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Target-dependent suppression of siRNA production modulates the levels of endogenous siRNAs in C. elegans germline.

Despite the prominent role of endo-siRNAs in transposon silencing, their expression is not limited to these "nonself" DNA elements. Transcripts of protein-coding genes ("self" DNA) in some cases also produce endo-siRNAs in yeast, plants, and animals (Piatek and Werner 2014). How cells distinguish these two populations of siRNAs to prevent unwanted silencing of active genes in animals is not well understood. To address this question, we inserted various self-gene or gfp fragments into an LTR retrotransposon that produces abundant siRNAs and examined the propensity of these gene fragments to produce ectopic siRNAs in C. elegans germline. We found that fragments of germline genes are generally protected from production of ectopic siRNAs. This phenomenon, which we termed "target-directed suppression of siRNA production" (or siRNA suppression), is dependent on the germline expression of target mRNA and requires germline P-granule components. We found that siRNA suppression can also occur to naturally produced endo-siRNAs. We suggest that siRNA suppression plays an important role in regulating siRNA expression and preventing self-genes from aberrant epigenetic silencing.