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Catalytic domain of type IIs restriction endonuclease BfiI and similar proteins Catalytic domain of a novel type IIs restriction endonuclease BfiI and similar proteins. Type II restriction endonucleases are components of restriction modification (RM) systems that protect bacteria and archaea against invading foreign DNA. They usually function as homodimers or homotetramers that cleave DNA at defined sites of 4 to 8 bp in length, and they require Mg2+, not ATP or GTP, for catalysis. Unlike all other restriction enzymes known to date, BfiI is unique in cleaving DNA at fixed positions downstream of an asymmetric sequence in the absence of Mg2+. BfiI consists of two discrete domains with distinct functions: an N-terminal catalytic domain with non-specific nuclease activity and dimerization function that is more closely related to Nuc, an EDTA-resistant nuclease from the phospholipase D (PLD) superfamily; and a C-terminal domain that specifically recognizes its target sequences, 5'-ACTGGG-3'. BfiI presumably evolved through domain fusion of a DNA recognition domain to the catalytic Nuc-like domain from the PLD superfamily. Most PLD enzymes have two copies of the conserved HKD motif (H-x-K-x(4)-D, where x represents any amino acid residue) in a single polypeptide chain and both are required for catalytic activity. However, BfiI contains only one HKD motif per protein chain and forms a functionally active homodimer which has two DNA-binding surfaces located at the C-terminal domains but only one active site, located at the dimer interface between the two N-terminal catalytic domains that contain the two HKD motifs from both subunits. BfiI utilizes a single active site to cut both DNA strands, which represents a novel mechanism for the scission of double-stranded DNA. It uses a histidine residue from the HKD motif in one subunit as the nucleophile for the cleavage of the target phosphodiester bond in both of the anti-parallel DNA strands, while the symmetrically-related histidine residue from the HKD motif of the opposite subunit acts as the proton donor/acceptor during both strand-scission events. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",