ABSTRACT
To date, 3015 restriction-modification (R/M) systems have been identi fied from many different species of bacteria, across virtually all bacter ial genera [1]. R/M systems consist of two enzymic activities, which act to defend bacterial cells against foreign DNA entering the cell [2]. One is a modification methyltransferase that methylates a particular base within a specific DNA sequence, the recognition site for that particular R/M system. The other is a restriction endonuclease that recognizes the same DNA sequence as the methyltransferase and cleaves DNA in re sponse to this sequence [3]. However, the restriction enzyme cannot cleave DNA previously modified by the methyltransferase. Thus, nei ther the genomic DNA of the cell nor invading DNA with methylated sites is cleaved, but invading DNA that carries unmethylated sites is destroyed by the restriction enzyme unless the methyltransferase mod ifies every site before the endonuclease has cleaved any one site [4].
