Orateur
Description
Nucleosomes and DNA-loop extruding SMC protein-complexes such as condensin and cohesin shape the 3D genome in eukaryotes, but their mutual functional relationships remain largely unclear. Using the fission yeast Schizosaccharomyces pombe as model, we investigated the interplays between nucleosomes and condensin-mediated mitotic chromosome assembly. We found that purified condensin fails to bind DNA wrapped into nucleosomes, regardless of whether they contain canonical or variant histones, whereas subnucleosomal particles permit condensin binding. In vivo, we found that condensin associates with the histone chaperone FACT and the nucleosome remodeler Chd1/Hrp1, both key players in transcription-coupled nucleosome dynamics. Acute depletion of FACT subunit Spt16 in metaphase-arrested cells causes an extensive loss of nucleosomes, likely due to the role played by FACT in restoring nucleosome architecture in the wake of mitotic transcription. This nucleosome loss increases the frequency of mitotic DNA loops and enhances mitotic chromosome condensation in a condensin-dependent manner, yet without increasing condensin occupancy. Lack of Chd1/Hrp1 or reduced histone gene dosage similarly facilitates condensin activity. Depleting RNA Pol II further revealed that condensin does not merely benefit from transcription-mediated nucleosome remodeling. Our results suggest that nucleosomes act as barriers to condensin-mediated loop extrusion in vivo and that condensin relies on dedicated nucleosome remodelling to bypass this obstruction.