PICKLE-mediated nucleosome condensing drives H3K27me3 spreading for the inheritance of Polycomb memory during differentiation [RNA-Seq]

Nucleation and spreading of H3K27me3 are critical steps for the initiation and maintenance, respectively, of mitotically inheritable Polycomb-mediated repressive chromatin states in cell identity memory, in both animals and plants. Although a semiconservative read-and-write mechanism is proposed to play a key role in H3K27me3 propagation, the specific determinant and underlying mechanism by which the Polycomb Repressive Complex 2 (PRC2) accesses unmodified nucleosomes for propagating H3K27me3 remain enigmatic. Using Arabidopsis thaliana as a model, we show that the chromatin remodeling activity of PICKLE (PKL) has a specialized role in H3K27me3 spreading to safeguard cell identity during differentiation. PKL specifically binds to H3K27me3 spreading regions but not nucleation sites of Polycomb target genes and physically interacts with the MSI1 subunit of PRC2. Loss of PKL abolishes the occupancy of the PRC2 catalytic subunit CLF in spreading regions and leads to aberrant differentiation. Nucleosome condensing endowed by the ATPase function of PKL ensures that unmodified nucleosomes are accessible to PRC2 catalytic activity. Our findings highlight that PKL-dependent nucleosome compaction is critical for PRC2-mediated H3K27me3 read-and-write functions in H3K27me3 spreading, thus revealing a mechanism by which repressive chromatin domains are established and propagated. Overall design: Examination of global RNA expression in 14-day-old wt and various mutants seedlings.