Restriction Defects: McrA, McrBC, and Mrr all restrict methylcytosine-containing DNA methylated by the CpG methylase (M.Sss I) (1). In addition, McrA and McrBC specifically restrict DNA modified by different sets of more sequence-specific cytosine methylases (2).
Mrr also restricts many methyladenine-containing sequences (3,4). Restriction by each of these three restriction systems individually (tested with λ DNA modified with the Sss I methylase and then packaged into phage) is: McrA, 250-fold; McrBC, 100-fold; Mrr, 200-fold. Cumulative restriction when all three are present in the same cell is 5,000-fold (1). In addition, EcoK restricts DNA not modified at K sites by 10-fold to 5,000-fold, depending on the number of sites (5). ER1821 lacks all four activities.
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