Site-specific tyrosine recombinase, which acts by catalyzing the cutting and rejoining of the recombining DNA molecules. Essential to convert dimers of the bacterial chromosome into monomers to permit their segregation at cell division (PubMed:17630835). Required for resolution of chromosome dimers frequently formed by transformation (PubMed:25569614). FtsK is required for recombination. In contrast to the XerC-XerD complex present in non-streptococcaceae bacteria, XerS acts as a single recombinase required to recombine difSL recombination site. Belongs to the 'phage' integrase family. XerS subfamily.