The Mrr protein of Escherichia coli K12 is a cryptic type IV restriction endonuclease with specificity for methylated DNA. Recently it was discovered that endogenous activation of E. coli Mrr could be triggered by high pressure stress, resulting in the generation of double strand breaks in the host chromosome and concomitant induction of the SOS response. In this report we focused on Mrr activity of Salmonella Typhimurium LT2, and although we surprisingly found no evidence of high pressure induced activation, a large number of constitutively activated Mrr mutants could be isolated when the mrr gene was routinely cloned in an expression vector. Analysis of several spontaneous mutants revealed different single mutations that rendered the Mrr protein constitutively active. Moreover, a spontaneous S. Typhimurium mutant could be isolated that displayed an increased basal SOS induction because of a point mutation in the chromosomal mrr gene. Based on these findings the physiological role of Mrr in the cell is discussed.