The standard perception of a restriction enzyme is of an endonuclease that recognizes a short palindrome of DNA and which cleaves both strands of the DNA at fixed locations in that sequence, in 'To whom correspondence should be addressed. a reaction that requires only Mg2+ ions as a cofactor [l]. The perception thus refers to the type I1 restriction enzymes, as opposed to the type I and the type 111 systems [2]. Many of the type I1 restriction enzymes conform to this perception [3]. Their recognition sites are often symmetrical 0 1999 Biochemical Society
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