Site-specific tyrosine recombinase, which acts by catalyzing the cutting and rejoining of the recombining DNA molecules. Binds cooperatively to specific DNA consensus sequences that are separated from XerC binding sites by a short central region, forming the heterotetrameric XerC-XerD complex that recombines DNA substrates. The complex is essential to convert dimers of the bacterial chromosome into monomers to permit their segregation at cell division. It also contributes to the segregational stability of plasmids at ColE1 xer (or cer) and pSC101 (or psi) sites. In the complex XerD specifically exchanges the bottom DNA strands (By similarity). During recombination, the heterotetrameric complex catalyzes two consecutive pairs of strand exchanges, implying that specific pairs of active sites are sequentially switched on and off in the recombinase tetramer to ensure that appropriate DNA strands will be exchanged at both reaction steps. FtsK plays a central role in this catalytic state switch that turns recombinase on and off reciprocally. The reciprocal C-terminal interaction between XerC and XerD may also participate in the enzymatic switch process. Forms a cyclic heterotetrameric complex composed of two molecules of XerC and two molecules of XerD, in which XerC interacts with XerD via its C-terminal region, XerD interacts with XerC via its C-terminal region and so on. Belongs to the 'phage' integrase family. XerD subfamily.