N-terminus of Drosophila MSL1 is critical for dosage compensation

The male-specific dosage compensation complex (DCC), which consists of five proteins and two non-coding roX RNAs, is necessary for the transcriptional enhancement of X-linked genes to compensate for the sex chromosome monosomy in Drosophila XY males, compared with XX females. The MSL1 and MSL2 proteins form the heterotetrameric core of DCC and are critical for the specific recruitment of the DCC to the high-affinity “entry” sites (HAS) on the X chromosome. Here we demonstrated that the N-terminal region of MSL1 is critical for its stability and functions. Amino acid deletions and substitutions in the N-terminal region of MSL1 strongly affect both interaction with roX2 RNA and DCC binding to HAS on the X chromosome. In particular, substitution of the conserved N-terminal amino-acids 3-7 in MSL1GS has an affect on dosage compensation similar to inactivation of genes encoding roX RNAs. MSL1GS binds to promoters like MSL1WT but does not co-bind with MSL2 and MSL3 to X chromosomal HAS. However, over-expression of MSL2 partially restores the functional activity of MSL1GS in dosage compensation. Thus, the interaction of MSL1 with roX RNA is critical for the efficient assembly of DCCs on HAS of the male X chromosome. ChIP-seq signal of the MSL1, MSL2 and MSL3 proteins occupancy in adult males and females represented as wt, M1[wt], M1[wt]_M2, M1[Δ41-85]_M2, M1[Δ1-15]_M2 and M1[2*G;3*S]_M2 lines.