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. Binds 1 [4Fe-4S] cluster per subunit. It may be important for protein stability, since mutation of the Cys that binds the cofactor leads to a colorless, insoluble protein. In culture expression is high during early log phase (1 hour), decreases and then rises again in late stationary phase (16 hours). During infection of mouse bone marrow-derived macrophages (BMDM) expression is maximal after 2 hours, when the bacteria is expected to be in the phagosome. No effect on expression of bacterial lipoprotein FTN_1103. Bacteria are as virulent in mice as wild-type bacteria. Part of a type II CRISPR-Cas system. Belongs to the CRISPR-associated exonuclease Cas4 family.