Drosophila MSL complex globally acetylates H4 Lys16 on the male X chromosome for dosage compensation

Drosophila MSL complex binds the single male X chromosome to upregulate gene expression to equal that from the two female X chromosomes. However, it has been puzzling that ~25% of transcribed genes on the X do not stably recruit MSL complex. Here, we find that almost all active genes on the X are associated with robust H4 Lys16 acetylation (H4K16ac), the histone modification catalyzed by MSL complex. The distribution of H4K16ac is much broader than that of MSL complex, and our results favor the idea that chromosome-wide H4K16ac reflects transient association of MSL complex, occurring through spreading or chromosomal looping. Our results parallel those of localized Polycomb repressive complex and its more broadly distributed H3K27me3 chromatin mark, suggesting a common principle for the establishment of active and silenced chromatin domains.