Cleavage restriction-modification systems

Two types of restriction-modification system have been found in bacteria. In type I systems, the methy-lase and R. e. are both associated with a complex containing three different polypeptide chains an a-chain with R.e activity, a P-chain with methylase activity and a y-chain with the recognition site for the DNA sequence. Type I systems require S-adenosyl-t-me-thionine and ATP for both R.e. and methylase activities they are less specific, and cleavage sites may be random and far removed (1,000 base pairs) from the 5 side of the recognition site. In type II systems, me-thylases and R.e. are separate, 5-adenosyl-L-methio-... 

Restriction-modification is a term for bacterial enzyme systems that cleave DNA sequences. Each system consists of two distinct enzyme activities a DNA methylase and an endonuclease that catalyzes the double-strand DNA break. Type I restriction endonuclease systems have both methylase and nuclease activities in one protein molecule, which contains three subunits. One subunit contains the nuclease, one the methylase, and one a sequence recognition determinant. The recognition site is not symmetrical, and cleavage occurs some distance (up to 10 kbp) away from the recognition site, although methylation occurs within the recognition site. 

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