[Weakening of type I restriction in E. coli: the effect of dam mutation]

The host-controlled EcoK-restriction of unmodified phages lambda.0 and T7ocr. is 100-fold alleviated in dam- mutants of E. coli. In addition the EcoK modification activity is considerably decreased in dam- strains. The I and III types restriction (EcoB, EcoD, EcoK and EcoP1) were relieved in dam- mutants, but no alleviation of EcoRI restriction occurred in dam- strains. We interpret the alleviation of the I type restriction in dam- mutants as consequence of induction of the function, which interferes with the I type restriction systems.