Fig. 2

piRNA mediates mRNA regulation by transposons. a piRNA precursors are transcribed from piRNA clusters that include retrotransposon sequences (the green box). Mature piRNAs (short green lines) associated with Piwi proteins guide the Piwi-piRNA complex to complementary transposon sequences predominately in 3′ UTR and occasionally to 5′ UTR or CDS of specific mRNAs [67,68,69]. The targeted mRNAs are degraded through Piwi-slicing and other mechanisms (see Fig. 3). Alternatively, piRNAs are generated from mRNA that contains transposon sequences (the orange box) in the 3′ UTR. These piRNAs (orange short lines) in turn target corresponding mRNAs and mediate their degradation [73,74,75]. b The piRNA-target RNA pairing rule as exemplified by C. elegans studies. Other organisms appear to follow a similar rule