Translation and codon usage regulate Argonaute slicer activity to trigger germline small RNA biogenesis [ncRNA-seq]

In Caenorhabditis elegans germline, thousands of mRNAs are concomitantly expressed with antisense 22G-RNAs, which are loaded into the Argonaute CSR-1. Despite their essential functions for animal fertility and embryonic development, how CSR-1 22G-RNAs are produced remain unknown. Here, we show that CSR-1 slicer activity is primarily involved in triggering the synthesis of small RNAs on the coding sequences of germline mRNAs and post-transcriptionally regulates a fraction of targets. CSR-1-cleaved mRNAs prime the RNA-dependent RNA polymerase, EGO-1 to synthesize 22G-RNAs concurrently with ribosome translation in the cytoplasm, in contrast to other 22G-RNAs mostly synthesized in germ-granules. Moreover, differences in germline mRNA codon optimality and ribosome occupancy antagonistically affect the abundance of CSR-1 22G-RNAs and the stability of their mRNA targets. We propose that codon usage differences encoded into mRNA sequences might be a conserved strategy adopted in eukaryotes to regulate small RNA biogenesis and Argonaute targeting. 37 samples were analyzed by sRNA-seq, including replicates (sample name contains 'rep').