LSM2-8 and XRN-2 contribute to the silencing of H3K27me3-marked genes through targeted RNA decay

In fission yeast and plants, RNA-processing pathways contribute to constitutive and facultative heterochromatin silencing, complementing well-characterized pathways of transcriptional repression. However, it was unclear whether this additional level of regulation occurs in metazoans. Here we describe a pathway of silencing in C. elegans somatic cells, in which the highly conserved, RNA binding complex LSM2-8 selectively silences heterochromatic reporters and endogenous genes bearing the Polycomb mark H3K27me3. Importantly, the LSM2-8 complex works cooperatively with XRN-2, a 5-3 exoribonuclease, and disruption of the pathway leads to mRNA stabilization. This selective LSM2-8-mediated RNA degradation does not target nor depend on H3K9me2/me3, unlike previously described pathways of heterochromatic RNA degradation. Intriguingly, the loss of LSM2-8 coincides with a localized drop in H3K27me3 levels on lsm-8-sensitive loci only. Together this defines a mechanism of RNA degradation that selectively targets a subset of H3K27me3-marked genes, revealing an unrecognized layer of regulation for facultative heterochromatin in animals.