odd number of crossovers segregation of dif sites away from midcell division site
activation of XerO by FtsK XerC also active
Holliday junction intermediate division site activation of XerO by FtsK XerC also active
FIGURE 11-16 Regulation of chromosome segregation by FtsK. Just before cell division, the newiy replicated origins, shown in green, move to the poles of the cell, whereas the replication terminus that includes df, shown as a tnangie, typically remains localized at the midcell. Wlien the dif site is replicated, the two daughter dii sites can recombine to form a Holliday junction, which is resolved by XerC. If tlie replicated chromosome forms monomers, segregation will break the synaptic complex and the df sites will move away from the midcell location before division. In contrast, it the chromosome forms a dimer (right panel), the synaptic complex remains trapped at midcell and allows access to FtsK, which is localized to the cell division site FtsK then activates XerD. XerO mediated recombination, followed by XeiC-mediated recombination, then allows resolution of the dmers into monomers for cell division. (Source: Barre et al. 2001. froc. Net Acad. Sa U.SA 98: 8189, f 5, p. B194.)
segregation of dif sites away from midcell
FtsK is an ATFase that ¡racks along DNA. II functions as a "DNA-pumping protein" similar to the RuvB protein that promotes DNA branch migration during homologous recombination (discussed in Chapter 10). FtsK js also a membrane-bound protein that is localized in the cell at the site where cell division occurs. It functions to move DNA away from the center of the ceil prior to division so that the cell can divide at this site (Figure VI-16),
This localization of FtsK to the division site is key to how the cells insure that XerD is activated specifically when a dimerit chromosome is present. In this case, the chromosome will be "stuck" in the middle of the dividing cell as one half of the chromosome dimer is moved into each daughter celts The two dif sites in this dimer, with bound XerCD proteins, therefore interact with FisK. In this manner, site-specific recombination is regulated to occur at the right time and place with respect to the cell division cycle.
There Are Other Mechanisms to Direct Recombination to Specific Segments of DNA
Although we have limited our discussion to conservative site-specific recombination, there are other recombination events that occur at specific sequences and serve similar biological functions. Some of these reactions, ior example, mating type switching in yeasl, occur by a targeted gene-conversion event, as we described in Chapter 10. The gene rearrangements responsible Tor assembly of gene segments encoding critical proteins for the vertebrate immune system—known as V(D)J recombination—also occurs at specific sites. This reaction is mechanistically similar to transposition, however, and therefore is considered later in this chapter.
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