Determination of the recognition sites of cytosine DNA-methylases from Escherichia coli SK

Two different cytosine DHA-methylases, N<inf>I</inf> and G<inf>II</inf>, are present in Escherichia coli SK. The G<inf>II</inf> methylase recognizes the five-member symmetric sequence: 5′…NpCpCpApGpGpN…3′. This sequence is identical with the recognition site of the hsp II type determined by RII plasmide but, in contrast to RII methylase, the G<inf>II</inf> enzyme methylates cytosine located on the 5′ side of the site. By analogy with the isoshizomery of the restricting endonucleases, RII and G<inf>II</inf> DNA methylaeses may be called isomethymers which recognize the same site but methylate different bases. Since the phage of the SK and hsp II phenotypes is effectively restricted in respective cells it may be assumed that the isomethymeric modification does not provide any protection against the corresponding restrictases. N<inf>I</inf> methylase recognizes the five-member symmetric site which represents an inverted sequence of the GTT site: 5′…HpGpGpApCpCpN…3′. In this ease cytosine at the 3′-end of the recognition site is methylated.