"Eukaryotic nuclei are organised into functional compartments, – local microenvironments that are enriched in certain molecules or biochemical activities and therefore specify localised functional outputs. Our study seeks to unveil fundamental principles of co-regulation of genes in a chromo¬somal compartment and the preconditions for homeostasis of such a compartment in the dynamic nuclear environment.The dosage-compensated X chromosome of male Drosophila flies satisfies the criteria for a functional com¬partment. It is rendered structurally distinct from all other chromosomes by association of a regulatory ribonucleoprotein ‘Dosage Compensation Complex’ (DCC), enrichment of histone modifications and global decondensation. As a result, most genes on the X chromosome are co-ordinately activated. Autosomal genes inserted into the X acquire X-chromosomal features and are subject to the X-specific regulation.We seek to uncover the molecular principles that initiate, establish and maintain the dosage-compensated chromosome. We will follow the kinetics of DCC assembly and the timing of association with different types of chromosomal targets in nuclei with high spatial resolution afforded by sub-wavelength microscopy and deep sequencing of DNA binding sites. We will characterise DCC sub-complexes with respect to their roles as kinetic assembly intermediates or as representations of local, functional heterogeneity. We will evaluate the roles of a DCC- novel ubiquitin ligase activity for homeostasis.Crucial to the recruitment of the DCC and its distribution to target genes are non-coding roX RNAs that are transcribed from the X. We will determine the secondary structure ‘signatures’ of roX RNAs in vitro and determine the binding sites of the protein subunits in vivo. By biochemical and cellular reconstitution will test the hypothesis that roX-encoded RNA aptamers orchestrate the assembly of the DCC and contribute to the exquisite targeting of the complex."