CRISPR (clustered regularly interspaced short palindromic repeat), is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain sequences complementary to antecedent mobile elements and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). Involved in the integration of spacer DNA into the CRISPR cassette (By similarity). Functions as a dsDNA-specific endonuclease, acting on circular and linear DNA; has no activity on ssRNA or ssDNA. Binds 1 Mg(2+) per homodimer, shared between the Asp-8 pair; divalent cations support activity in decreasing order, Mg(2+) >> Mn(2+) > Fe(2+) > Ni(2+) > Ca(2+). Inhibited by EDTA and Zn(2+). Activated by high concentrations of monovalent cation with a preference for K(+) over Na(+). Optimum pH is 7-10. Homodimer, forms a heterotetramer with a Cas1 homodimer. Belongs to the CRISPR-associated endoribonuclease Cas2 protein family.