MukB acts as a macromolecular clamp in DNA condensation.

Correct folding of the chromosome into its highly ordered structure requires the action of condensins. The multisubunit condensins are highly conserved from bacteria to humans, and at their core they contain the characteristic V-shaped dimer of structural maintenance of chromosome proteins. The mechanism of DNA rearrangements by condensins remains unclear. Using magnetic tweezers, we show that bacterial condensin MukB acts as an ATP-modulated macromolecular assemblage in DNA condensation. Condensation occurs in a highly cooperative manner, resulting in the formation of force-resilient clusters. ATP regulates nucleation but not propagation of the clusters and seems to play a structural role. MukB clusters can further interact with each other, thereby bringing distant DNA segments together. The resulting activity has not previously been described among DNA-remodeling machines and may explain how the protein can organize the global structure of the chromosome.