Fig 4.

Model of exchange of the DNA region flanked by oriT and Xer recombination sites. (A) Two plasmids (black and red) form a cointegrate through oriT site-specific recombination mediated by the nickase supplied by a resident plasmid that includes a mob gene. R, ARG box; C, XerC-binding site; D, XerD-binding site. (B) The cointegrate is resolved through Xer site-specific mediated recombination. Whether the accessory sequences are needed to facilitate formation of a synaptic complex is still unknown. Double-stranded DNA is shown only at the core recombination site to show the exchange of strands. XerC mediates the exchange of the first pair of strands at an undetermined nucleotide inside the XerC-binding site and forms a Holliday junction. Resolution of the Holliday junction may occur through XerD-mediated exchange of the second pair of strands or through a Xer-independent process such as replication. Since we do not know the mechanism of resolution of the Holliday junction we cannot predict the final nucleotide sequence of the newly formed Xer recombination sites. For this reason we indicate them in blue and green. Resolution of the Holliday junction results in two plasmids that have swapped the region limited by oriT and the Xer recombination site.