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 spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). A cyclic nucleotide-activated dsDNase. In the presence of 3',3',3'-cyclic AMP-AMP-AMP (cAAA), and to a much lesser extent 3',3',3'-cyclic AMP-AMP-GMP (cAAG) and cyclic-di-AMP (c-di-AMP), endonucleolytically degrades dsDNA. The nuclease digests dsDNA to about 50 bp lengths. Not stimulated by cGAMP or linear di-AMP (PubMed:31932164). DNA has been modeled to contact a pair of juxtaposed active sites (one from each layer of the hexamer), accounting for cleavage on both strands (By similarity). Activated by cAAA and to a lesser extent cAA; cAAA binding promotes hexamerization which is probably necessary for nuclease activation. Forms homotrimers and homohexamers; in the presence of cAAA only the homohexamers of face-to-face associated trimers are formed (PubMed:31932164). The 2 cAAA-binding sites are on the exterior of the hexamer at the three-way junction, there are maximally 2 cyclic nucleotides per hexamer (Probable). Encoded in a type III-B CRISPR locus. Belongs to the NucC endonuclease family.