FtsK-dependent and -independent pathways of Xer site-specific recombination

Homologous recombination between circular chromosomes generates dimers that cannot be segregated at: cell division. Escherichia. coli Xer site-specifi c recombination converts chromosomal and plasmid dimers to monomers. Two re combinases, XerC and XerD, act at the E. coli chromosomal recombination sit e, dif, and at related sites in plasmids. We demonstrate that Xer recombina tion at plasmid dif sites occurs efficiently only when FtsK is present and under conditions that allow chromosomal dimer formation, whereas recombinat ion at the plasmid sites cer and psi is independent of these factors. We pr opose that the chromosome dimer- and FtsK-dependent process that: activates Xer recombination at plasmid dif also activates Xer recombination at chrom osomal dif. The defects in chromosome segregation that result from mutation of the FtsK C-terminus are attributable to the failure of Xer recombinatio n to resolve chromosome dimers. to monomers. Conditions that lead to FtsK-i ndependent Xer recombination support the hypothesis that FtsK acts on Holli day junction Xer recombination inter; mediates.