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). This may be a 5' to 3' ssDNA exonuclease (By similarity). 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. It may be important for protein stability, since mutation of the Cys that bind the cofactor leads to a colorless, insoluble protein. Can form a Cascis complex with Cas1/2 and Csa1. Slightly induced by 20 J/m2 ultraviolet light. Member of the csa1-cas1/2-cas4 operon. Belongs to the CRISPR-associated exonuclease Cas4 family.