Piwi and piRNA in Germline and Epigenetic Regulation

Genes are transcribed to generate proteins for biological functions. However, not all transcripts encode proteins. Transfer RNAs and ribosomal RNAs play essential roles in translational processes. In addition, various long and small non-coding RNAssmall non-coding RNA have been discovered. For example, small interfering RNAs and microRNAs regulate genes by posttranscriptional gene silencing. This chapter focuses on a class of small non-coding RNA called Piwi RNA (piRNApiRNA). PiwiPiwi is a protein required to maintain germlinegermline stem cells in invertebrate and vertebrate species. We will discuss how the piwi gene was found and how piRNAs were identified as RNAs associated with Piwi family proteins. Drosophila and mammalian cells express more than 105 different piRNAs. Like other small non-coding RNAs, piRNA transcripts must be processed into small mature piRNA. We will describe a model for the biogenesis of piRNA. Piwi and piRNA play diverse roles in cell cycle, differentiation, and apoptosis. In particular, Piwi and piRNA are critical for protecting genome integrity by repressing transposable elements. Such repression is mediated by the recruitment of chromatin-modifying enzymes to the Piwi-piRNA complex. Piwi and piRNA also participate in the epigenetic activation of gene expression. Piwi and piRNA provide insights into the expanding roles of small RNAs in gene regulation.