Functional analysis of three Arabidopsis ARGONAUTES using slicer-defective mutants

[EN] In RNA-directed silencing pathways, ternary complexes result from small RNA-guided ARGONAUTE (AGO) associating with target transcripts. Target transcripts are often silenced through direct cleavage (slicing), destabilization through slicer-independent turnover mechanisms, and translational repression. Here, wild-type and active-site defective forms of several Arabidopsis thaliana AGO proteins involved in posttranscriptional silencing were used to examine several AGO functions, including small RNA binding, interaction with target RNA, slicing or destabilization of target RNA, secondary small interfering RNA formation, and antiviral activity. Complementation analyses in ago mutant plants revealed that the catalytic residues of AGO1, AGO2, and AGO7 are required to restore the defects of Arabidopsis ago1-25, ago2-1, and zip-1 (AGO7-defective) mutants, respectively. AGO2 had slicer activity in transient assays but could not trigger secondary small interfering RNA biogenesis, and catalytically active AGO2 was necessary for local and systemic antiviral activity against Turnip mosaic virus. Slicer-defective AGOs associated with miRNAs and stabilized AGO-miRNA-target RNA ternary complexes in individual target coimmunoprecipitation assays. In genome-wide AGO-miRNA-target RNA coimmunoprecipitation experiments, slicer-defective AGO1-miRNA associated with target RNA more effectively than did wild-type AGO1-miRNA. These data not only reveal functional roles for AGO1, AGO2, and AGO7 slicer activity, but also indicate an approach to capture ternary complexes more efficiently for genome-wide analyses.We thank Goretti Nguyen for excellent technical assistance. A. C. was supported by a postdoctoral fellowship from the Ministerio de Ciencia e Innovacion (BMC-2008-0188). H.G.-R. was the recipient of a Helen Hay Whitney Postdoctoral fellowship (F-972). This work was supported by grants from the National Science Foundation (MCB-1231726), the National Institutes of Health (AI043288), and Monsanto Corporation.Carbonell, A.; Fahlgren, N.; García-Ruíz, H.; Gilbert, KB.; Montgomery, TA.; Nguyen, T.; Cuperus, JT.... (2012). Functional analysis of three Arabidopsis ARGONAUTES using slicer-defective mutants. The Plant Cell. 24(9):3613-3629. https://doi.org/10.1105/tpc.112.099945S36133629249Allen, E., Xie, Z., Gustafson, A. M., & Carrington, J. C. (2005). microRNA-Directed Phasing during Trans-Acting siRNA Biogenesis in Plants. Cell, 121(2), 207-221. doi:10.1016/j.cell.2005.04.004Aukerman, M. J., & Sakai, H. (2003). Regulation of Flowering Time and Floral Organ Identity by a MicroRNA and Its APETALA2-Like Target Genes. The Plant Cell, 15(11), 2730-2741. doi:10.1105/tpc.016238Axtell, M. J., Jan, C., Rajagopalan, R., & Bartel, D. P. (2006). A Two-Hit Trigger for siRNA Biogenesis in Plants. Cell, 127(3), 565-577. doi:10.1016/j.cell.2006.09.032Baek, D., Villén, J., Shin, C., Camargo, F. D., Gygi, S. P., & Bartel, D. P. (2008). The impact of microRNAs on protein output. Nature, 455(7209), 64-71. doi:10.1038/nature07242Baumberger, N., & Baulcombe, D. C. (2005). Arabidopsis ARGONAUTE1 is an RNA Slicer that selectively recruits microRNAs and short interfering RNAs. Proceedings of the National Academy of Sciences, 102(33), 11928-11933. doi:10.1073/pnas.0505461102Brodersen, P., Sakvarelidze-Achard, L., Bruun-Rasmussen, M., Dunoyer, P., Yamamoto, Y. Y., Sieburth, L., & Voinnet, O. (2008). Widespread Translational Inhibition by Plant miRNAs and siRNAs. Science, 320(5880), 1185-1190. doi:10.1126/science.1159151Chekanova, J. A., Gregory, B. D., Reverdatto, S. V., Chen, H., Kumar, R., Hooker, T., … Belostotsky, D. A. (2007). Genome-Wide High-Resolution Mapping of Exosome Substrates Reveals Hidden Features in the Arabidopsis Transcriptome. Cell, 131(7), 1340-1353. doi:10.1016/j.cell.2007.10.056Chen, H.-M., Chen, L.-T., Patel, K., Li, Y.-H., Baulcombe, D. C., & Wu, S.-H. (2010). 22-nucleotide RNAs trigger secondary siRNA biogenesis in plants. Proceedings of the National Academy of Sciences, 107(34), 15269-15274. doi:10.1073/pnas.1001738107Chen, X. (2004). A MicroRNA as a Translational Repressor of APETALA2 in Arabidopsis Flower Development. Science, 303(5666), 2022-2025. doi:10.1126/science.1088060Chi, S. W., Zang, J. B., Mele, A., & Darnell, R. B. (2009). Argonaute HITS-CLIP decodes microRNA–mRNA interaction maps. Nature, 460(7254), 479-486. doi:10.1038/nature08170Clough, S. J., & Bent, A. F. (1998). Floral dip: a simplified method forAgrobacterium-mediated transformation ofArabidopsis thaliana. The Plant Journal, 16(6), 735-743. doi:10.1046/j.1365-313x.1998.00343.xCuperus, J. T., Carbonell, A., Fahlgren, N., Garcia-Ruiz, H., Burke, R. T., Takeda, A., … Carrington, J. C. (2010). Unique functionality of 22-nt miRNAs in triggering RDR6-dependent siRNA biogenesis from target transcripts in Arabidopsis. Nature Structural & Molecular Biology, 17(8), 997-1003. doi:10.1038/nsmb.1866Curtis, M. D., & Grossniklaus, U. (2003). A Gateway Cloning Vector Set for High-Throughput Functional Analysis of Genes in Planta. Plant Physiology, 133(2), 462-469. doi:10.1104/pp.103.027979Dunoyer, P., Himber, C., & Voinnet, O. (2005). DICER-LIKE 4 is required for RNA interference and produces the 21-nucleotide small interfering RNA component of the plant cell-to-cell silencing signal. Nature Genetics, 37(12), 1356-1360. doi:10.1038/ng1675Eulalio, A., Huntzinger, E., & Izaurralde, E. (2008). Getting to the Root of miRNA-Mediated Gene Silencing. Cell, 132(1), 9-14. doi:10.1016/j.cell.2007.12.024Fahlgren, N., Sullivan, C. M., Kasschau, K. D., Chapman, E. J., Cumbie, J. S., Montgomery, T. A., … Carrington, J. C. (2009). Computational and analytical framework for small RNA profiling by high-throughput sequencing. RNA, 15(5), 992-1002. doi:10.1261/rna.1473809Filipowicz, W., Bhattacharyya, S. N., & Sonenberg, N. (2008). Mechanisms of post-transcriptional regulation by microRNAs: are the answers in sight? Nature Reviews Genetics, 9(2), 102-114. doi:10.1038/nrg2290Gandikota, M., Birkenbihl, R. P., Höhmann, S., Cardon, G. H., Saedler, H., & Huijser, P. (2007). The miRNA156/157 recognition element in the 3′ UTR of the Arabidopsis SBP box gene SPL3 prevents early flowering by translational inhibition in seedlings. The Plant Journal, 49(4), 683-693. doi:10.1111/j.1365-313x.2006.02983.xGarcia-Ruiz, H., Takeda, A., Chapman, E. J., Sullivan, C. M., Fahlgren, N., Brempelis, K. J., & Carrington, J. C. (2010). Arabidopsis RNA-Dependent RNA Polymerases and Dicer-Like Proteins in Antiviral Defense and Small Interfering RNA Biogenesis during Turnip Mosaic Virus Infection  . The Plant Cell, 22(2), 481-496. doi:10.1105/tpc.109.073056Gasciolli, V., Mallory, A. C., Bartel, D. P., & Vaucheret, H. (2005). Partially Redundant Functions of Arabidopsis DICER-like Enzymes and a Role for DCL4 in Producing trans-Acting siRNAs. Current Biology, 15(16), 1494-1500. doi:10.1016/j.cub.2005.07.024Guo, H., Ingolia, N. T., Weissman, J. S., & Bartel, D. P. (2010). Mammalian microRNAs predominantly act to decrease target mRNA levels. Nature, 466(7308), 835-840. doi:10.1038/nature09267Harvey, J. J. W., Lewsey, M. G., Patel, K., Westwood, J., Heimstädt, S., Carr, J. P., & Baulcombe, D. C. (2011). An Antiviral Defense Role of AGO2 in Plants. PLoS ONE, 6(1), e14639. doi:10.1371/journal.pone.0014639Havecker, E. R., Wallbridge, L. M., Hardcastle, T. J., Bush, M. S., Kelly, K. A., Dunn, R. M., … Baulcombe, D. C. (2010). TheArabidopsisRNA-Directed DNA Methylation Argonautes Functionally Diverge Based on Their Expression and Interaction with Target Loci  . The Plant Cell, 22(2), 321-334. doi:10.1105/tpc.109.072199Hendrickson, D. G., Hogan, D. J., McCullough, H. L., Myers, J. W., Herschlag, D., Ferrell, J. E., & Brown, P. O. (2009). Concordant Regulation of Translation and mRNA Abundance for Hundreds of Targets of a Human microRNA. PLoS Biology, 7(11), e1000238. doi:10.1371/journal.pbio.1000238Hunter, C., Sun, H., & Poethig, R. S. (2003). The Arabidopsis Heterochronic Gene ZIPPY Is an ARGONAUTE Family Member. Current Biology, 13(19), 1734-1739. doi:10.1016/j.cub.2003.09.004Huntzinger, E., & Izaurralde, E. (2011). Gene silencing by microRNAs: contributions of translational repression and mRNA decay. Nature Reviews Genetics, 12(2), 99-110. doi:10.1038/nrg2936Iki, T., Yoshikawa, M., Nishikiori, M., Jaudal, M. C., Matsumoto-Yokoyama, E., Mitsuhara, I., … Ishikawa, M. (2010). In Vitro Assembly of Plant RNA-Induced Silencing Complexes Facilitated by Molecular Chaperone HSP90. Molecular Cell, 39(2), 282-291. doi:10.1016/j.molcel.2010.05.014Jaubert, M., Bhattacharjee, S., Mello, A. F. S., Perry, K. L., & Moffett, P. (2011). ARGONAUTE2 Mediates RNA-Silencing Antiviral Defenses against Potato virus X in Arabidopsis    . Plant Physiology, 156(3), 1556-1564. doi:10.1104/pp.111.178012Ji, L., Liu, X., Yan, J., Wang, W., Yumul, R. E., Kim, Y. J., … Chen, X. (2011). ARGONAUTE10 and ARGONAUTE1 Regulate the Termination of Floral Stem Cells through Two MicroRNAs in Arabidopsis. PLoS Genetics, 7(3), e1001358. doi:10.1371/journal.pgen.1001358Kim, V. N., Han, J., & Siomi, M. C. (2009). Biogenesis of small RNAs in animals. Nature Reviews Molecular Cell Biology, 10(2), 126-139. doi:10.1038/nrm2632Lanet, E., Delannoy, E., Sormani, R., Floris, M., Brodersen, P., Crété, P., … Robaglia, C. (2009). Biochemical Evidence for Translational Repression by Arabidopsis MicroRNAs. The Plant Cell, 21(6), 1762-1768. doi:10.1105/tpc.108.063412Langmead, B., Trapnell, C., Pop, M., & Salzberg, S. L. (2009). Ultrafast and memory-efficient alignment of short DNA sequences to the human genome. Genome Biology, 10(3), R25. doi:10.1186/gb-2009-10-3-r25Leung, A. K. L., Young, A. G., Bhutkar, A., Zheng, G. X., Bosson, A. D., Nielsen, C. B., & Sharp, P. A. (2011). Genome-wide identification of Ago2 binding sites from mouse embryonic stem cells with and without mature microRNAs. Nature Structural & Molecular Biology, 18(2), 237-244. doi:10.1038/nsmb.1991Llave, C., Xie, Z., Kasschau, K. D., & Carrington, J. C. (2002). Cleavage of Scarecrow-like mRNA Targets Directed by a Class of Arabidopsis miRNA. Science, 297(5589), 2053-2056. doi:10.1126/science.1076311Lobbes, D., Rallapalli, G., Schmidt, D. D., Martin, C., & Clarke, J. (2006). SERRATE: a new player on the plant microRNA scene. EMBO reports, 7(10), 1052-1058. doi:10.1038/sj.embor.7400806Mallory, A., & Vaucheret, H. (2010). Form, Function, and Regulation of ARGONAUTE Proteins. The Plant Cell, 22(12), 3879-3889. doi:10.1105/tpc.110.080671Manavella, P. A., Koenig, D., & Weigel, D. (2012). Plant secondary siRNA production determined by microRNA-duplex structure. Proceedings of the National Academy of Sciences, 109(7), 2461-2466. doi:10.1073/pnas.1200169109Matranga, C., Tomari, Y., Shin, C., Bartel, D. P., & Zamore, P. D. (2005). Passenger-Strand Cleavage Facilitates Assembly of siRNA into Ago2-Containing RNAi Enzyme Complexes. Cell, 123(4), 607-620. doi:10.1016/j.cell.2005.08.044Mi, S., Cai, T., Hu, Y., Chen, Y., Hodges, E., Ni, F., … Qi, Y. (2008). Sorting of Small RNAs into Arabidopsis Argonaute Complexes Is Directed by the 5′ Terminal Nucleotide. Cell, 133(1), 116-127. doi:10.1016/j.cell.2008.02.034Montgomery, T. A., Howell, M. D., Cuperus, J. T., Li, D., Hansen, J. E., Alexander, A. L., … Carrington, J. C. (2008). Specificity of ARGONAUTE7-miR390 Interaction and Dual Functionality in TAS3 Trans-Acting siRNA Formation. Cell, 133(1), 128-141. doi:10.1016/j.cell.2008.02.033Montgomery, T. A., Yoo, S. J., Fahlgren, N., Gilbert, S. D., Howell, M. D., Sullivan, C. M., … Carrington, J. C. (2008). AGO1-miR173 complex initiates phased siRNA formation in plants. Proceedings of the National Academy of Sciences, 105(51), 20055-20062. doi:10.1073/pnas.0810241105Morel, J.-B., Godon, C., Mourrain, P., Béclin, C., Boutet, S., Feuerbach, F., … Vaucheret, H. (2002). Fertile Hypomorphic ARGONAUTE (ago1) Mutants Impaired in Post-Transcriptional Gene Silencing and Virus Resistance. The Plant Cell, 14(3), 629-639. doi:10.1105/tpc.010358Peragine, A. (2004). SGS3 and SGS2/SDE1/RDR6 are required for juvenile development and the production of trans-acting siRNAs in Arabidopsis. Genes & Development, 18(19), 2368-2379. doi:10.1101/gad.1231804Qi, Y., He, X., Wang, X.-J., Kohany, O., Jurka, J., & Hannon, G. J. (2006). Distinct catalytic and non-catalytic roles of ARGONAUTE4 in RNA-directed DNA methylation. Nature, 443(7114), 1008-1012. doi:10.1038/nature05198Rajagopalan, R., Vaucheret, H., Trejo, J., & Bartel, D. P. (2006). A diverse and evolutionarily fluid set of microRNAs in Arabidopsis thaliana. Genes & Development, 20(24), 3407-3425. doi:10.1101/gad.1476406Scholthof, H. B., Alvarado, V. Y., Vega-Arreguin, J. C., Ciomperlik, J., Odokonyero, D., Brosseau, C., … Moffett, P. (2011). Identification of an ARGONAUTE for Antiviral RNA Silencing in Nicotiana benthamiana        . Plant Physiology, 156(3), 1548-1555. doi:10.1104/pp.111.178764Song, J.-J., Smith, S. K., Hannon, G. J., & Joshua-Tor, L. (2004). Crystal Structure of Argonaute and Its Implications for RISC Slicer Activity. Science, 305(5689), 1434-1437. doi:10.1126/science.1102514Souret, F. F., Kastenmayer, J. P., & Green, P. J. (2004). AtXRN4 Degrades mRNA in Arabidopsis and Its Substrates Include Selected miRNA Targets. Molecular Cell, 15(2), 173-183. doi:10.1016/j.molcel.2004.06.006Wang, L., Si, Y., Dedow, L. K., Shao, Y., Liu, P., & Brutnell, T. P. (2011). A Low-Cost Library Construction Protocol and Data Analysis Pipeline for Illumina-Based Strand-Specific Multiplex RNA-Seq. PLoS ONE, 6(10), e26426. doi:10.1371/journal.pone.0026426Wang, X.-B., Jovel, J., Udomporn, P., Wang, Y., Wu, Q., Li, W.-X., … Ding, S.-W. (2011). The 21-Nucleotide, but Not 22-Nucleotide, Viral Secondary Small Interfering RNAs Direct Potent Antiviral Defense by Two Cooperative Argonautes in Arabidopsis thaliana    . The Plant Cell, 23(4), 1625-1638. doi:10.1105/tpc.110.082305Wang, Y., Juranek, S., Li, H., Sheng, G., Wardle, G. S., Tuschl, T., & Patel, D. J. (2009). Nucleation, propagation and cleavage of target RNAs in Ago silencing complexes. Nature, 461(7265), 754-761. doi:10.1038/nature08434Wu, L., & Belasco, J. G. (2008). Let Me Count the Ways: Mechanisms of Gene Regulation by miRNAs and siRNAs. Molecular Cell, 29(1), 1-7. doi:10.1016/j.molcel.2007.12.010Xie, Z., Allen, E., Wilken, A., & Carrington, J. C. (2005). DICER-LIKE 4 functions in trans-acting small interfering RNA biogenesis and vegetative phase change in Arabidopsis thaliana. Proceedings of the National Academy of Sciences, 102(36), 12984-12989. doi:10.1073/pnas.0506426102Yang, L., Wu, G., & Poethig, R. S. (2011). Mutations in the GW-repeat protein SUO reveal a developmental function for microRNA-mediated translational repression in Arabidopsis. Proceedings of the National Academy of Sciences, 109(1), 315-320. doi:10.1073/pnas.1114673109Yoshikawa, M. (2005). A pathway for the biogenesis of trans-acting siRNAs in Arabidopsis. Genes & Development, 19(18), 2164-2175. doi:10.1101/gad.1352605Zhang, X., Zhao, H., Gao, S., Wang, W.-C., Katiyar-Agarwal, S., Huang, H.-D., … Jin, H. (2011). Arabidopsis Argonaute 2 Regulates Innate Immunity via miRNA393∗-Mediated Silencing of a Golgi-Localized SNARE Gene, MEMB12. Molecular Cell, 42(3), 356-366. doi:10.1016/j.molcel.2011.04.010Zhu, H., Hu, F., Wang, R., Zhou, X., Sze, S.-H., Liou, L. W., … Zhang, X. (2011). Arabidopsis Argonaute10 Specifically Sequesters miR166/165 to Regulate Shoot Apical Meristem Development. Cell, 145(2), 242-256. doi:10.1016/j.cell.2011.03.024Zisoulis, D. G., Lovci, M. T., Wilbert, M. L., Hutt, K. R., Liang, T. Y., Pasquinelli, A. E., & Yeo, G. W. (2010). Comprehensive discovery of endogenous Argonaute binding sites in Caenorhabditis elegans. Nature Structural & Molecular Biology, 17(2), 173-179. doi:10.1038/nsmb.174

Specificity of ARGONAUTE7-miR390 Interaction and Dual Functionality in TAS3 Trans-Acting siRNA Formation

SummaryTrans-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

Identification of MIR390a precursor processing defective mutants in Arabidopsis by direct genome sequencing

Transacting siRNA (tasiRNA) biogenesis in Arabidopsis is initiated by microRNA (miRNA) –guided cleavage of primary transcripts. In the case of TAS3 tasiRNA formation, ARGONAUTE7 (AGO7)– miR390 complexes interact with primary transcripts at two sites, resulting in recruitment of RNA-DEPENDENT RNA POLYMERASE6 for dsRNA biosynthesis. An extensive screen for Arabidopsis mutants with specific defects in TAS3 tasiRNA biogenesis or function was done. This yielded numerous ago7 mutants, one dcl4 mutant, and two mutants that accumulated low levels of miR390. A direct genome sequencing-based approach to both map and rapidly identify one of the latter mutant alleles was developed. This revealed a G-to-A point mutation (mir390a-1) that was calculated to stabilize a relatively nonpaired region near the base of the MIR390a foldback, resulting in misprocessing of the miR390/ miR390* duplex and subsequent reduced TAS3 tasiRNA levels. Directed substitutions, as well as analysis of variation at paralogous miR390-generating loci (MIR390a and MIR390b), indicated that base pair properties and nucleotide identity within a region 4–6 bases below the miR390/miR390* duplex region contributed to the efficiency and accuracy of precursor processing

Functional Analysis of Three Arabidopsis

In RNA-directed silencing pathways, ternary complexes result from small RNA-guided ARGONAUTE (AGO) associating with target transcripts. Target transcripts are often silenced through direct cleavage (slicing), destabilization through slicer-independent turnover mechanisms, and translational repression. Here, wild-type and active-site defective forms of several Arabidopsis thaliana AGO proteins involved in posttranscriptional silencing were used to examine several AGO functions, including small RNA binding, interaction with target RNA, slicing or destabilization of target RNA, secondary small interfering RNA formation, and antiviral activity. Complementation analyses in ago mutant plants revealed that the catalytic residues of AGO1, AGO2, and AGO7 are required to restore the defects of Arabidopsis ago1-25, ago2-1, and zip-1 (AGO7-defective) mutants, respectively. AGO2 had slicer activity in transient assays but could not trigger secondary small interfering RNA biogenesis, and catalytically active AGO2 was necessary for local and systemic antiviral activity against Turnip mosaic virus. Slicer-defective AGOs associated with miRNAs and stabilized AGO-miRNA-target RNA ternary complexes in individual target coimmunoprecipitation assays. In genome-wide AGO-miRNA-target RNA coimmunoprecipitation experiments, slicer-defective AGO1-miRNA associated with target RNA more effectively than did wild-type AGO1-miRNA. These data not only reveal functional roles for AGO1, AGO2, and AGO7 slicer activity, but also indicate an approach to capture ternary complexes more efficiently for genome-wide analyses

The ERI-6/7 Helicase Acts at the First Stage of an siRNA Amplification Pathway That Targets Recent Gene Duplications

Endogenous small interfering RNAs (siRNAs) are a class of naturally occuring regulatory RNAs found in fungi, plants, and animals. Some endogenous siRNAs are required to silence transposons or function in chromosome segregation; however, the specific roles of most endogenous siRNAs are unclear. The helicase gene eri-6/7 was identified in the nematode Caenorhabditis elegans by the enhanced response to exogenous double-stranded RNAs (dsRNAs) of the null mutant. eri-6/7 encodes a helicase homologous to small RNA factors Armitage in Drosophila, SDE3 in Arabidopsis, and Mov10 in humans. Here we show that eri-6/7 mutations cause the loss of 26-nucleotide (nt) endogenous siRNAs derived from genes and pseudogenes in oocytes and embryos, as well as deficiencies in somatic 22-nucleotide secondary siRNAs corresponding to the same loci. About 80 genes are eri-6/7 targets that generate the embryonic endogenous siRNAs that silence the corresponding mRNAs. These 80 genes share extensive nucleotide sequence homology and are poorly conserved, suggesting a role for these endogenous siRNAs in silencing of and thereby directing the fate of recently acquired, duplicated genes. Unlike most endogenous siRNAs in C. elegans, eri-6/7–dependent siRNAs require Dicer. We identify that the eri-6/7–dependent siRNAs have a passenger strand that is ~19 nt and is inset by ~3–4 nts from both ends of the 26 nt guide siRNA, suggesting non-canonical Dicer processing. Mutations in the Argonaute ERGO-1, which associates with eri-6/7–dependent 26 nt siRNAs, cause passenger strand stabilization, indicating that ERGO-1 is required to separate the siRNA duplex, presumably through endonucleolytic cleavage of the passenger strand. Thus, like several other siRNA–associated Argonautes with a conserved RNaseH motif, ERGO-1 appears to be required for siRNA maturation

SNPC-1.3 is a sex-specific transcription factor that drives male piRNA expression in C. elegans.

Piwi-interacting RNAs (piRNAs) play essential roles in silencing repetitive elements to promote fertility in metazoans. Studies in worms, flies, and mammals reveal that piRNAs are expressed in a sex-specific manner. However, the mechanisms underlying this sex-specific regulation are unknown. Here we identify SNPC-1.3, a male germline-enriched variant of a conserved subunit of the small nuclear RNA-activating protein complex, as a male-specific piRNA transcription factor in Caenorhabditis elegans. SNPC-1.3 colocalizes with the core piRNA transcription factor, SNPC-4, in nuclear foci of the male germline. Binding of SNPC-1.3 at male piRNA loci drives spermatogenic piRNA transcription and requires SNPC-4. Loss of snpc-1.3 leads to depletion of male piRNAs and defects in male-dependent fertility. Furthermore, TRA-1, a master regulator of sex determination, binds to the snpc-1.3 promoter and represses its expression during oogenesis. Loss of TRA-1 targeting causes ectopic expression of snpc-1.3 and male piRNAs during oogenesis. Thus, sexually dimorphic regulation of snpc-1.3 expression coordinates male and female piRNA expression during germline development

The ERI-6/7 Helicase Acts at the First Stage of an siRNA Amplification Pathway That Targets Recent Gene Duplications

Endogenous small interfering RNAs (siRNAs) are a class of naturally occuring regulatory RNAs found in fungi, plants, and animals. Some endogenous siRNAs are required to silence transposons or function in chromosome segregation; however, the specific roles of most endogenous siRNAs are unclear. The helicase gene eri-6/7 was identified in the nematode Caenorhabditis elegans by the enhanced response to exogenous double-stranded RNAs (dsRNAs) of the null mutant. eri-6/7 encodes a helicase homologous to small RNA factors Armitage in Drosophila, SDE3 in Arabidopsis, and Mov10 in humans. Here we show that eri-6/7 mutations cause the loss of 26-nucleotide (nt) endogenous siRNAs derived from genes and pseudogenes in oocytes and embryos, as well as deficiencies in somatic 22-nucleotide secondary siRNAs corresponding to the same loci. About 80 genes are eri-6/7 targets that generate the embryonic endogenous siRNAs that silence the corresponding mRNAs. These 80 genes share extensive nucleotide sequence homology and are poorly conserved, suggesting a role for these endogenous siRNAs in silencing of and thereby directing the fate of recently acquired, duplicated genes. Unlike most endogenous siRNAs in C. elegans, eri-6/7–dependent siRNAs require Dicer. We identify that the eri-6/7–dependent siRNAs have a passenger strand that is ∼19 nt and is inset by ∼3–4 nts from both ends of the 26 nt guide siRNA, suggesting non-canonical Dicer processing. Mutations in the Argonaute ERGO-1, which associates with eri-6/7–dependent 26 nt siRNAs, cause passenger strand stabilization, indicating that ERGO-1 is required to separate the siRNA duplex, presumably through endonucleolytic cleavage of the passenger strand. Thus, like several other siRNA–associated Argonautes with a conserved RNaseH motif, ERGO-1 appears to be required for siRNA maturation

PIWI Associated siRNAs and piRNAs Specifically Require the Caenorhabditis elegans HEN1 Ortholog henn-1

Small RNAs—including piRNAs, miRNAs, and endogenous siRNAs—bind Argonaute proteins to form RNA silencing complexes that target coding genes, transposons, and aberrant RNAs. To assess the requirements for endogenous siRNA formation and activity in Caenorhabditis elegans, we developed a GFP-based sensor for the endogenous siRNA 22G siR-1, one of a set of abundant siRNAs processed from a precursor RNA mapping to the X chromosome, the X-cluster. Silencing of the sensor is also dependent on the partially complementary, unlinked 26G siR-O7 siRNA. We show that 26G siR-O7 acts in trans to initiate 22G siRNA formation from the X-cluster. The presence of several mispairs between 26G siR-O7 and the X-cluster mRNA, as well as mutagenesis of the siRNA sensor, indicates that siRNA target recognition is permissive to a degree of mispairing. From a candidate reverse genetic screen, we identified several factors required for 22G siR-1 activity, including the chromatin factors mes-4 and gfl-1, the Argonaute ergo-1, and the 3′ methyltransferase henn-1. Quantitative RT–PCR of small RNAs in a henn-1 mutant and deep sequencing of methylated small RNAs indicate that siRNAs and piRNAs that associate with PIWI clade Argonautes are methylated by HENN-1, while siRNAs and miRNAs that associate with non-PIWI clade Argonautes are not. Thus, PIWI-class Argonaute proteins are specifically adapted to associate with methylated small RNAs in C. elegans

Functional Analysis of Three \u3ci\u3eArabidopsis\u3c/i\u3e ARGONAUTES Using Slicer-Defective Mutants

In RNA-directed silencing pathways, ternary complexes result from small RNA-guided ARGONAUTE (AGO) associating with target transcripts. Target transcripts are often silenced through direct cleavage (slicing), destabilization through slicerindependent turnover mechanisms, and translational repression. Here, wild-type and active-site defective forms of several Arabidopsis thaliana AGO proteins involved in posttranscriptional silencing were used to examine several AGO functions, including small RNA binding, interaction with target RNA, slicing or destabilization of target RNA, secondary small interfering RNA formation, and antiviral activity. Complementation analyses in ago mutant plants revealed that the catalytic residues of AGO1, AGO2, and AGO7 are required to restore the defects of Arabidopsis ago1-25, ago2-1, and zip-1 (AGO7-defective) mutants, respectively. AGO2 had slicer activity in transient assays but could not trigger secondary small interfering RNA biogenesis, and catalytically active AGO2 was necessary for local and systemic antiviral activity against Turnip mosaic virus. Slicer-defective AGOs associated with miRNAs and stabilized AGO-miRNA-target RNA ternary complexes in individual target coimmunoprecipitation assays. In genome-wide AGO-miRNA-target RNA coimmunoprecipitation experiments, slicerdefective AGO1-miRNA associated with target RNA more effectively than did wild-type AGO1-miRNA. These data not only reveal functional roles for AGO1, AGO2, and AGO7 slicer activity, but also indicate an approach to capture ternary complexes more efficiently for genome-wide analyses