Nucleoside triphosphate-dependent restriction enzymes

The known nucleoside triphosphate-dependent restriction enzymes are hetero- oligomeric proteins that behave as molecular machines in response to their target sequences. They translocate DNA in a process dependent on the hydrol ysis of a nucleoside triphosphate. For the ATP-dependent type I and type II I restriction and modification systems, the collision of translocating comp lexes triggers hydrolysis of phosphodiester bonds in unmodified DNA to gene rate double-strand breaks. Type I endonucleases; break the DNA at unspecifi ed sequences remote from the target sequence, type III endonucleases at a f ixed position close to the target sequence. Type I and type III restriction and modification (R-M) systems are notable for effective post-translationa l control of their endonuclease activity. For some type I enzymes, this con trol is mediated by proteolytic degradation of that subunit of the complex which is essential for DNA translocation and breakage. This control, lackin g in the well-studied type II R-M systems, provides extraordinarily effecti ve protection of resident DNA should it acquire unmodified target sequences . The only well-documented GTP-dependent restriction enzyme, McrBC, require s methylated target sequences for the initiation of phosphodiester bond cle avage.