Fig 7.

Evolution of an ancestral Bsr3-Par system. The Par system of Burkholderia cepacia complex chromosome 3 (C3) is proposed to be the most likely ancestor. Its ParB has a relatively wide binding specificity, and its parS comprises the features of the other systems: GTT ends (red circles), CACGTG core (red rectangle), and symmetry (red convergent arrows). An ancestral plasmid form would have been captured and frozen by C3 but would have spread on other plasmid replicons and diverged to generate the Par systems 12J, 12D, and G4, each with a parS having lost one feature. Where evolution has proceeded in different species, without the requirement for compatibility, ParB-parS cross-reaction can be detected (arrows). When managing two replicons in the same species (e.g., B. vietnamiensis G4 and R. pickettii 12D), Par systems must diverge for the replicons to be compatible and ParB-parS cross-reaction is detected barely or not at all (ball lines).