Stable Heritable Germline Silencing Directs Somatic Silencing at an Endogenous Locus
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AbstractDouble stranded RNA (dsRNA) introduced into C. elegans can move throughout the worm to initiate RNAi-mediated silencing in recipient cells. DsRNA movement, both within a worm and between generations, requires the conserved dsRNA-transporting protein SID-1. I have found that a transgenic multi-copy array of the sid-1 upstream intergenic region (promoter and 5' UTR) silences sid-1 and upstream genes in the germline and soma. This is surprising for two reasons. First, arrays containing multiple copies of promoters have never been shown to stably silence endogenous genes. Second, heritable epigenetic gene silencing has not previously been described in the soma in C. elegans. Once silenced, the sid-1 locus can remain silenced for up to 13 generations in the absence of the array. Reminiscent of paramutation, a silenced sid-1 locus can silence a naive locus introduced by mating in the germline. Surprisingly, transmission of silencing does not require transmission of chromatin. Instead, small interfering RNAs (siRNAs) mediate sid-1 silencing in the germline; worms containing the sid-1 promoter array have a three-fold increase in small RNAs targeting the sid-1 locus. This increase is maintained even in the absence of the array indicating that these small RNAs are likely the transgenerational signal that epigenetically silences sid-1 in the germline. Genetic analysis reveals that efficient initiation of sid-1 silencing partially requires the piRNA-stabilizing Argonaute PRG-1 and that maintenance of sid-1 silencing requires HRDE-1, an Argonaute that stabilizes secondary siRNAs in the germline nuclear RNAi pathway. Further, I identify the specific endogenous piRNA that is required for efficient initiation of silencing. While the germline nuclear RNAi pathway is required for promoter-mediated sid-1 silencing in the germline, the somatic nuclear RNAi pathway is not required for sid-1 silencing in the soma. Instead, I find that multi-generational somatic silencing of sid-1 specifically requires several chromatin modifying enzymes, leading to the intriguing possibility that the mechanisms of transgenerational silencing in the soma and germline are distinct.
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