Fig. 1

Biogenesis mechanisms and characteristics of piRNAs. (A) Five origins of primary piRNAs. It has been confirmed that the generation of piRNAs can be divided into five groups depending on their origin: (a) transposon-derived, (b) mRNA-derived, (c) tRNA-derived, (d) lncRNA-derived, and (e) snoRNA-derived piRNAs. (B) Sense and antisense transcripts of the primary piRNAs are produced by small nucleotide sequences from clusters of piRNA genes. (C) Primary piRNAs are cleaved by Zuc in the YB body, and the 5′ fragments of the intermediate sequence interact with a PIWI protein to form the piRNA/PIWI complex and thus intermediate piRNAs. The 2′-hydroxyl at the 3′-end of the intermediate piRNA is methylated by Hen1 to form a mature piRNA/PIWI complex. (D) The primary piRNA binds to AGO3 or AUB proteins through the “ping-pong cycle” to form complexes in the nucleus. Given the complementary piRNA sequences of these two complexes, the piRNA/AGO3 complex splits target RNA in the cytoplasm to produce corresponding new sequences, which can then be used as substrates to bind to AUB proteins in the nucleus to form new piRNA/AUB complexes. TSS transcription start site; Zuc zucchini