LAUSR

Tethering DNA for packing purposes Condensin protein complexes are critical for chromosome segregation and compaction. They form ring-shaped structures that encircle and topologically constrain DNA strands. Wang et al. show that Bacillus subtilis condensin complexes hold the two arms of the circular chromosome together (see the Perspective by Sherratt). The complexes seem to do this by encircling individual DNA duplexes and then tethering the two duplexes together by “handcuffing.” The complexes actively travel along the DNA and function to enlarge DNA loops processively, leading to chromosome compaction. Science, this issue p. 524; see also p. 460 Condensin ring complexes function to extrude DNA loops by tethering DNA strands together and actively moving along them. Structural maintenance of chromosomes (SMC) complexes play critical roles in chromosome dynamics in virtually all organisms, but how they function remains poorly understood. In the bacterium Bacillus subtilis, SMC-condensin complexes are topologically loaded at centromeric sites adjacent to the replication origin. Here we provide evidence that these ring-shaped assemblies tether the left and right chromosome arms together while traveling from the origin to the terminus (>2 megabases) at rates >50 kilobases per minute. Condensin movement scales linearly with time, providing evidence for an active transport mechanism. These data support a model in which SMC complexes function by processively enlarging DNA loops. Loop formation followed by processive enlargement provides a mechanism by which condensin complexes compact and resolve sister chromatids in mitosis and by which cohesin generates topologically associating domains during interphase.