Normal Patterns of Histone H3K27 Methylation Require the Histone Variant H2A.Z in Neurospora crassa .

Neurospora crassacontains a minimal Polycomb repression system, which provides rich opportunities to explore Polycomb-mediated repression across eukaryotes and enables genetic studies that can be difficult in plant and animal systems. Polycomb Repressive Complex 2 is a multi-subunit complex that deposits mono-, di-, and trimethyl groups on lysine 27 of histone H3, and trimethyl H3K27 is a molecular marker of transcriptionally repressed facultative heterochromatin. In mouse embryonic stem cells and multiple plant species, H2A.Z has been found to be colocalized with H3K27 methylation. H2A.Z is required for normal H3K27 methylation in these experimental systems, though the regulatory mechanisms are not well understood. We report here thatNeurospora crassamutants lacking H2A.Z or SWR-1, the ATP-dependent histone variant exchanger, exhibit a striking reduction in levels of H3K27 methylation. RNA-sequencing revealed downregulation ofeed, encoding a subunit of PRC2, in anhH2Azmutant compared to wild type, and overexpression of EED in a Delta hH2Az;Delta eedbackground restored most H3K27 methylation. Reducedeedexpression leads to region-specific losses of H3K27 methylation, suggesting that differential dependence on EED concentration is critical for normal H3K27 methylation at certain regions in the genome.