Molecular Biology

RISC Requires an Armi

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Science  02 Apr 2004:
Vol. 304, Issue 5667, pp. 19
DOI: 10.1126/science.304.5667.19a

RNA silencing underlies a number of important and highly related gene regulatory phenomena: RNA interference (RNAi), posttranscriptional gene silencing, regulation by microRNAs (miRNAs), and so forth. The extent to which these RNA silencing mechanisms are involved in cellular and developmental processes, hinted at in bioinformatics screens for miRNA function, is only gradually becoming apparent, with the latest evidence coming from the screen for genes involved in specifying embryonic axes in Drosophila described by Cook et al.

This screen identified the gene armitage (armi) as being required for proper axis formation in the Drosophila egg. Revealingly, armi was found to encode a protein similar to the helicase SDE3, a known component of RNA silencing in Arabidopsis. In oocytes mutant for armi, the polarization of the oocyte cytoskeleton, which is essential for the asymmetric localization of messenger RNAs (mRNAs) critical for development and axis formation, is deranged. One of these mRNAs codes for the Oskar protein, which is itself required for patterning. In these mutant oocytes, Tomari et al. show that the normally translationally repressed osk mRNA was expressed, as was the stellate gene, which is normally silenced by RNAi. The link between armi and RNA silencing was further demonstrated by showing that extracts from armi mutant oocytes were defective in RNAi in vitro because they did not properly assemble the RNA-induced silencing complex (RISC). — GR

Cell 116, 817; 831 (2004).

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