Fig. 8.

Model depicting antagonistic roles of Nbr and Hen1 in the modulation of piRNA 3′ ends. There is a delicate balance in the mechanism by which Nbr and Hen1 modulate piRNA 3′ ends. Mature piRNAs of ∼23-30 nt are bound to PIWI clade Argonaute proteins (ovals), including Piwi, a nodal protein that is crucial in piRNA pathways. We provide evidence that Piwi can interact with Nbr. Our data reveal interplay between Nbr-mediated exoribonuclytic trimming and Hen1-dependent methylation that antagonizes trimming, thus linking two genes with opposing activity in shaping piRNA 3′ ends in the biogenesis of piRNAs. Nbr might represent just one of a large cohort of additional, as yet unknown, factors with roles in trimming piRNA 3′ ends.