During replication of the circular Escherichia coli genome, chromosome dimers can form. These dimers must be resolved into monomers before they can be segregated into daughter cells. Resolution occurs at dif sites by a site-specific recombinase, XerCD. Another multimeric protein complex, FtsK, assists in dimer resolution by bringing the two dif sites together. FtsK also associates with the cell membrane and helps to partition DNA into daughter cells by acting as a pump that drives DNA through the septum.

A recent study (Pease et al., Science 307, 586–590, 2005) has uncovered interesting new facets of the movement of FtsK on the DNA. This single molecule study confirms that FtsK is the premier speedster of the cell. It translocates along DNA at rates of up to 5 kb per second without unwinding the DNA—making FtsK the fastest motor protein yet identified. It also has muscle, able to maintain high speed against significant load.

FtsK was observed to move bidirectionally without losing contact with DNA, suggesting that there is more than one motor in the complex (schematized as purple triangle and green square). While both motors seem to be capable of bidirectional movement, only one is active at a time. This leads to accumulation of supercoiled DNA between the motors. For actual translocation to occur, the supercoiled DNA must be released through the rear motor (green square).

Yet while FtsK has the potential to move bidirectionally on random DNA, net movement was unidirectional on DNA containing a dif site. Particularly notable is that the net movement occurred in the same direction on all molecules. To examine what specifies this directionality, regions of chromosomal DNA surrounding the dif site were tested. In each case, when FtsK bound to the DNA it traveled toward the dif site (movement indicated by arrows). But FtsK did not stop at dif. Instead, it was observed to go beyond dif, and then seemed to hit a site that caused it to reverse direction and travel back toward dif again. Thus, the end result is that FtsK reaches a state where it oscillates around the dif site.

It is proposed that there exist short, asymmetric sequences that signal when the active motor is moving toward dif; this would direct the FtsK movement to facilitate dimer resolution. An implication of this model is that these sites must be oriented in opposite directions from the origin of replication. This concept parallels the regulation of the activity of E. coli RecBCD helicase by an asymmetric octameric sequence (chi). Genomic analysis has revealed that chi sequences are oriented predominantly in the direction away from the origin of replication, which may be rationalized with the function of RecBCD in repairing broken replication forks.