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Nucleic Acids Research Advance Access published online on June 8, 2009
Nucleic Acids Research, doi:10.1093/nar/gkp478
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© 2009 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Structural Biology

Extensive DNA mimicry by the ArdA anti-restriction protein and its role in the spread of antibiotic resistance

Stephen A. McMahon1, Gareth A. Roberts2, Kenneth A. Johnson1, Laurie P. Cooper2, Huanting Liu1, John H. White2, Lester G. Carter1, Bansi Sanghvi2, Muse Oke1, Malcolm D. Walkinshaw3, Garry W. Blakely3, James H. Naismith1,* and David T. F. Dryden2,*

1Centre for Biomolecular Science, The University, St Andrews, KY16 9ST, 2School of Chemistry and 3School of Biological Sciences, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3JJ, UK

*To whom correspondence should be addressed. Tel: +44 (0)131 650 4735; Email: david.dryden{at}ed.ac.uk Correspondence may also be addressed to James H. Naismith. Tel: +44 (0)1334 463792; Email: naismith{at}st-and.ac.uk

Received March 2, 2009. Revised May 13, 2009. Accepted May 18, 2009.

The ardA gene, found in many prokaryotes including important pathogenic species, allows associated mobile genetic elements to evade the ubiquitous Type I DNA restriction systems and thereby assist the spread of resistance genes in bacterial populations. As such, ardA contributes to a major healthcare problem. We have solved the structure of the ArdA protein from the conjugative transposon Tn916 and find that it has a novel extremely elongated curved cylindrical structure with defined helical grooves. The high density of aspartate and glutamate residues on the surface follow a helical pattern and the whole protein mimics a 42-base pair stretch of B-form DNA making ArdA by far the largest DNA mimic known. Each monomer of this dimeric structure comprises three alpha–beta domains, each with a different fold. These domains have the same fold as previously determined proteins possessing entirely different functions. This DNA mimicry explains how ArdA can bind and inhibit the Type I restriction enzymes and we demonstrate that 6 different ardA from pathogenic bacteria can function in Escherichia coli hosting a range of different Type I restriction systems.

The authors wish it to be known that, in their opinion, the first two authors should be regarded as joint First Authors.


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