Background: The male-specific lethal (MSL) complex of Drosophila remodels the chromatin of the X chromosome in males to enhance the level of transcription of most X-linked genes, and thereby achieve dosage compensation. The core complex consists of five proteins and one of two non-coding RNAs. One of the proteins, MOF (males absent on the first), is a histone acetyltransferase that specifically acetylates histone H4 at lysine 16. Another protein, maleless (MLE), is an ATP-dependent helicase with the ability to unwind DNA/RNA or RNA/RNA substrates in vitro. Recently, we showed that the ATPase activity of MLE is sufficient for the hypertranscription of genes adjacent to a high-affinity site by MSL complexes located at that site. The helicase activity is required for the spreading of the complex to the hundreds of positions along the X chromosome, where it is normally found. In this study, to further understand the role of MLE in the function of the MSL complex, we analyzed its relationship to the other complex components by creating a series of deletions or mutations in its putative functional domains, and testing their effect on the distribution and function of the complex in vivo.
Results: The presence of the RB2 RNA-binding domain is necessary for the association of the MSL3 protein with the other complex subunits. In its absence, the activity of the MOF subunit was compromised, and the complex failed to acetylate histone H4 at lysine 16. Deletion of the RB1 RNA-binding domain resulted in complexes that maintained substantial acetylation activity but failed to spread beyond the high-affinity sites. Flies bearing this mutation exhibited low levels of roX RNAs, indicating that these RNAs failed to associate with the proteins of the complex and were degraded, or that MLE contributes to their synthesis. Deletion of the glycine-rich C-terminal region, which contains a nuclear localization sequence, caused a substantial level of retention of the other MSL proteins in the cytoplasm. These data suggest that the MSL proteins assemble into complexes or subcomplexes before entering the nucleus.
Conclusions: This study provides insights into the role that MLE plays in the function of the MSL complex through its association with roX RNAs and the other MSL subunits, and suggests a hypothesis to explain the role of MLE in the synthesis of these RNAs.