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