piRNA - Piwi-interacting RNAs

Transposable DNA elements, or transposons, show random movement throughout the genome. These insertions that disrupt a gene can result in genomic instability, which is dangerous to a cell.  

In somatic cells, transposon induced genomic instability remains limited to a single generation; however, in germ cells, these changes can be passed to future generations leading to harmful effects. 

Germ cell-specific transposable elements are silenced by small non-coding regulatory RNAs known as piwi-interacting RNA or piRNA.

piRNAs are essential for proper germ cell development, and their absence can cause infertility in animals. 

piRNA are a class of silencing RNAs that differ from miRNA and siRNA by three defining characteristics: length, processing mechanism, and binding with Argonaute subfamily proteins.

piRNA are 24 to 32 nucleotides in length, longer than both miRNA and siRNA which are usually 20 to 25 nucleotides long. 

piRNA is processed from single-stranded mRNA without Dicer while both miRNA and siRNA are processed from double-stranded RNA by Dicer. 

Each of these three types of non-coding RNA bind to Argonaute family proteins, but piRNA bind to the piwi subfamily while miRNA and siRNA bind to the AGO subfamily of proteins.

piRNA originate from piRNA clusters, specific regions of the genome that are rich in transposons. 

Two phases have been proposed for the biogenesis of piRNA: the primary processing pathway and the amplification loop. 

In the primary processing pathway, the transcripts from piRNA clusters are used to produce piRNA. These are loaded onto selected piwi proteins to form piRISC, an alternative form of the RNA induced silencing complex. 

Primary piRNA then takes part in an amplification loop to rapidly increase the concentration of the piRNA. 

piRISC binds and cleaves the complementary target RNA creating the 5’ end of a premature secondary pi-RNA.  The 3’ end of the pi-RNA is processed further by other piwi proteins to resulting in a mature secondary pi-RNA. 

This process repeats itself resulting in the amplification of both sense and antisense piRNA.