Histone H4 acetyl-methyllysine marks accessible chromatin that resists compaction

Regulatory DNA often remains accessible even under conditions of widespread chromatin compaction. Specific factors and histone modifications mark these regions and help maintain local accessibility. Here, we examine the genomic landscape of acetyl-methyllysine (Kacme), a recently discovered histone post-translational modification. Across multiple systems, Kacme is highly enriched at sites of accessible chromatin, including active promoters, enhancers, silencers, and CTCF-binding sites. We find that Kacme is selectively retained at loci that resist condensation during mitosis, marks XIST and escapee regions on the inactive X chromosome in female cells, and demarcates the boundaries of broad heterochromatin domains. Using the KDM6A/6B inhibitor GSK-J4 to elevate H3K27me3 levels, we show that Kacme-marked insulator elements block heterochromatin spreading and protect adjacent genes from transcriptional repression. Taken together, our findings define the chromatin features associated with Kacme and support a model in which Kacme safeguards chromatin accessibility, particularly at loci that remain active despite compaction and silencing pressures.