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Microbiology and Molecular Biology Reviews, June 2006, p. 296-316, Vol. 70, No. 2
1092-2172/06/$08.00+0     doi:10.1128/MMBR.00048-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

ISCR Elements: Novel Gene-Capturing Systems of the 21st Century?

Mark A. Toleman,* Peter M. Bennett, and Timothy R. Walsh

Bristol Centre for Antimicrobial Research and Evaluation, Department of Cellular and Molecular Medicine, School of Medical Sciences, University Walk, University of Bristol, Bristol BS8 1TD, United Kingdom

"Common regions" (CRs), such as Orf513, are being increasingly linked to mega-antibiotic-resistant regions. While their overall nucleotide sequences show little identity to other mobile elements, amino acid alignments indicate that they possess the key motifs of IS91-like elements, which have been linked to the mobility ent plasmids in pathogenic Escherichia coli. Further inspection reveals that they possess an IS91-like origin of replication and termination sites (terIS), and therefore CRs probably transpose via a rolling-circle replication mechanism. Accordingly, in this review we have renamed CRs as ISCRs to give a more accurate reflection of their functional properties. The genetic context surrounding ISCRs indicates that they can procure 5' sequences via misreading of the cognate terIS, i.e., "unchecked transposition." Clinically, the most worrying aspect of ISCRs is that they are increasingly being linked with more potent examples of resistance, i.e., metallo-ß-lactamases in Pseudomonas aeruginosa and co-trimoxazole resistance in Stenotrophomonas maltophilia. Furthermore, if ISCR elements do move via "unchecked RC transposition," as has been speculated for ISCR1, then this mechanism provides antibiotic resistance genes with a highly mobile genetic vehicle that could greatly exceed the effects of previously reported mobile genetic mechanisms. It has been hypothesized that bacteria will surprise us by extending their "genetic construction kit" to procure and evince additional DNA and, therefore, antibiotic resistance genes. It appears that ISCR elements have now firmly established themselves within that regimen.


* Corresponding author. Mailing address: Department of Cellular and Molecular Medicine, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, United Kingdom. Phone: 0117 9288819. Fax: 0117 9287896. E-mail: mark.toleman{at}bristol.ac.uk.


Microbiology and Molecular Biology Reviews, June 2006, p. 296-316, Vol. 70, No. 2
1092-2172/06/$08.00+0     doi:10.1128/MMBR.00048-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.




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