Abstract

PIWI -family proteins and their associated small RNAs (piRNAs) act in an evolutionarily conserved innate immune mechanism that provides an essential protection for germ cell genomes against the activity of mobile genetic elements. piRNA populations comprise a molecular definition of transposons that permits them to be distinguished from host genes and selectively silenced. piRNAs can be generated in two distinct ways. Primary piRNAs emanate from discrete genomic loci, termed piRNA clusters, and appear to be derived from long, single-stranded precursors. The biogenesis of primary piRNAs involves at least two nucleolytic steps. Zucchini cleaves piRNA cluster transcripts to generate monophosphorylated piRNA 5’ ends. piRNA 3’ ends are likely formed by exonucleolytic trimming, after a piRNA precursor is loaded into its PIWI partner. Secondary piRNAs arise during the adaptive ping-pong cycle, with their 5’ termini being formed by the activity of PIW Is themselves. At least in Drosophila, piRNAs are maternally deposited and transmit an epigenetic signal essential for the effective control of at least some transposable elements. Our continuing efforts combine genetics, biochemistry, structural biology, and evolutionary and computational approaches to understand how the piRNA pathway effectively discriminates self from non-self at the genomic level.