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J Mol Biol. 2008 Nov 28;383(5):970-81. doi: 10.1016/j.jmb.2008.06.005. Epub 2008 Jun 7.

The Orf18 gene product from conjugative transposon Tn916 is an ArdA antirestriction protein that inhibits type I DNA restriction-modification systems.

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School of Chemistry, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3JJ, UK.


Gene orf18, which is situated within the intercellular transposition region of the conjugative transposon Tn916 from the bacterial pathogen Enterococcus faecalis, encodes a putative ArdA (alleviation of restriction of DNA A) protein. Conjugative transposons are generally resistant to DNA restriction upon transfer to a new host. ArdA from Tn916 may be responsible for the apparent immunity of the transposon to DNA restriction and modification (R/M) systems and for ensuring that the transposon has a broad host range. The orf18 gene was engineered for overexpression in Escherichia coli, and the recombinant ArdA protein was purified to homogeneity. The protein appears to exist as a dimer at nanomolar concentrations but can form larger assemblies at micromolar concentrations. R/M assays revealed that ArdA can efficiently inhibit R/M by all four major classes of Type I R/M enzymes both in vivo and in vitro. These R/M systems are present in over 50% of sequenced prokaryotic genomes. Our results suggest that ArdA can overcome the restriction barrier following conjugation and so helps increase the spread of antibiotic resistance genes by horizontal gene transfer.

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