The small RNA world

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Abstract
Small regulatory RNAs are generated via processing of longer double-stranded RNA (dsRNA) precursors by an RNaseIII-like enzyme termed Dicer (Bernstein et al., 2001). Despite the overall conservation of RNA-mediated silencing mechanisms, this key enzyme has diversified structurally and numerically in different taxa (Schauer et al., 2002). While there is a single Dicer protein in mammals, Caenorhabditis elegans and fission yeast, Drosophila melanogaster has two, and in Arabidopsis thaliana there are four dicer-like (DCL) proteins (Schauer et al., 2002). Two Arabidopsis DCL proteins (DCL1 and DCL4) contain one or more predicted nuclear localization signals (NLS), suggesting both nuclear and cytoplasmic pathways for processing dsRNA in plants. Indeed, there is evidence that DCL1 is a nuclear protein (Papp et al., 2003). Nuclear processing of dsRNA might also occur in other organisms: one of the two Dicers in Drosophila contains a predicted NLS as does the single mammalian Dicer (Schauer et al., 2002). Although the mammalian Dicer has been reported to be located in the cytoplasm (Billy et al., 2001), a recent study has suggested that it can process nonpolyadenylated dsRNA in the nucleus (Shinagawa and Ishii, 2003). Therefore, both nuclear (N) and cytoplasmic (C) Dicer activities are depicted here, the caveat being that both forms may not be present in all organisms. Although not illustrated here, Dicer acts in complexes with other proteins, including members of the Argonaute family (Carmell et al., 2002) and possibly HEN1 (Park et al., 2002; Boutet et al., 2003), to produce small RNAs.