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The Escherichia coli SMC complex, MukBEF, forms clusters of molecules that interact with the decatenase topisomerase IV and which are normally associated with the chromosome replication origin region (ori). Here we demonstrate an additional ATP-hydrolysis-dependent association of MukBEF with the replication termination region (ter). Consistent with this, MukBEF interacts with MatP, which binds matS sites in ter. MatP displaces wild-type MukBEF complexes from ter, thereby facilitating their association with ori, and limiting the availability of topoisomerase IV (TopoIV) at ter. Displacement of MukBEF is impaired when MukB ATP hydrolysis is compromised and when MatP is absent, leading to a stable association of ter and MukBEF. Impairing the TopoIV-MukBEF interaction delays sister ter segregation in cells lacking MatP. We propose that the interplay between MukBEF and MatP directs chromosome organization in relation to MukBEF clusters and associated topisomerase IV, thereby ensuring timely chromosome unlinking and segregation.

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The bacterial septum-located DNA translocase FtsK coordinates circular chromosome segregation with cell division. Rapid translocation of DNA by FtsK is directed by 8-base-pair DNA motifs (KOPS), so that newly replicated termini are brought together at the developing septum, thereby facilitating completion of chromosome segregation. Translocase functions reside in three domains, α, β and γ. FtsKαβ are necessary and sufficient for ATP hydrolysis-dependent DNA translocation, which is modulated by FtsKγ through its interaction with KOPS. By solving the FtsKγ structure by NMR, we show that γ is a winged-helix domain. NMR chemical shift mapping localizes the DNA-binding site on the γ domain. Mutated proteins with substitutions in the FtsKγ DNA-recognition helix are impaired in DNA binding and KOPS recognition, yet remain competent in DNA translocation and XerCD-dif site-specific recombination, which facilitates the late stages of chromosome segregation.The 4.6-million-base-pair (bp) circular Escherichia coli chromosome is replicated bidirectionally from a unique origin (oriC), with replication termination occurring in a broad ter region opposite oriC (reviewed in refs. 1,2). Therefore, replication divides the chromosome into two equal arms or replichores, which locate to separate halves of the E. coli cell and behave as units of chromosome segregation 3 . Each replichore is transcribed predominantly in the same direction as replication and contains distinctive asymmetric, base composition-skewed sequences on each strand. These have been implicated in recombination, genome stability and chromosome processing 1,2,4,5 . Separation and segregation of newly replicated sister chromosomes can be compromised by catenation and chromosome dimer formation. Site-specific recombination mediated by the XerCD recombinase occurs between newly replicated sister dif sites, located in ter, and acts in sister-chromosome segregation by converting any chromosome dimers to monomers and by © 2006 Nature Publishing Group Correspondence should be addressed to D.J.S. (sherratt@bioch.ox.ac.uk).. 4 These authors contributed equally to this work. AUTHOR CONTRIBUTIONS V.S., molecular biology and manuscript preparation. M.D.A., biochemistry, NMR and manuscript preparation. C.d.B., molecular biology. R.B., molecular biology. L.K.A., project direction and manuscript preparation. S.M.F., NMR. M.B., NMR. J.L., molecular biology, structural biology, project direction and manuscript preparation. D.J.S., project conception, project direction and manuscript preparation.Supplementary information is available on the Nature Structural & Molecular Biology website. COMPETING INTERESTS STATEMENTThe authors declare that they have no competing financial interests. Fig. 1a,b), containing helices H1, H2 and H3 and the C-terminal wing. We have not assigned or constrained secondary structure in the wing; in other WHD proteins, the wing usually has very few residues in true β-sheet conformation. WHDs are a subtype of helix-turn-helix DNA-binding proteins in ...

The Escherichia coli SMC complex, MukBEF, acts in chromosome segregation. MukBEF shares the distinctive architecture of other SMC complexes, with one prominent difference; unlike other kleisins, MukF forms dimers through its N-terminal domain. We show that a 4-helix bundle adjacent to the MukF dimerisation domain interacts functionally with the MukB coiled-coiled ‘neck’ adjacent to the ATPase head. We propose that this interaction leads to an asymmetric tripartite complex, as in other SMC complexes. Since MukF dimerisation is preserved during this interaction, MukF directs the formation of dimer of dimer MukBEF complexes, observed previously in vivo. The MukF N- and C-terminal domains stimulate MukB ATPase independently and additively. We demonstrate that impairment of the MukF interaction with MukB in vivo leads to ATP hydrolysis-dependent release of MukBEF complexes from chromosomes.