catalytic state are lacking. Here we report the cryo-electron microscopy structure of hDICER bound to pre-miRNA in a dicing state and uncover the structural basis of pre-miRNA processing. hDICER undergoes large conformational changes to attain the active state. The helicase domain becomes flexible, which allows the binding of pre-miRNA to the catalytic valley. The double-stranded RNA-binding domain relocates and anchors pre-miRNA in a specific position through both sequence-independent and sequence-specific recognition of the newly identified ‘GYM motif’ 3 . The DICER-specific PAZ helix is also reoriented to accommodate the RNA. Furthermore, our structure identifies a configuration of the 5′ end of pre-miRNA inserted into a basic pocket. In this pocket, a group of arginine residues recognize the 5′ terminal base (disfavouring guanine) and terminal monophosphate; this explains the specificity of hDICER and how it determines the cleavage site. We identify cancer-associated mutations in the 5′ pocket residues that impair miRNA biogenesis. Our study reveals how hDICER recognizes pre-miRNAs with stringent specificity and enables a mechanistic understanding of hDICER-related diseases. The active-state structure of human DICER bound to pre-miRNA reveals the structural basis for the specificity of DICER in how it selects substrates in a sequence dependent manner, and sheds light on DICER-related diseases.

Small RNAs play important roles during plant development by regulating transcript levels of target mRNAs, maintaining genome integrity, and reinforcing DNA methylation. Dicer-like 5 ( Dcl5 ) is proposed to be responsible for precise slicing in many monocots to generate diverse 24-nt phased, secondary small interfering RNAs (phasiRNAs), which are exceptionally abundant in meiotic anthers of diverse flowering plants. The importance and functions of these phasiRNAs remain unclear. Here, we characterized several mutants of dcl5 , including alleles generated by the clustered regularly interspaced short palindromic repeats (CRISPR)– Cas9 system and a transposon-disrupted allele. We report that dcl5 mutants have few or no 24-nt phasiRNAs, develop short anthers with defective tapetal cells, and exhibit temperature-sensitive male fertility. We propose that DCL5 and 24-nt phasiRNAs are critical for fertility under growth regimes for optimal yield. Small RNAs act to regulate gene or transposon activity during plant development. Here, the authors show that maize Dicer-like 5 is required for 24-nt phased, secondary small interfering RNA production in anthers and that dicer-like 5 mutants show abnormal tapetal development and temperature-sensitive sterility.