Type III restriction endonuclease

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 III restriction endonuclease system resemble more closely the type I systems than the type II systems. Type III enzymes contain both nuclease and methylase activities in a two-subunit enzyme. They differ from type I enzymes in that they do not require ATP, they modify just one DNA strand, and the cleavage site is fairly close to the recognition site. 

There are three types of restriction endonucleases, designated I, II, and III. Types I and HI are generally large, multisubunit complexes containing both the endonucle-... 

These enzymes are extraordinarily abundant over 1200 restriction endonucleases had been isolated and characterized by early 1990. Of three classes defined, type II restriction enzymes, which generally cut within their recognition sequences, have found uses in a host of biomedical research and diagnostic applications to be discussed below. Type 1 enzymes cut nonspecifically many nucleotides distal to specific recognition sequences and contain both restriction enzyme and DNA modification (see below) activities on different subunits of multienzyme complexes. Type III restriction enzymes share the multienzyme aspeas of type I enzymes but vary in other properties such as ATPase activity and cofactor requirements. 

Type II restriction-modification systems differ from their type I and type III counterparts in that the endonuclease and DNA methylase activities are conducted by two separate enzymes (not a single multisubunit complex). The restriction endonuclease cleaves both strands of the DNA duplex within a defined recognition sequence, while the companion DNA methylase methylates a specific base within the same recognition sequence. In contrast... 

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