Editing Restriction enzyme (section)

=== Type III ===
Type III restriction enzymes (e.g., EcoP15) recognize two separate non-palindromic sequences that are inversely oriented. They cut DNA about 20–30 base pairs after the recognition site.<ref name="pmid11557806">{{cite journal | vauthors = Dryden DT, Murray NE, Rao DN | title = Nucleoside triphosphate-dependent restriction enzymes | journal = Nucleic Acids Research | volume = 29 | issue = 18 | pages = 3728–41 | date = September 2001 | pmid = 11557806 | pmc = 55918 | doi = 10.1093/nar/29.18.3728 }}</ref> These enzymes contain more than one subunit and require AdoMet and ATP cofactors for their roles in DNA methylation and restriction digestion, respectively.<ref name="pmid1734285">{{cite journal | vauthors = Meisel A, Bickle TA, Krüger DH, Schroeder C | title = Type III restriction enzymes need two inversely oriented recognition sites for DNA cleavage | journal = Nature | volume = 355 | issue = 6359 | pages = 467–9 | date = January 1992 | pmid = 1734285 | doi = 10.1038/355467a0 | bibcode = 1992Natur.355..467M | s2cid = 4354056 }}</ref> They are components of [[Prokaryote|prokaryotic]] DNA restriction-modification [[Mechanism (biology)|mechanisms]] that protect the organism against invading foreign DNA. Type III enzymes are hetero-oligomeric, multifunctional [[protein]]s composed of two subunits, Res ({{uniProt|P08764}}) and Mod ({{uniProt|P08763}}). The Mod subunit recognises the DNA sequence specific for the system and is a modification [[methyltransferase]]; as such, it is functionally equivalent to the M and S subunits of type I restriction endonuclease. Res is required for restriction digestion, although it has no [[enzyme|enzymatic]] activity on its own. Type III enzymes recognise short 5–6 bp-long asymmetric DNA sequences and cleave 25–27 bp [[Upstream and downstream (DNA)|downstream]] to leave short, single-stranded 5' protrusions. They require the presence of two inversely oriented unmethylated recognition sites for restriction digestion to occur. These enzymes [[DNA methylation|methylate]] only one strand of the DNA, at the N-6 position of adenine residues, so newly replicated DNA will have only one strand methylated, which is sufficient to protect against restriction digestion. Type III enzymes belong to the beta-subfamily of [[DNA methyltransferase|N6 adenine methyltransferases]], containing the nine [[protein motif|motif]]s that characterise this family, including [[sequence motif|motif]] I, the [[S-adenosyl-L-methionine|AdoMet]] binding pocket (FXGXG), and motif IV, the [[catalytic]] region (S/D/N (PP) Y/F).<ref name="pmid15121719"/><ref name="pmid12595133">{{cite journal | vauthors = Bourniquel AA, Bickle TA | title = Complex restriction enzymes: NTP-driven molecular motors | journal = Biochimie | volume = 84 | issue = 11 | pages = 1047–59 | date = November 2002 | pmid = 12595133 | doi = 10.1016/S0300-9084(02)00020-2 }}</ref>