for reproduction). Thus, this species has multiple condensin I complexes that have different regulatory subunits and display distinct nuclear localization. This is a very unique property that is not found in other species. Prokaryotic types Prokaryotic species also have condensin-like complexes that play an important role in chromosome (nucleoid) organization and segregation. The prokaryotic condensins can be classified into two types: SMC-ScpAB and MukBEF. Many eubacterial and archaeal species have SMC-ScpAB, whereas a subgroup of eubacteria (known as Gammaproteobacteria) including Escherichia coli has MukBEF. ScpA and MukF belong to a family of proteins called "kleisins", whereas ScpB and MukF have recently been classified into a new family of proteins named "kite". Despite highly divergent primary structures of their corresponding subunits between SMC-ScpAB and MukBEF, it is reasonable to consider that the two complexes play similar if not identical functions in prokaryotic chromosome organization and dynamics, based on their molecular architecture and their defective cellular phenotypes. Both complexes are therefore often called prokaryotic (or bacterial) condensins. Recent studies report the occurrence of a third complex related to MukBEF (termed MksBEF) in some bacterial species. Molecular mechanisms Molecular structures SMC dimers that act as the core subunits of condensins display a highly characteristic V-shape, each arm of which is composed of anti-parallel coiled-coils (Figure 3; see SMC proteins for details). The length of each coiled-coil arm reaches ~50 nm, which corresponds to the length of ~150 bp of double-stranded DNA (dsDNA). In eukaryotic condensin I and II complexes, a kleisin subunit bridges the two head domains of an SMC dimer, and binds to two HEAT repeat subunits (Figure 1). Early studies elucidated the structure of parts of bacterial condensins, such as MukBEF and SMC-ScpA. In eukaryotic complexes, several structures of subcomplexes and subdomains have been reported, including the hinge and arm domains of an SMC2-SMC4 dimer, a CAP-G(ycg1)/CAP-H(brn1) subcomplex, and a CAP-D2(ycs4)/CAP-H(brn1) subcomplex. A recent cryo-EM study has shown that condensin undergoes large conformational changes that are coupled with ATP-binding and hydrolysis by its SMC subunits. On the other hand, fast-speed atomic force microscopy has demonstrated that the arms of an SMC dimer is far more flexible than was expected. Molecular activities Condensin I purified from Xenopus egg extracts is a DNA-stimulated ATPase and displays the ability to introduce positive superhelical tension into dsDNA in an ATP-hydrolysis-dependent manner (positive supercoiling activity). Similar activities have been detected in condensins from other organisms. The