One Sentence Summary: Single-molecule imaging reveals that eukaryotic condensin is a highly processive DNA-translocating motor complex. Abstract:Condensin plays crucial roles in chromosome organization and compaction, but the mechanistic basis for its functions remains obscure. Here, we use single-molecule imaging to demonstrate that Saccharomyces cerevisiae condensin is a molecular motor capable of ATP hydrolysis-dependent translocation along double-stranded DNA. Condensin's translocation activity is rapid and highly processive, with individual complexes traveling an average distance of ≥10 kilobases at a velocity of ~60 base pairs per second. Our results suggest that condensin may take steps comparable in length to its ~50-nanometer coiled-coil subunits, suggestive of a translocation mechanism that is distinct from any reported DNA motor protein. The finding that condensin is a mechanochemical motor has important implications for understanding the mechanisms of chromosome organization and condensation.Main Text: Structural maintenance of chromosomes (SMC) complexes are the major organizers of chromosomes in all living organisms (1, 2). These protein complexes play essential roles in sister chromatid cohesion, chromosome condensation and segregation, DNA replication, DNA damage repair, and gene expression. A distinguishing feature of SMC complexes is their large ring-like architecture, the circumference of which is made up of two SMC protein coiled-coil proteins and a single kleisin subunit (Fig.1A) (1-4). The ~50-nm long antiparallel coiled-coils are connected at one end by a stable dimerization interface, referred to as the hinge domain, and at the other end by globular ATP-binding cassette (ABC) family ATPase domains (5). The ATPase domains are bound by a protein of the kleisin family, along with additional accessory subunits, which vary for different types of SMC complexes (Fig. 1A). The relationship between SMC peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/137711 doi: bioRxiv preprint first posted online May. 13, 2017; 2 structures and their functions in chromosome organization is not completely understood (6), but many models envision that the coiled-coil domains allow the complexes to topologically embrace DNA (1-4). Given the general resemblance to myosin and kinesin, some early models postulated that SMC proteins might be mechanochemical motors (7-10).SMC complexes are thought to regulate genome architecture by physically linking distal chromosomal loci, but how these bridging interactions might be formed remains unknown (1, 2, 11). An early model suggested that many three-dimensional (3D) features of eukaryotic chromosomes might be explained by DNA loop extrusion (Fig. 1B) (12), and recent polymer dynamics simulations have shown that loop extrusion can recapitulate the formation of topologically associating domains (TADs), chromatin compaction, and sister chromatid segr...