Finding the end of a circle: how bacteria process the end of circular chromosomes

The processing of the replication termination region is crucial for the segregation of circular bacterial chromosomes. It includes the removal of catenation links by TopoIV and the resolution of dimeric chromosomes to monomers by XerCD/dif recombination. The FtsK DNA-translocase controls these last steps of chromosome segregation. It is associated with the cell division septum and translocates sister chromosomes using the KOPS DNA motifs to orient its activity. KOPS are specific octameric motifs over-represented on the chromosome and whose orientation is highly biased from the replication origin to the opposite dif site, following the two chromosome replichores. FtsK is thought to control TopoIV activity and finally reaches the dif site to induce XerCD/dif recombination. We will report our recent findings of how these activities are coordinated during the cell cycle.

We have investigated the effects of important asymmetry of the two replichores in E. coli. We show that large chromosome inversions from the dif position disturbs the ongoing post-replicative events resulting in inhibition of both cell division and cell elongation. This is accompanied by alterations of the segregation pattern of loci located at the inversion endpoints, particularly of the new replichore junction. None of these defects is suppressed by restoration of termination of replication opposite oriC, indicating that they are more likely due to the asymmetry of replichore polarity than to asymmetric replication. Strikingly, DNA translocation by FtsK, which processes the terminal junction of the replichores during cell division, becomes essential in inversion-carrying strains. Inactivation of the FtsK translocation activity leads to aberrant cell morphology, strongly suggesting that it controls membrane synthesis at the division septum. Our results reveal that FtsK mediates a reciprocal control between processing of the replichore polarity junction and cell division.

We have used XerCD/dif recombination to probe the interaction of chromosome loci with translocating FtsK. We show that FtsK acts in a ~400 kb region around the dif position. Measurement of the relative times at which different loci segregate shows that this region is subjected to an elevated level of post-replicative cohesion between sister-chromosomes. Both cohesion and interaction with FtsK can be uncoupled from the zone where replication terminates. Cohesion is controlled by FtsK and TopoIV, strongly suggesting that these two proteins collaborate to the establishment and release of cohesion during segregation of this specific chromosome region.