The Determinant and the Genes in Acinetobacter Plasmids Are Each Part of Discrete Modules Flanked by Inversely Oriented p (XerC-XerD) Sites.

The tetracycline resistance determinant and the macrolide resistance genes and were found in an 18.2-kb plasmid, pS30-1, recovered from a global clone 2 (GC2) isolate from Singapore, that conferred resistance to tetracycline and erythromycin. pS30-1 also contains and genes encoding MOB family proteins, but attempts to mobilize pS30-1 utilizing a coresident conjugative plasmid were unsuccessful. Eight p sites, consisting of inversely oriented binding sites for the XerC and XerD recombinases separated by 6 bp, were detected in pS30-1. The determinant and the gene pair are each surrounded by two p sites in inverse orientation. Identical regions in different contexts and many previously unnoticed p sites were found in a number of different plasmids in GenBank, showing that the and modules are mobile. A putative toxin/antitoxin system, a gene encoding a serine recombinase, and open reading frames of unknown function were also part of modules in pS30-1. In general, modules with internal XerC or XerD sites alternate. Two copies of IS (94% identical to IS) in pS30-1 were inserted 5 bp from a XerC site, and this appears to be the preferred insertion site for this insertion sequence (IS) group. Apparently, plasmids exploit the XerC-XerD recombinases to mobilize DNA units containing resistance and other genes, via an uncharacterized mechanism. The and modules add to the module and the module redefined here, bringing the total of resistance gene-containing modules in plasmids to four.