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Interplay between recombination, cell division and chromosome structure during chromosome dimer resolution in Escherichia coli

Chromosome dimers form in bacteria by recombination between circular chromosomes. Resolution of dimers is a highly integrated process involving recombination between dif sites catalysed by the XerCD recombinase, cell division and the integrity of the division septum‐associated FtsK protein and the presence of dif inside a restricted region of the chromosome terminus, the dif activity zone (DAZ). We analyse here how these phenomena collaborate. We show that (i) both inter‐ and intrachromosomal recombination between dif sites are activated by their presence inside the DAZ; (ii) the DAZ‐specific activation only occurs in conditions supporting the formation of chromosome dimers; (iii) overexpression of FtsK leads to a general increase in dif recombination irrespective of dif location; (iv) overexpression of FtsK does not improve the ability of dif sites inserted outside the DAZ to resolve chromosome dimers. Our results suggest that the formation of an active XerCD‐FtsK–dif complex is restricted to when a dimer is present, the features of chromosome organization that determine the DAZ playing a central role in this control.