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). A ssDNA exonuclease that has 5' to 3' activity, yielding 5'-OH and 3'-phosphate groups. Has Mn(2+)-dependent endonuclease activity on circular ssDNA. Can unwind dsDNA; unwinding does not require ATP. exonucleolytic cleavage in the 5'- to 3'-direction to yield nucleoside 3'-phosphates. Binds 1 [2Fe-2S] cluster per subunit. Not required for nuclease activity, since mutation of the Cys residues leads to a colorless but active protein. Mn(2+) required for ssDNA cleavage activity. Can also utilize Co(2+) and to a lesser extent Mg(2+). Optimum pH is 7-9. Optimum temperature is 50-70 degrees Celsius. Monomer. Belongs to the CRISPR-associated exonuclease Cas4 family.