What is a Type III Restriction Enzyme?

Type III restriction enzymes are a group of endonucleases that recognize a non-pallindromic sequence, comprising two inversely oriented sites. Learn more about these poorly understood enzymes.


Type III restriction enzymes are infrequently used in molecular biology, as they have few relevant applications. Therefore, they are less well characterized than Type II enzymes.

While the mechanism of Type III RM systems is uncertain, the enzymes themselves consist of two proteins that function as a single protein complex: the methyltransferase, or M protein, which contains the specificity domain, and the restriction endonuclease, or R protein.

Type III enzymes recognize a 5-6 base, non-palindromic sequence, and require 2 inversely oriented recognition sites for cleavage.

Once bound, the enzyme slides along the DNA, and upon encountering a second complex, cleaves downstream of the recognition site, typically 25-28 bases away.

Type III enzymes require ATP, S-adenosyl methionine, and magnesium ions.

EcoP15I is currently the only Type III restriction enzyme commercially available. For more information about restriction endonucleases, visit www.NEBrestrictionenzymes.com, or go to REBASE, the restriction enzyme database, at REBASE.neb.com.

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