A MOTIF CONSERVED AMONG THE TYPE-I RESTRICTION-MODIFICATION ENZYMES AND ANTIRESTRICTION PROTEINS - A POSSIBLE BASIS FOR MECHANISM OF ACTIONOF PLASMID-ENCODED ANTIRESTRICTION FUNCTIONS

Antirestriction proteins Ard encoded by some self-transmissible plasmi ds specifically inhibit restriction by members of all three families o f type I restriction-modification (R-M) systems in E.coli. Recently, w e have identified the amino acid region, 'antirestriction' domain, tha t is conserved within different plasmid and phage T7-encoded antirestr iction proteins and may be involved in interaction with the type I R-M systems. In this paper we demonstrate that this amino acid sequence s hares considerable similarity with a well-known conserved sequence (th e Argos repeat) found in the DNA sequence specificity (S) polypeptides of type I systems. We suggest that the presence of these similar moti fs in restriction and antirestriction proteins may give a structural b asis for their interaction and that the antirestriction action of Ard proteins may be a result of the competition between the 'antirestricti on' domains of Ard proteins and the similar conserved domains of the S subunits that are believed to play a role in the subunit assembly of type I R-M systems.