Piwi-interacting RNAs

Piwi is the general name for a group of proteins that are part of the Argonaute gene family. The Piwi proteins are expressed mostly in the germ line cells and have been observed to play a role in repressing the expression and transposition of transposable elements, although there is also evidence that some early development genes are regulated by piRNA.

piRNA refers to Piwi-interacting RNA which are RNA molecules about 21-30 nucleotides in length. The piRNAs act to regulate gene expression by forming a complex with Piwi. In some ways piRNA is similar to miRNA and siRNA but there are some differences, aside from the obvious difference in length (siRNA and miRNA tend to be about 22-24 nt long). One is that piRNA is generated from a single stranded RNA (called a pre-piRNA) without the involvement of Dicer (which is involved in siRNA and miRNA generation). Second is that piRNA is not involved in RNA interference. Instead, piRNA/Piwi complexes have been implicated in RNA-induced epigenetic silencing (if you want to add to the growing list of RNA tags, this is sometimes given as RNAe).