Piwi-interacting RNAs (piRNA) are a class small of RNA molecules that are expressed uniquely in mammalian spermatogenic cell lines. These are 26–31 nucleotides long and bigger that miRNAs and siRNAs. They are so named because of their capability of forming RNA-protein complexes with Piwi proteins. Piwi proteins are part of the family of Argonaute proteins, which are defined by the PAZ (Piwi Argonaut and Zwille) domain and the PIWI domain. Argonaute proteins interact with small RNAs through PAZ and PIWI domains. A small RNA guides the Argonaute protein to its target molecule, which leads to gene silencing. MIWI, MIWI2 and MILI, three Piwi subfamily proteins, are essential for spermatogenesis in mice. piRNAs are involved in RNA silencing via the formation of RISC. The biogenesis pathway of piRNAs has not been clearly elucidated yet, but they are generated from junk DNA. A review on piRNA can be found here.
In a recent report scientists at Yale University have shown that piRNAs play pivotal role in regulating gene function. They discovered more than 13,000 Piwi-associated piRNAs in fruit flies. Out of these one was found to interact with Piwi, which finally binds to the chromatin and regulates the activity of the gene. Read the full story here.