Small silencing RNAs: an expanding universe

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Abstract
Like nearly all biological mechanisms, small RNA-directed pathways are both elegantly simple — small RNA guides use sequence complementarity to identify their targets — and shockingly complex, with myriad proteins required to excise small RNA guides from much longer precursors and still more required to carry out small RNA-directed functions. Despite this complexity, the defining features of small silencing RNAs are their short length (∼20–30 nucleotides) and their association with members of the Argonaute family of proteins. Small interfering RNAs are typically 21 nucleotides and they are derived from dsRNA, the nearly universal eukaryotic signal for 'foreignness'. Small interfering RNAs can be both exogenous and endogenous in origin in plants and animals, and provide an epigenetic component of resistance against biotic and abiotic stress. MicroRNAs 'tune' and regulate development and many other biological processes in plants and animals. MicroRNAs are encoded in the genome and typically repress their mRNA targets by partial base-pairing, and hence have the potential to regulate many distinct mRNA targets. Piwi-interacting RNAs participate in a feed-forward amplification loop that monitors and silences transposon expression in the germ line. Small RNA pathways, although distinct, share protein components and repress and enhance each other. Such 'cross talk' between small RNA pathways is poorly understood.