Functional Analysis of the XerC and XerD Site-Specific Recombinases: Potential Role in Dissemination of Resistance Genes.

Modules composed of a resistance gene flanked by Xer site-specific recombination sites, the vast majority of which were found in , are thought to behave as elements that facilitate horizontal dissemination. The and genes were cloned, and the recombinant clones used to complement the cognate mutants. The complemented strains supported the resolution of plasmid dimers, and, as is the case with and plasmids, the activity was enhanced when the cells were grown in a low osmolarity growth medium. Binding experiments showed that the partially purified XerC and XerD proteins (XerC and XerD) bound synthetic Xer site-specific recombination sites, some of them with a nucleotide sequence deduced from existing plasmids. Incubation with suicide substrates resulted in the covalent attachment of DNA to a recombinase, probably XerC, indicating that the first step in the recombination reaction took place. The results described show that XerC and XerD are functional proteins and support the hypothesis that they participate in horizontal dissemination of resistant genes among bacteria.