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 with 5' to 3' and perhaps 3' to 5' activity. Binds 1 [4Fe-4S] cluster per subunit. May not be required for nuclease activity. Mn(2+) required for nuclease activity. Can form a Cascis complex with Cas4 and Csa1. Slightly induced by 20 J/m2 ultraviolet light. Member of the csa1-cas1/2-cas4 operon. Belongs to the CRISPR-associated protein Csa1 family.