FtsK functions in the processing of a Holliday junction intermediate during bacterial chromosome segregation

In bacteria with circular chromosomes, homologous recombination can generat e chromosome dimers that cannot be segregated to daughter cells at cell div ision. Xer site-specific recombination at dif, a 28-bp site located in the replication terminus region of the chromosome, converts dimers to monomers through the sequential action of the XerC and XerD recombinases. Chromosome dimer resolution requires that dif is positioned correctly in the chromoso me, and the activity of FtsK, a septum-located protein that coordinates cel l division with chromosome segregation. Here, we show that cycles of XerC-m ediated strand exchanges form and resolve Holliday junction intermediates b ack to substrate irrespective of whether conditions support a complete reco mbination reaction. The C-terminal domain of FtsK is sufficient to activate the exchange of the second pair of strands by XerD, allowing both intra- a nd intermolecular recombination reactions to go to completion. Proper posit ioning of dif in the chromosome and of FtsK at the septum is required to se nse the multimeric state of newly replicated chromosomes and restrict compl ete Xer reactions to dimeric chromosomes.