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) (By similarity). This protein is a 5' to 3' partially processive exonuclease that cleaves off single mononucleotides. Has a marked preference for ssDNA, although in vitro it also acts on dsDNA and ssRNA. Has low endonuclease activity with circular ssDNA. Binds ssDNA and can unwind dsDNA; unwinding does not require ATP. exonucleolytic cleavage in the 5'- to 3'-direction to yield nucleoside 3'-phosphates. Mg(2+) or Mn(2+) required for ssDNA cleavage activity. Can also utilise Cu(2+). Binds 1 [4Fe-4S] cluster per subunit (PubMed:24171432). It may be important for protein stability, since mutation of the Cys that bind the cofactor leads to a colorless, insoluble protein. Optimum pH is 9-10. Optimum temperature is 65-70 degrees Celsius. Homodecamer, formed by the assembly of dimers into a decameric toroid, with the active sites oriented towards the central tunnel. Belongs to the CRISPR-associated exonuclease Cas4 family.