The vast majority of bacteria have a circular chromosome(s). Having a circular chromosome has many advantages such as having no DNA ends to protect from exonucleases. However, it also has a major disadvantage- a recombination event during or after replication that results in a crossover will convert the two monomeric circular chromsomes into a single dimeric molecule. A chromosome dimer cannot be segregated at cell division and is lethal unless resolved. Bacteria have evolved a site-specific recombination mechanism to introduce a second crossover to resolve the dimers back to monomers: the Xer system. Xer site-specific recombination requires two tyrosine recombinase proteins, XerC and XerD, the recombination site (28bp) called dif, and the DNA translocase FtsK. FtsK acts at the division septum to clear DNA from the site of cell division, and in the presence of a chromosome dimer, pumps the DNA so that the two dif sites are brought together (synapsis), and then interacts with the XerD recombinase to activate recombination.

The interaction between FtsK and XerD has been narrowed down to the very C-terminal domain of FtsK, called γ, but the molecular detail of this interaction has been unknown until now. We have solved the structure of XerD interacting with FtsK-γ by X-ray crystallography. The interaction appears relatively weak, but has been confirmed with site-directed mutagenesis of both the interface on FtsK-γ and XerD, to produce a system that is no longer capable of activating recombination. The mechanism behind how the interaction leads to activation of catalysis is under further investigation.