<P> Type IIB restriction enzymes (e.g., BcgI and BplI) are multimers, containing more than one subunit . They cleave DNA on both sides of their recognition to cut out the recognition site . They require both AdoMet and Mg cofactors . Type IIE restriction endonucleases (e.g., NaeI) cleave DNA following interaction with two copies of their recognition sequence . One recognition site acts as the target for cleavage, while the other acts as an allosteric effector that speeds up or improves the efficiency of enzyme cleavage . Similar to type IIE enzymes, type IIF restriction endonucleases (e.g. NgoMIV) interact with two copies of their recognition sequence but cleave both sequences at the same time . Type IIG restriction endonucleases (e.g., Eco57I) do have a single subunit, like classical Type II restriction enzymes, but require the cofactor AdoMet to be active . Type IIM restriction endonucleases, such as DpnI, are able to recognize and cut methylated DNA . Type IIS restriction endonucleases (e.g., FokI) cleave DNA at a defined distance from their non-palindromic asymmetric recognition sites; this characteristic is widely used to perform in - vitro cloning techniques such as Golden Gate cloning . These enzymes may function as dimers . Similarly, Type IIT restriction enzymes (e.g., Bpu10I and BslI) are composed of two different subunits . Some recognize palindromic sequences while others have asymmetric recognition sites . </P> <P> 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 . These enzymes contain more than one subunit and require AdoMet and ATP cofactors for their roles in DNA methylation and restriction, respectively . They are components of prokaryotic DNA restriction - modification mechanisms that protect the organism against invading foreign DNA . Type III enzymes are hetero - oligomeric, multifunctional proteins composed of two subunits, Res and Mod . 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, although it has no enzymatic activity on its own . Type III enzymes recognise short 5--6 bp - long asymmetric DNA sequences and cleave 25--27 bp downstream to leave short, single - stranded 5' protrusions . They require the presence of two inversely oriented unmethylated recognition sites for restriction to occur . These enzymes methylate only one strand of the DNA, at the N - 6 position of adenosyl residues, so newly replicated DNA will have only one strand methylated, which is sufficient to protect against restriction . Type III enzymes belong to the beta - subfamily of N6 adenine methyltransferases, containing the nine motifs that characterise this family, including motif I, the AdoMet binding pocket (FXGXG), and motif IV, the catalytic region (S / D / N (PP) Y / F). </P> <P> Type IV enzymes recognize modified, typically methylated DNA and are exemplified by the McrBC and Mrr systems of E. coli . </P> <P> Type V restriction enzymes (e.g., the cas9 - gRNA complex from CRISPRs) utilize guide RNAs to target specific non-palindromic sequences found on invading organisms . They can cut DNA of variable length, provided that a suitable guide RNA is provided . The flexibility and ease of use of these enzymes make them promising for future genetic engineering applications . </P>

Can pcr act as molecular scissors to insert new nucleotides at specific dna locations