Alleviation of type I restriction in adenine methylase (dam) mutants of Escherichia coli

Summary The host-controlled EcoK-restriction of unmodified phage .O is alleviated in dam mutants of Escherichia coli by 100- to 300-fold. In addition, the EcoK modification activity is substantially decreased in dam^- strains. We show that type I restriction (EcoB, EcoD and EcoK) is detectably alleviated in dam mutants. However, no relief of EcoRI restriction (Type II) occurs in dam^- strains and only a slight effect of dam mutation on EcoP1 restriction (Type III) is observed. We interpret the alleviation of the type I restriction in dam^- strains to be a consequence of induction of the function which interferes with type I restriction systems.