Fighting an old war with a new weapon--silencing transposons by Piwi-interacting RNA

Discovered six decades ago, transposons are known to selfishly multiply within and between chromosomes. Although they may play a creative role in building new functional parts of the genome, transposons usually cause insertional mutagenesis and/or turn nearby genes on or off. To maintain genome integrity, cells use a variety of strategies to defend against the proliferation of transposons. A class of small noncoding RNA, discovered seven years ago and called piRNA, is a new player in the war to silence transposons. piRNA is made via two biogenesis pathways: the primary processing pathway and the ping-pong amplification loop. These pathways are critically involved in transposon RNA degradation, DNA methylation, and histone modification machinery that represses transposons. In this review, we briefly introduce transposon-caused genomic instability and summarize our current understanding of the piRNA pathway, focusing on its key function in transposon silencing.