Figure 2.

Schematic representation of miRNA (left) and piRNA (right) biogenesis. Primary miRNAs (pri-miRNAs) product by RNA POL II can be monocistronic if they carry just one mir or polycistronic if they contain multiple miRNAs. While in the nucleus, pri-miRNAs are cleaved by the Drosha/DGR8 complex into 65nt precursor miRNAs (pre-miRNAs) and then exported by the Exportin5/RAN-GTP complex into the cytoplasm where they are in turn cleaved by the Dicer/TRBP/PACT complex which thus produces a miRNA duplex ready to be incorporated into the RISC complex; only one strand of the miRNA is preferentially selected to be coupled with the RISC complex. On the right, piRNA precursors are produced through the primary processing pathway, transcribed by POL II and exported into the cytosol where they are cut into mature piRNAs. Then, mature piRNAs in complex with PIWI proteins migrate to the nucleus where they silence TEs. In addition, MIWI2 and MILI coupled with mature piRNAs, cleave transcripts bearing sites complementary to the piRNA sequence, thus amplifying mature piRNA species through the ping-pong pathway.