[Antimodification activity of the ArdA and Ocr proteins]

The ArdA and Ocr antirestriction proteins, whose genes are in transmissible plasmids (ardA) and bacteriophage genomes (0.3 (ocr)), specifically inhibit type I restriction-modification enzymes. The Ocr protein (T7 bacteriophage) was shown to inhibit both restriction (endonuclease) and modification (methylase) activities of the EcoKI enzyme in a broad range of intracellular concentrations (starting from 10-20 molecules per cell). In contrast to Ocr, the ArdA protein (ColIb-P9 transmissible plasmid) inhibited both of the EcoKI activities only at high intracellular concentrations (30000-40000 molecules per cell). When the ArdA concentration was several fold lower, only endonuclease activity of EcoKI was inhibited. It was assumed that a poorer ArdA ability to inhibit EcoKI modification activity is related to the substantial difference in life cycle between transmissible plasmids (symbiosis with the bacterial cell) and bacteriophages (infection and lysis of bacteria). The Ocr and ArdA mutants that inhibited exclusively endonuclease activity of EcoKI were obtained. Antirestriction proteins incapable of homodimerization were assumed to inhibit only endonuclease activity of type I restriction-modification enzymes.