Abstract
Modules composed of a resistance gene flanked by Xer site-specific recombination sites, the vast majority of which were found in Acinetobacter baumannii, are thought to behave as elements that facilitate horizontal dissemination. The A. baumannii xerC and xerD genes were cloned, and the recombinant clones used to complement the cognate Escherichia coli mutants. The complemented strains supported the resolution of plasmid dimers, and, as is the case with E. coli and Klebsiella pneumoniae plasmids, the activity was enhanced when the cells were grown in a low osmolarity growth medium. Binding experiments showed that the partially purified A. baumannii XerC and XerD proteins (XerCAb and XerDAb) bound synthetic Xer site-specific recombination sites, some of them with a nucleotide sequence deduced from existing A. baumannii plasmids. Incubation with suicide substrates resulted in the covalent attachment of DNA to a recombinase, probably XerCAb, indicating that the first step in the recombination reaction took place. The results described show that XerCAb and XerDAb are functional proteins and support the hypothesis that they participate in horizontal dissemination of resistant genes among bacteria.
Highlights
Site-specific recombination mediated by the tyrosine recombinases XerC and XerD (XerCDSSR) participates in a wide array of genetic processes in bacteria
XerCD SSR is critical for stabilization of numerous plasmids by resolving multimers that otherwise would lead to segregational instability [4,5]
At least for some Escherichia coli and Klebsiella pneumoniae plasmids, XerCD SSR activity levels depend on the osmolarity of the environment that produce modifications in the supercoiling density [7,8,9,10]
Summary
Site-specific recombination mediated by the tyrosine recombinases XerC and XerD (XerCDSSR) participates in a wide array of genetic processes in bacteria. After activation by FtsK, XerC and XerD catalyze the resolution of dimeric chromosomes formed by homologous recombination as a consequence of repaired, broken, or stalled replication forks [1,2,3]. XerCD SSR is critical for stabilization of numerous plasmids by resolving multimers that otherwise would lead to segregational instability [4,5]. In this case, the recombination reaction requires the presence of architectural proteins, like PepA and ArgR or ArcA, to form of a synaptic complex that acts as a topological filter, permitting the resolution but not formation of multimers [6]. Many genetic elements take advantage of XerCD SSR to integrate into the bacterial chromosome. These elements, the vast majority of them phages, are known as IMEX (Integrative Mobile Elements that integrate through XerCD SSR), Antibiotics 2020, 9, 405; doi:10.3390/antibiotics9070405 www.mdpi.com/journal/antibiotics
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