DamID for Cid14 in wild-type and swi6∆

HP1 proteins are major components of heterochromatin, which is generally perceived to be an inert and transcriptionally inactive chromatin structure. Yet, HP1 binding to chromatin is highly dynamic and robust silencing of heterochromatic genes can involve RNA processing. Here we demonstrate by a combination of in vivo and in vitro experiments that the fission yeast HP1Swi6 protein guarantees tight repression of heterochromatic genes through RNA sequestration and degradation. Stimulated by positively charged residues in the hinge region, RNA competes with methylated histone H3K9 for binding to the chromodomain of HP1Swi6. Hence, HP1Swi6 binding to RNA is incompatible with stable heterochromatin association. We propose a model in which an ensemble of HP1Swi6 proteins functions as a heterochromatin-specific checkpoint capturing and priming heterochromatic RNAs for the RNA degradation machinery. Sustaining a functional checkpoint requires continuous exchange of HP1Swi6 within heterochromatin, which explains the dynamic localization of HP1 proteins on heterochromatin. Methylation levels in strains expressing Dam-Cid14 fusions were compared to those in a Dam only expressing strain. 2 biological replicates for each strain.