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Science 20 September 2002:
Vol. 297. no. 5589, p. 1953
DOI: 10.1126/science.297.5589.1953o
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This Week in Science
MicroRNAs (miRNAs), found in scores in animals and now plants, and small intermediate RNAs (siRNAs) are essentially identical in size, ~22 nucleotides, and are generated by the same enzyme, Dicer. miRNAs lin-41 and let-7 are known to down-regulate the translation of target mRNAs by binding to imperfectly matched binding sites in their 3'-untranslated regions, the mechanism by which all known animal miRNAs are likely to work. siRNAs, which are fully complementary with their target sites, are intermediates in RNA interference, the outcome of which is degradation of the target RNA. Two reports shed light on how this functional distinction can be achieved (see the Perspective by Baulcombe). Hutvágner and Zamore (p. 2056) show that, both in vitro and in vivo, it is purely the degree of complementarity of the let-7 miRNA to its target binding site that determines whether it will function as an miRNA or an siRNA. They also show that the siRNA-protein complex that directs cleavage of the target RNA can function as a multiple turnover enzyme. Llave et al. (p. 2053) show that at least one plant miRNA (from Arabidopsis) has a perfect match with several protein-coding target RNAs. In a similar manner to the siRNA seen in RNAi, and unlike the characterized animal miRNAs, miRNA 39 can direct the cleavage of these targets.
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Science. ISSN 0036-8075 (print), 1095-9203 (online)
