The H4K20 demethylase DPY-21 regulates the dynamics of condensin DC binding

Condensin is a multi-subunit SMC complex that binds to and compacts chromosomes. Unlike cohesin, in vivo regulators of condensin binding dynamics remain unclear. Here we addressed this question using C. elegans condensin DC, which specifically binds to and represses transcription of both X chromosomes in hermaphrodites for dosage compensation. Mutants of several chromatin modifiers that regulate H4K20me and H4K16ac cause varying degrees of X chromosome derepression. We used fluorescence recovery after photobleaching (FRAP) to analyze how these modifiers regulate condensin DC binding dynamics in vivo. We established FRAP using the SMC4 homolog DPY-27 and showed that a well-characterized ATPase mutation abolishes its binding. The greatest effect on condensin DC dynamics was in a null mutant of the H4K20me2 demethylase DPY-21, where the mobile fraction of the complex reduced from ∼30% to 10%. In contrast, a catalytic mutant of dpy-21 did not regulate condensin DC mobility. Separation of catalytic and non-catalytic activity is also supported by Hi-C data in the dpy-21 null mutant. Together, our results indicate that DPY-21 has a non-catalytic role in regulating the dynamics of condensin DC binding, which is important for transcription repression

The H4K20 demethylase DPY-21 regulates the dynamics of condensin DC binding

Condensin is a multi-subunit SMC complex that binds to and compacts chromosomes. Here we addressed the regulation of condensin binding dynamics using C. elegans condensin DC, which represses X chromosomes in hermaphrodites for dosage compensation. We established fluorescence recovery after photobleaching (FRAP) using the SMC4 homolog DPY-27 and showed that a well-characterized ATPase mutation abolishes its binding. Next, we performed FRAP in the background of several chromatin modifier mutants that cause varying degrees of X-chromosome derepression. The greatest effect was in a null mutant of the H4K20me2 demethylase DPY-21, where the mobile fraction of condensin DC reduced from ∼30% to 10%. In contrast, a catalytic mutant of dpy-21 did not regulate condensin DC mobility. Hi-C data in the dpy-21 null mutant showed little change compared to wild type, uncoupling Hi-C measured long-range DNA contacts from transcriptional repression of the X chromosomes. Together, our results indicate that DPY-21 has a non-catalytic role in regulating the dynamics of condensin DC binding, which is important for transcription repression