Characterization of plasmid pAX22, encoding VIM-1 metallo-β-lactamase, reveals a new putative mechanism of In70 integron mobilization.
OBJECTIVE
VIM-type enzymes are among the most widespread acquired metallo-β-lactamases among Gram-negative pathogens. Integron In70 is a class 1 integron that has emerged as a successful genetic support for blaVIM-1 (one of the most prevalent blaVIM allelic variants) in Gram-negative non-fermenters, and is usually chromosome borne. The objective of this study was to characterize plasmid pAX22 from Achromobacter xylosoxidans AX22, which represents the only In70-harbouring plasmid known so far, to gather insights into the mechanisms of evolution and dissemination of In70-like elements.


METHODS
The complete sequence of pAX22 was obtained by pyrosequencing and assembled with Roche Newbler software. The draft sequence, completed using a PCR-based strategy, was annotated via the BASys tool and compared with known sequences using BLAST algorithms.


RESULTS
The backbone of pAX22 showed significant similarity with that of pNOR-2000, a blaVIM-2-harbouring plasmid from Pseudomonas aeruginosa, and with the TnCP23 transposon. The three elements differed from each other mainly by the class 1 integron cassette arrays and by some integron-associated structures. In pAX22, In70 was associated with a novel putative transposon, Tn7017, composed of a defective Tn402-like transposon carrying In70 and the ISPa17 insertion sequence.


CONCLUSIONS
Plasmid pAX22 belongs to a lineage of plasmids circulating among Gram-negative non-fermenters. In70 was probably acquired by pAX22 by transposition of Tn7017, revealing a novel putative mechanism of In70 mobilization. Our results highlight the potential role that ISPa17 could have in mobilizing defective Tn402-like transposons carrying class 1 integrons.
Department of Medical Biotechnologies, University of Siena, Siena, Italy.
Vincenzo
V
Di Pilato
Simona
S
Pollini
Gian Maria
GM
Rossolini
D042441:Achromobacter denitrificans / Q000235:genetics*; D004251:DNA Transposable Elements; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D023061:Gene Order; D006801:Humans; D032023:Integrons; D008969:Molecular Sequence Data; D010957:Plasmids; D011550:Pseudomonas aeruginosa / Q000235:genetics; D017422:Sequence Analysis, DNA; D012689:Sequence Homology, Nucleic Acid; D026801:Synteny; D001618:beta-Lactamases / Q000235:genetics
D004251:DNA Transposable Elements; D004269:DNA, Bacterial; C417264:VIM-1 metallo-beta-lactamase; D001618:beta-Lactamases
Gram negative; insertion sequence; transposon