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). A ssDNA exonuclease that has both 5' to 3' and 3' to 5' 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. Mn(2+) required for nuclease activity. Can also utilize Co(2+) and to a lesser extent Mg(2+) or Ni(2+). Binds 1 [4Fe-4S] cluster per subunit. Not required for nuclease activity, since mutation of the Cys residues leads to a colorless but active protein. Optimum pH is 7-9. Homodimer. Belongs to the CRISPR-associated protein Csa1 family.