The essential process of dosage compensation, which corrects for the imbalance in Xlinked gene expression between XX females and XY males, represents a key model for how genes are targeted for coordinated regulation. However, the mechanism by which dosage compensation complexes identify the Xchromosome during early development remained unknown because of the difficulty of sexing embryos prior to zygotic transcription. We used meiotic drive to sex Drosophila embryos prior to zygotic transcription and ChIPseq to measure dynamics of dosage compensation factor targeting. The Drosophila MaleSpecific Lethal dosage compensation complex (MSLc) requires the ubiquitous zincfinger protein ChromatinLinked Adaptor for MSL Proteins (CLAMP) to identify the Xchromosome. We observe a multistage process in which MSLc first identifies CLAMP binding sites throughout the genome followed by concentration at the strongest Xlinked MSLc sites. We provide insight into the dynamic mechanism by which a large transcription complex identifies its binding sites during early development.

The binding of Drosophila male-specific lethal (MSL) dosage compensation complex exclusively to male X chromosome provides an excellent model system to understand mechanisms of selective recruitment of protein complexes to chromatin. Previous studies showed that the male-specific organizer of the complex, MSL2, and ubiquitous DNA-binding protein CLAMP are key players in the specificity of X chromosome binding. The CXC domain of MSL2 binds to genomic sites of MSL complex recruitment. Here we demonstrated that MSL2 directly interacts with the Nterminal zinc-finger domain of CLAMP. CLAMP-MSL2 and CXC-DNA interactions are cooperatively involved in recruitment of MSL complex to the X chromosome. KeywordsMSL2/ Dosage compensation/ sex determination/ transcription factor/ zinc finger C2H2