Dr. Mikiko Siomi and colleagues from the Institute for Genome Rsesearch in Japan have identified the elusive small RNA partners of Piwi proteins - lending new insight into the gene silencing pathways mediated by small RNAs in Drosophila.
Piwi (P-element-induced wimpy testis) is a member of the Drosophila Argonaute protein family, which is expressed specifically in fly testis and ovaries, and is necessary for germ stem cell self-renewal. It has recently been shown that Piwi is responsible for the silencing of retrotransposons in the testis.
Now, Dr. Siomi and colleagues demonstrate that in Drosophila ovaries, Piwi specifically associates with a distinct class of 25-29 nucleotide-long small RNAs, called repeat-associated siRNAs (rasiRNAs). They further show that Piwi is able to cleave target RNAs in vitro - what the authors refer to as Slicer activity. According to the authors, their result suggest "that Piwi functions in nuclear RNA silencing as Slicer by associating specifically with the rasiRNAs," thereby introducing a third, and novel, pathway of gene silencing in Drosophila.