The dosage compensation complex (DCC) of Drosophila identifies its X chromosomal binding sites with exquisite selectivity. The principles that assure this vital targeting are known from the D. melanogaster model: DCC-intrinsic specificity of DNA binding, cooperativity with the CLAMP protein, and non-coding roX2 RNA transcribed from the X chromosome. We found that in D. virilis, a species separated from melanogaster by 40 million years of evolution, all principles are active, but contribute differently to X-specificity. In melanogaster, the DCC subunit MSL2 evolved intrinsic DNA-binding selectivity for rare PionX sites, which mark the X chromosome. In virilis, PionX sites are abundant and not X-enriched. Accordingly, MSL2 lacks specific recognition. Here, roX2 RNA plays a more instructive role, counteracting a non-productive interaction of CLAMP and modulating DCC binding selectivity. Remarkably, roX2 triggers a low-diffusion chromatin binding mode characteristic of DCC. Evidently, X-specific regulation is achieved by divergent evolution of similar components.
Overall design: ChIP and DIP experiments with different target proteins and addition of accessory factors.
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