Functional polarization of the Escherichia coli chromosome terminus: the dif site acts in chromosome dimer resolution only when located between long stretches of opposite polarity

In Escherichia coli, chromosome dimers are generated by recombination betwe en circular sister chromosomes. Dimers are lethal unless resolved by a syst em that involves the XerC, XerD and FtsK proteins acting at a site (dif) in the terminus region. Resolution fails if dif is moved from its normal posi tion. To analyse this positional requirement, dif was transplaced to a vari ety of positions, and deletions and inversions of portions of the dif regio n were constructed. Resolution occurs only when dif is located at the conve rgence of multiple, oppositely polarized DNA sequence elements, inferred to lie in the terminus region. These polar elements may position dif at the c ell septum and be general features of chromosome organization with a role i n nucleoid dynamics.