Condensin is a SMC complex that is involved in genome organisation during interphase and regulates chromosome compaction during cell division. However, the factors that control the dynamics of condensin binding are not fully understood. In this work, Laura Breimann, Sevinç Ercan and co-workers (Breimann et al., 2022) address this issue in Caenorhabditis elegans XX hermaphrodites, where the X chromosome is repressed for dosage compensation by the X-specific condensin (termed condensin DC), which interacts with DPY-21, a histone demethylase that converts H4K20me2 into H4K20me1. Using a FRAP assay employing fluorescently labelled DPY-27, the authors follow condensin DC binding in vivo and demonstrate the importance of DPY-27 ATPase activity for condensin DC association with the X chromosomes. Interestingly, in null mutants for the histone demethylase DPY-21, the mobile fraction of condensin DC is markedly reduced, in contrast to when a catalytic mutant is used, suggesting that DPY-21 has a non-catalytic function that regulates the dynamics of condensin DC binding. Hi-C analysis in the dpy-21 null mutant revealed no significant changes in long-range DNA contacts, indicating that DPY-21 does not act by unloading condensin DC. Taken together, this work demonstrates that DPY-21 regulates both histone modifications and condensin DC mobility to repress X chromosome transcription.
