Linker histone dH1K27 dimethylation marks Drosophila heterochromatin independently of H3K9 methylation

Post-translational modifications (PTMs) of histones are important epigenetic determinants and specific core histones PTMs correlate with functional chromatin states. However, despite linker histone H1s are heavily post-translationally modified, little is known about the genomic distribution of H1s PTMs and their association with epigenetic chromatin states. Here, we address this question in Drosophila that encodes a single somatic linker histone, dH1. We previously reported that dH1 is dimethylated at K27 (dH1K27me2). Here, we show that dH1K27me2 is a major PTM of Drosophila heterochromatin. At mitosis, dH1K27me2 accumulates at pericentromeric heterochromatin, while, in interphase cells, it is also detected at intercalary heterochromatin. ChIPseq experiments show that dH1K27me2 enriched regions cluster at both the assembled and unassembled heterochromatin regions of all four Drosophila chromosomes. More than 98% of the dH1K27me2 enriched regions map to heterochromatic repetitive DNA elements, including transposable elements, simple DNA repeats and satellite DNAs. We also show that dH1K27me2 is independent of H3K9 methylation, as it is equally detected in flies carrying a H3K9R mutation. Moreover, dH1K27me2 is not affected by depletion of Su(var)3-9, HP1a and Su(var)4-20. Altogether these results suggest that dH1K27me2 is a novel epigenetic mark of Drosophila heterochromatin that acts upstream of the major Su(var)3- 9/HP1a pathway of heterochromatin formation.