Analysis of histone modification interplay reveals two distinct domains in facultative heterochromatin in Pyricularia oryzae.

Histone post-translational modifications (PTMs) interact in complex ways to regulate chromatin structure and gene expression. To investigate this interplay, we analyze ChIP-seq and RNA-seq data from knock-out mutants lacking enzymes responsible for H3K4me2/3, H3K9me3, or H3K27me3 in the phytopathogenic fungus Pyricularia oryzae. Loss of specific PTMs alters other PTMs and gene expression in a compartment-specific manner, with distinct effects across H3K4me2-rich euchromatin (EC), H3K27me3-rich facultative heterochromatin (fHC), H3K9me3-rich constitutive heterochromatin (cHC), and centromeres. We identify two distinct fHC subcompartments: K4-fHC, adjacent to EC, and K9-fHC, adjacent to cHC. Both contain poorly conserved genes, but K9-fHC harbors more transposable elements, while K4-fHC is more enriched for genes upregulated during infection, including effector-like genes. H3K27me3 levels in K4-fHC respond to changes in other PTMs, especially H3K9me3, and to environmental conditions. These findings suggest that K4-fHC functions as a reservoir of genes highly responsive to chromatin context and environmental cues.