(A) A working model for ParB network formation and concomitant DNA condensation incorporating ideas from this work and other published studies (see main text for details). At least two DNA:protein and three protein:protein interfaces have been identified (greek letters), which together result in the formation of ParB networks on DNA, but only dimers in free solution. In free solution, the interfaces supporting dimerisation of ParB are unclear, but scenarios in which the CTD is dimerised may be considered to be more likely because the purified CTD has been shown to be a tight dimer. (B) Hypothetical mechanisms to explain CTD-induced DNA condensation. Scenario 1: The CTD dimer (which can bind dsDNA but not support condensation itself) simply competes for the DNA substrate in the magnetic tweezers, displacing full length ParB and therefore decondensing the DNA. Alternatively, the CTD exchanges with full length ParB forming inactive heterodimers in free solution that can no longer exchange with the ParB network (Scenario 2) or heterooligomers that retain DNA binding activity but which are not able to condense DNA because important bridging interactions are ‘capped’ by the CTD (Scenario 3). In scenario 3, we cannot exclude the idea that the interaction with the truncated CTD somehow has an allosteric effect on a different bridging interface, but the simplest explanation is that the CTD is itself responsible for key bridging interactions between DNA segments.