A C. elegans Piwi, PRG-1, regulates 21U-RNAs during spermatogenesis.

Epigenetic regulation by diverse classes of small RNAs is mediated by the highly conserved Argonaute/Piwi family of proteins. Although Argonautes are broadly expressed, the Piwi subfamily primarily functions in the germ line. Piwi proteins are associated with germline-specific ribonucleoprotein (RNP) granules in Drosophila, zebrafish, and mouse. Depending on the species and on the specific family member, Piwis play important roles in spermatogenesis and/or in maintaining germ cell and stem cell totipotency. Piwis bind to a newly discovered class of small RNAs, called piRNAs. C. elegans contains a large set of Argonaute/Piwi-related proteins, including two closely related to piwi called prg-1 and prg-2. The function of prg-1 and prg-2 and whether piRNAs exist in C. elegans is unknown. Here, we demonstrate that the Piwi-like protein PRG-1 is localized to P granules in germ cells entering spermatogenesis and is required for successful spermatogenesis. Loss of prg-1 causes a marked reduction in expression of a subset of mRNAs expressed during spermatogenesis, and prg-1 mutant sperm exhibit extensive defects in activation and fertilization. Moreover, prg-1 activity is required for the presence of the small RNAs called 21U-RNAs. Our data suggest that PRG-1 promotes expression, processing, or stability of 21U-RNAs, which, in turn or in concert with PRG-1, promote proper expression of spermatogenesis transcripts.