Structural Basis of Condensin Recruitment for X Chromosome Repression

In Caenorhabditis elegans , the condensin I DC complex represses transcription from both X chromosomes in hermaphrodites to achieve dosage compensation. How condensin I DC is specifically recruited to the X chromosomes in coordination with sex determination and dosage compensation (SDC) proteins and how it modulates gene expression have, however, remained unresolved. Here, we identify SDC-3 as the key adaptor that directly binds the ‘elbow’ coiled-coil domain of the condensin I DC -specific SMC subunit DPY-27. Using cryo-electron microscopy, we determine the structure of the SDC-3 adaptor domain bound to an auto-inhibited condensin I DC holoenzyme. Disrupting this interaction compromises dosage compensation and diminishes condensin I DC enrichment on the X chromosomes. Upon overcoming auto-inhibition, condensin I DC exhibits robust DNA loop-extrusion activity comparable to that of canonical condensin. We propose that SDC-3-anchored condensin I DC exploits loop-extrusion to reorganize X-chromosome chromatin and mediate chromosome-wide transcriptional repression.