Fig. 2. One scheme by which FtsK50C (hexameric rings) might promote synapsis of directly repeated dif sites (red and green triangles) on a supercoiled DNA molecule, prior to activation of Xer recombination. Two units of FtsK50C form a double ring on DNA, by analogy with RuvB and other DNA translocases. Translocation by each ring reels in the distant dif sites and facilitates their synapsis and activation of recombination. We propose that once a XerCD-bound dif site interacts with a ring, its DNA translocation activity stalls. Models in which a single ring facilitates synapsis of dif sites are also possible, although the reaction is not as facile as that presented here. DNA translocation will generate separate positively and negatively supercoiled domains when two distant DNA segments interact simultaneously with a ring (Aussel et al., 2002). For simplicity, this differential supercoiling is not shown.