(A) Top panel; Schematics of possible configurations of MukB and MukF interactions in ATP-bound head-engaged asymmetric complexes of MukBEF. We favour the trans-configuration, identical to those of other SMC complexes. Bottom panel; possible unengaged head complexes. The cartoon on the left is a repeat of that in Figure 1, but additionally indicating that the MukF 4-helix bundles may interact with the same MukB molecule as its C-terminal domain (cis), or the partner MukB molecule (trans). On the right, is a cartoon indicating how daisy-chained multimers could form; these have not been detected in the studies here. (B) Rock- (or rope-) climber model for DNA transport and loop extrusion by MukBEF, modified from (Badrinarayanan et al., 2012a). For clarity only MukBF is shown. The paths of DNA (blue lines; not to scale) are hypothetical, although DNA interactions with the MukB head and hinge have been demonstrated (Kumar et al., 2017; Woo et al., 2009). The state shown in the middle panel (parentheses) could rarely exist if ATP binding and hydrolysis were to be coordinated between the two MukBEF dimers. For presentational simplicity, we have shown MukBEF ring opening through head disengagement, by release and transfer of the MukF C-terminal domain to the ‘cis-configuration’, although results here and elsewhere provide stronger support for ring opening through release of the MukF N-terminal domain from the MukB neck. In reality, the patterns of DNA binding and release, and the conformational changes in the complexes are likely to be more complicated, with both an upper and a lower chamber in each dimeric complex (for example, see Diebold-Durand et al., 2017; Uhlmann, 2016).