Two distinct transcriptional controls triggered by nuclear Piwi-piRISCs in the Drosophila piRNA pathway.

Transposons occupy a large proportion of eukaryotic genomes. Spontaneous movement of transposons within the genome leads to genomic mutations that are often life threatening. To avoid such outcomes, host species have acquired an elaborate molecular pathway during evolution, in which PIWI proteins and PIWI-interacting RNAs (piRNAs) collaboratively control transposons to maintain germline genome integrity. Studies of PIWI-piRNA-based transposon silencing have uncovered the molecular mechanisms underlying piRNA biogenesis and the silencing mode-of-action in the pathway. Interestingly, the bilateral events require transcriptional controls triggered by nuclear PIWI-piRNA complexes in positive and negative manners, respectively. This review summarizes the mechanisms of PIWI-piRNA-mediated transcriptional controls in Drosophila ovaries.