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RasiRNA
Repeat associated small interfering RNA (rasiRNA) is a class of small RNA that is involved in the RNA interference (RNAi) pathway. RasiRNA are in fact Piwi-interacting RNAs (piRNAs), which are small RNA molecules that interact with Piwi proteins.〔Gunawardane, L. S., K. Saito, K. M. Nishida, K. Miyoshi, Y. Kawamura, T. Nagami, H. Siomi, M. C. Siomi. 2007. A Slicer-Mediated Mechanism for Repeat-Associated siRNA 5’ End Formation in Drosophila. ''Science'' 315(5818): 1587-1590.〕 Piwi proteins are a clade of the Argonaute family of proteins. In the germline, RasiRNA is involved in establishing and maintaining heterochromatin structure, controlling transcripts that emerge from repeat sequences, and silencing transposons and retrotransposons.〔Dorner, S., A. Eulalio, E. Huntzinger, E. Izaurralde. 2007. Symposium on MicroRNAs and siRNAs: Biological Functions and Mechanisms. ''EMBO'' 8: 723-729.〕〔Klattenhoff, C., D. P. Bratu, N. McGinnis-Schultz, B. S. Koppetsch, H. A. Cook, W. E. Theurkauf. 2006. ''Drosophila'' rasiRNA Pathway Mutations Disrupt Embryonic Axis Specification through Activation of an ATR/Chk2 DNA Damage Response. ''Developmental Cell'' 12: 45-55.〕
== Classification ==
There are at least three Argonaute subfamilies that have been found in eukaryotes. Unlike the Ago subfamily which is present in animals, plants, and fission yeast, the Piwi subfamily has only been found in animals.〔Houwing, S., L. M. Kamminga, E. Berezikov, D. Cronembold, A. Girard, H. v. d. Elst, D. V. Filippov, H. Blaser, E. Raz, C. B. Moens, R. H. A. Plasterk, G. J. Hannon, B. W. Draper, R. F. Ketting. 2007. A Role for Piwi and piRNAs in Germ Cell Maintenance and Transposon Silencing in Zebrafish. ''Cell'' 129(1): 69-82.〕 RasiRNA has been observed in ''Drosophila'' and some unicellular eukaryotes but its presence in mammals has not been determined, unlike piRNA which has been observed in many species of invertebrates and vertebrates including mammals;〔Girard, A. R. Sachidanandam, G. Hannon, M. A. Carmell. 2006. A germline-specific class of small RNAs binds mammalian Piwi proteins. ''Nature'' 442: 199-202.〕 however, since proteins which associate with rasiRNA are found in both vertebrates and invertebrates, it is possible that active rasiRNA exist and have yet to be observed in other animals. RasiRNAs have been observed in Schizosaccharomyces pombe, a species of yeast, as well in some plants, neither of which have been observed to contain the Piwi Argonaute protein subfamily.〔Aravin, Alexei, Thomas Tuschl. 2005. Identification and characterization of small RNAs involved in RNA silencing. ''FEBS'' 579: 5830-5840.〕 It has been observed that both rasiRNA and piRNA are maternally linked, but more specifically it is the Piwi protein subfamily that is maternally linked and therefore leads to the observation that rasiRNA and piRNA are maternally linked.〔Tomari, Y., T. Du, B. Haley, D. S. Schwarz, R. Bennett, H. A. Cook, B. S. Koppetsch, W. E. Theurkauf, P. D. Zamore. 2004. RISC Assembly Defects in the ''Drosophila'' RNAi Mutant ''armitage''. ''Cell'' 116: 831-841.〕
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