Gene orf18, which is situated in the conjugative transposon Tn916 from the bacterial pathogen Enterococcus faecalis, encodes a putative ArdA (alleviation of restriction of DNA) protein. 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. Biophysical characterisation of ArdA demonstrated tight association between ArdA and the M.EcoKI. Also, ArdA was shown to efficiently inhibit restriction and modification by all four major classes of Type I R/M enzymes in vivo. Thus, ArdA can overcome the restriction barrier following conjugation and so helps to increase the spread of antibiotic resistance genes by horizontal gene transfer. The amino acid sequence of KlcA, from the incompatibility plasmid pBP136 from Bordetella pertussis, showed a high degree of similarity with the antirestriction protein ArdB from the IncN plasmid pKM101. In this study the solution structure of KlcA was solved with high-resolution NMR and its antirestriction function demonstrated. The structure of KlcA showed a rigid globular molecule with a novel fold. No antimodification function was observed for KlcA in vivo and the antirestriction function of KlcA has been successfully shown in vivo but not in vitro. Because no direct binding of KlcA to EcoKI was observed in vitro, the mechanism of the endonuclease blocking was assumed to be different from that of ArdA. Preliminary experiments including coimmunoprecipitation assays were conducted in order to elucidate the antirestriction mechanism of KlcA.