Type II restriction endonuclease, CstMI, obtainable from

In accordance with the present invention, there is provided a novel type II restriction endonuclease, obtainable from Corynebacterium striatum M82B, hereinafter referred to as “CstMI”, which endonuclease: (1) recognizes the nucleotide sequence 5′-AAGGAG-3′ in a double-stranded DNA molecule as shown below, 5′-AAGGAGN20↓-3′ 3′-TTCCTCN18↑-5′ (wherein G represents guanine, C represents cytosine, A represents adenine, T represents thymine and N represents either G, C, A, or T); (2) cleaves said sequence at the phosphodiester bonds between the 20th and the 21th nucleotides 3′ to the recognition sequence in the 5′-AAGGAG-3 strand of the DNA, and between the 18th and 19th nucleotides 5′ to the recognition sequence in the complement stand, 5′-CTCCTT-3′, to produce a 2 base 3′extension; and (3) possesses a second enzymatic activity that recognizes the same DNA sequence, 5′-AAGGAG-3′, but modifies this sequence by the addition of a methyl group to prevent cleavage by the CstMI endonuclease activity.