Outcomes of MSL complex mutations on X chromosome dosage compensation by ChIP

Confinement of the X chromosome into a territory for dosage compensation (DC) is a prime example of subnuclear compartmentalization for transcription regulation at the megabase scale. In D. melanogaster, two sex-specific non-coding (nc) RNAs roX1 and roX2 are transcribed from the X chromosome. They associate with the Male-specific lethal (MSL) complex, which by acetylating histone H4 lysine 16 (H4K16ac) confers approximately 2-fold upregulated expression of male X-linked genes. Current models explain X-over-autosome specificity based on the MSL2 subunit recognizing cis-regulatory DNA high-affinity sites (HAS). However, HAS motifs are also found on autosomes, indicating that additional factors have evolved to confer stable association of the MSL complex with the X. Here, we show that the low-complexity C-terminal domain (CTD) of MSL2 renders its recruitment to the X chromosome sensitive to roX ncRNAs. roX-MSL2-CTD form a stably condensated state, and functional analyses in Drosophila and mammalian cells reveal the critical importance of their interplay for DC in vivo. Replacing the CTD of mammalian MSL2 with that from Drosophila and expressing roX in cis is sufficient to nucleate ectopic DC in mammalian cells. Thus, the condensating nature of roX-MSL2CTD is the primary determinant for specific compartmentalization of the X in Drosophila. ChIP-seq profiles of H4K16ac and FLAG-tagged MSL2-transgenes in Drosophila melanogaster. Separated male and female third instar larva.