Figure 2.

ParB complex assembly at parS. (a) Schematic representation of chromosomally encoded ParB protein (dimer) with the indicated functions of individual domains. *- only confirmed for B. subtilis Spo0J. (b) Models of the ParB–ParB interactions involved in formation of the ParB nucleoprotein complexes around parS. (I) Adjacent ParB dimers may interact with each other to form 1D filaments around parS. (II) Interactions between ParB dimers associated with distal DNA fragments may lead to DNA bridging and looping. (III) ParB self-interactions provide a scaffold (cage), attracting and trapping additional ParB molecules. (c) A model illustrating ParB loading at parS and sliding [122]. Free CTP-ParB exists as a dimer in an open conformation. Binding to parS induces conformational changes involving the N-terminal ParB domains and the formation of “closed” ring-shaped molecules. Steric hindrance between HTH motifs interacting with parS in such a closed conformation may prompt the release of ParB rings from parS via their sliding on adjacent DNA and the loading of new ParB dimers at parS. Finally, switching from a closed to open conformation by an unknown mechanism (possibly involving CTP hydrolysis) may lead to ParB’s dissociation from the DNA. The parS sites are indicated in red.