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Piwi-interacting RNA (piRNA) is the largest class of small non-coding RNA molecules expressed in animal cells.〔Molecular Biology Select. Cell, 2006. 126(2): p. 223, 225-223, 225.〕〔Seto, A.G., R.E. Kingston, and N.C. Lau, The Coming of Age for Piwi Proteins. Molecular Cell, 2007. 26(5): p. 603-609.〕 piRNAs form RNA-protein complexes through interactions with piwi proteins. These piRNA complexes have been linked to both epigenetic and post-transcriptional gene silencing of retrotransposons and other genetic elements in germ line cells, particularly those in spermatogenesis.〔Siomi MC, Sato K, Pezic D, Aravin AA: PIWI-interacting small RNAs: the vanguard of genome defence. Nat Rev Mol Cell Biol 2011, 12:246-258.〕 They are distinct from microRNA (miRNA) in size (26–31 nt rather than 21–24 nt), lack of sequence conservation, and increased complexity.〔〔 It remains unclear how piRNAs are generated, but potential methods have been suggested, and it is certain their biogenesis pathway is distinct from miRNA and siRNA, while rasiRNAs are a piRNA subspecies.〔Klattenhoff, C. and W. Theurkauf, Biogenesis and germline functions of piRNAs. Development, 2008. 135(1): p. 3-9.〕 ==Characteristics== piRNAs have been identified in both vertebrates and invertebrates, and although biogenesis and modes of action do vary somewhat between species, a number of features are conserved. piRNAs have no clear secondary structure motifs,〔 the length of a piRNA is, by definition, between 26 and 31 nucleotides, and the bias for a 5’ uridine is common to piRNAs in both vertebrates and invertebrates. piRNAs in ''Caenorhabditis elegans'' have a 5’ monophosphate and a 3’ modification that acts to block either the 2’ or 3’ oxygen,〔Ruby, J.G., et al., Large-Scale Sequencing Reveals 21U-RNAs and Additional MicroRNAs and Endogenous siRNAs in C. elegans. 2006. 127(6): p. 1193-1207.〕 and this has also been confirmed to exist in ''Drosophila melanogaster'',〔Vagin, V.V., et al., A Distinct Small RNA Pathway Silences Selfish Genetic Elements in the Germline. Science, 2006. 313(5785): p. 320-324.〕 zebrafish,〔Houwing, S., et al., A Role for Piwi and piRNAs in Germ Cell Maintenance and Transposon Silencing in Zebrafish. Cell, 2007. 129(1): p. 69-82.〕 mice〔Kirino, Y. and Z. Mourelatos, Mouse Piwi-interacting RNAs are 2()-O-methylated at their 3() termini. Nat Struct Mol Biol, 2007. 14(4): p. 347-348.〕 and rats.〔 This 3’ modification is a 2’-O-methylation; the reason for this modification is not clear, but it has been suggested to increase piRNA stability.〔〔Faehnle, C.R. and L. Joshua-Tor, Argonautes confront new small RNAs. Current Opinion in Chemical Biology, 2007. 11(5): p. 569-577.〕 It is thought that there are many hundreds of thousands of different piRNA species found in mammals.〔Das, P.P., et al., Piwi and piRNAs Act Upstream of an Endogenous siRNA Pathway to Suppress Tc3 Transposon Mobility in the Caenorhabditis elegans Germline. Molecular Cell, 2008. 31(1): p. 79-90.〕 Thus far, over 50,000 unique piRNA sequences have been discovered in mice and more than 13,000 in ''D. melanogaster''.〔Lin, H., et al., The role of the piRNA pathway in stem cell self-renewal. Developmental Biology, 2008. 319(2): p. 479-479.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Piwi-interacting RNA」の詳細全文を読む スポンサード リンク
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