Figure 7: Model of the interplay between MukBEF, TopoIV and MatP in chromosome positioning, unlinking and segregation.

Outer circle (red arrows; clockwise from top). In wild-type newborn cells, MukBEF clusters and associated TopoIV are positioned at midcell by an unknown mechanism. ori associates with these clusters. MukBEF complexes load onto DNA in ATP hydrolysis-dependent reactions. Complexes that encounter MatP-matS undergo ATP-hydrolysis-dependent dissociation from ter, thereby depleting the ter region from MukBEF whether in single complexes or in clusters, (and thereby facilitating the ori association). After replication initiation, MukBEF clusters localize to the nucleoid quarter positions, followed by the newly segregated oris⁴. The process that directs the ori to the MukBEF clusters is unknown, but appears to require multiple cycles of ATP binding and hydrolysis, could involve repeated interactions with the chromosome by a ‘rock-walker’¹³ or ‘zipping’^47,48 mechanism, and may facilitate chromosome ‘organization’. Directional ‘zipping’ of the chromosome through MukBEF clusters, would result in active segregation or translocation of newly replicated oris to the cell quarter positions. MukBEF complexes interact with TopoIV when not associated with MatP-matS, thereby enhancing decatenation other than in regions bound by MatP-matS. Inner circle (dark arrows; clockwise from top). In ΔmatP cells, the positioned MukBEF clusters associate with ori and ter, because the lack of MatP prevents displacement of ter from MukBEF complexes. Consequently, the MukBEF–TopoIV interaction leads to enhanced decatenation at ter and precocious segregation of newly replicated ters.