Fig. 1

Biogenic pathways of piRNA in mice. PiRNA precursor is transcribed from piRNA cluster, protein coding region or intergenic region, and it is further cut into small pieces that varies in length. After RNA pieces associate specifically with PIWI, they undergo 3′ end trimming process. The 3′ end is further modified by the RNA methyltransferase Hen1, which marks the maturation of primary piRNA. The secondary biogenesis process of piRNA is known as ping-pong cycle: mili firstly binds to primary piRNA, which is followed by the association between primary piRNA and mRNA through base-paring. Mili cut the paired mRNA between 10 and 11 nt position at 5′ end. The resultant RNA piece binds to miwi2 and goes through the same 3′ end trimming and modification procedure as described above. The miwi2-associated piRNA recognize target RNA in a similar manner, which is again followed by 5′ end trimming, mili association, 3′ end trimming and modification. The cycle then repeats