The proteolytic control of restriction activity in Escherichia coli K‐12

The endonuclease activity of EcoKI is regulated by the ClpXP‐dependent degradation of the subunit that is essential for restriction, but not modification. We monitored proteolysis in mutants blocked at different steps in the restriction pathway. Mutations that prevent DNA translocation render EcoKI refractory to proteolysis, whereas those that permit DNA translocation, but block endonuclease activity, do not. Although proteolysis alleviates restriction in a mutant that lacks modification activity, some restriction activity remains; our evidence indicates residual EcoKI associated with the membrane fraction. ClpXP protects the bacterial chromosome, but little effect was detected on unmodified foreign DNA within the cytoplasm of a restriction‐proficient cell. The molecular basis for the distinction between unmodified resident and foreign DNA remains to be determined.