A Nuclear RNA Degradation Pathway Helps Silence Polycomb/H3K27me3-Marked Loci in Caenorhabditis elegans

In fission yeast and plants, RNA-processing pathways contribute to heterochromatin silencing, complementing well-characterized pathways of transcriptional repression. However, it was unclear whether this additional level of regulation occurs in metazoans. In a genetic screen, we uncovered a pathway of silencing in Caenorhabditis elegans somatic cells, whereby the highly conserved, RNA-binding complex LSM2-8 contributes to the repression of heterochromatic reporters and endogenous genes bearing the Polycomb mark H3K27me3. Importantly, the LSM2-8 complex works cooperatively with a 5'-3' exoribonuclease, XRN-2, and disruption of the pathway leads to selective mRNA stabilization. LSM2-8 complex-mediated RNA degradation does not target nor depend on H3K9me2/me3, unlike previously described pathways of heterochromatic RNA degradation. Up-regulation of lsm-8-sensitive loci coincides with a localized drop in H3K27me3 levels in the lsm-8 mutant. Put into the context of epigenetic control of gene expression, it appears that targeted RNA degradation helps repress a subset of H3K27me3-marked genes, revealing an unappreciated layer of regulation for facultative heterochromatin in animals.