Host-induced restriction and modification

A phage whose DNA is unmodified infects a bacterium with a restriction system that recognizes the DNA sequence 5’–GAATTC–3’ (step 1).

Most phage DNA molecules are cleaved by the restriction nuclease (step 2), but the few that become methylated first on the innermost A are protected from attack (step 3).

The phages that emerge contain modifie (methylated) DNA (step 4).

Because they are not vulnerable to restriction by the host nuclease, they are able to overcome the bacterium’s defense system when they reinfect the same bacterial strain.