Figure 1.

RNAa in C. elegans. piRNA genes encoded in the genome as clusters are individually transcribed in a process that requires a conserved upstream motif. The resulted piRNA precursors are then processed into mature 21U-RNAs in the cytoplasm through several undefined steps. The 21U-RNAs form a complex with PRG-1 protein and guide the complex to non-perfectly matched targets on endogenous mRNAs where PRG-1 further recruits RdRP to generate secondary 22G-RNAs. After being loaded by CSR-1, the 22G-RNAs then target CSR-1 to cognate nascent transcripts to interact with RNAP II and local chromatin where CSR-1 can further recruit histone modifying enzymes such as histone methyltransferases (HMTs) to promote epigenetic activation (H3K4me or H3K36me3).