Mechanism of selective incorporation of lncRNA into a chromatin modifier

The MLE DExH helicase and the roX lncRNAs are essential components of the chromatin modifying Dosage Compensation Complex (DCC) in Drosophila. To explore the mechanism of ribonucleoprotein complex assembly, we designed vitRIP, an unbiased, transcriptome-wide in vitro assay that reveals RNA binding specificity. We found that MLE has intrinsic specificity for U-rich sequences and tandem stem-loop structures. In vitro, the helicase binds and remodels many RNAs beyond its main target, roX2. Unwinding of roX2 by the helicase triggers their selective association with the DCC, via the MSL2 subunit. Whereas the core DCC alone does not show intrinsic RNA binding specificity, the presentation of remodeled roX2 by MLE induces a highly selective RNA binding surface in the unstructured C-terminus of MSL2. The exquisite selectivity of roX2 incorporation into the DCC thus originates from intimate cooperation between the helicase and the core DCC involving two distinct RNA selection principles and their mutual refinement. Overall design: RNA-seq of in vitro and in vivo RIP experiments. IP and Input samples in at least 2 biological replicates per condition.