ABSTRACT
Homologues of T3 SAMase and T7 ocr have been found in related bacteriophages, but are not particularly widespread in nature, however, other families of antirestriction proteins are more common.
The ArdA family of proteins are found on bacterial chromosomes, on conjugative plasmids and conjugative transposons such as Tn916.22 Each ArdA gene encodes a protein about 20 kDa in size which, in common with ocr is highly acidic: this strongly implies that it is a DNA mimic. The cur rent model for ArdA action is that it is expressed early in conjugation in the recipient cytoplasm22 and inhibits Type I REases before they can act on the double-stranded form of the invading DNA molecule. The mechanism of action of ArdA proteins is not known but biochemical characterisation has begun. In particular, recent work33,34 suggests that it may be able to interact differentially between EcoKI REase and MTase, inhibiting REase, but permitting MTase to modify the conjugated DNA as a consequence of its lower affinity for the MTase.
