Condensin DC loads and spreads from recruitment sites to create loop-anchored TADs inC. elegans

Condensins are molecular motors that compact DNA via linear translocation. InC. elegans, the X-chromosome harbors a specialized condensin that participates in dosage compensation (DC). Condensin DC is recruited to and spreads from a small number of<underline>r</underline>ecruit<underline>e</underline>lements on the<underline>X</underline>-chromosome (rex) and is required for the formation of topologically associating domains (TADs). We take advantage of autosomes that are largely devoid of condensin DC and TADs to address howrexsites and condensin DC give rise to the formation of TADs. When an autosome and X-chromosome are physically fused, despite the spreading of condensin DC into the autosome, no TAD was created. Insertion of a strongrexon the X-chromosome results in the TAD boundary formation regardless of sequence orientation. When the samerexis inserted on an autosome, despite condensin DC recruitment, there was no spreading or features of a TAD. On the other hand, when a“super rex”composed of sixrexsites or three separaterexsites are inserted on an autosome, recruitment and spreading of condensin DC led to formation of TADs. Therefore, recruitment to and spreading fromrexsites are necessary and sufficient for recapitulating loop-anchored TADs observed on the X-chromosome. Together our data suggest a model in whichrexsites are both loading sites and bidirectional barriers for condensin DC, a one-sided loop-extruder with movable inactive anchor.