The Assembly and Control of the XerCD-dif Recombination Machinery Studied at the Single Molecule Level

Tyrosine recombinases are well known to catalyse site-specific DNA recombination in bacteria, archeae and eukaryotes. In bacteria, these recombinases are extensively used for programmed integration, excision and inversion of DNA segments. XerC and XerD form together a highly conserved tyrosine recombinase devoted to recombine dif sites, located in the terminal domain of circular bacterial chromosome. XerCD-dif recombination resolves chromosome dimers to monomers before segregation and is thus required for the faithful segregation of sister chromosomes in daughter cells. To do so, its activity is precisely tuned and controlled during the bacterial cell cycle. In E. coli, XerCD/dif activity is controlled at two levels: i) the direction of the recombination reaction (from dimer to monomer and not the inverse) and ii) the timing of the reaction (coupled to septation). Both controls necessitate the cell division protein FtsK. The way this septal DNA translocase acts on XerCD/dif recombination is not completely understood but involves the control of the assembly of the nucleoproteic complex where recombination takes place.To understand the XerCD-dif recombination and its FtsK-mediated control, we study the assembly of the recombination complexes on single DNA molecules. We will present the Tethered Particule Motion experimental setup we are using as well as our findings.