Cloning and characterization of the genes encoding theMspl restriction modification system

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Abstract
The genes encoding the Msp l restriction modification system, which recognizes the sequence 5′CCGG, have been cloned into pUC9. Selection was based on expression of the cloned methylase gene which renders plasmid DNA insensitive to Msp l cleavage in vitro . Initially, an insert of 15kb was obtained which, upon subcloning, yielded a 3 kb Eco RI to Hind III insert, carrying the genes for both the methylase and the restriction enzyme. This insert has been sequenced. Based upon the sequence, together with appropriate subclones, it is shown that the two genes are transcribed divergently with the methylase gene encoding a polypeptide of 418 amino acids, while the restriction enzyme is composed of 262 amino acids. Comparison of the sequence of the Msp l methylase with other cytosine methylases shows a striking degree of similarity. Especially noteworthy is the high degree of similarity with the Hha I and Eco RII methylases.