CBASS (cyclic oligonucleotide-based antiphage signaling system) provides immunity against bacteriophage. The CD-NTase protein synthesizes cyclic nucleotides in response to infection; these serve as specific second messenger signals. The signals activate a diverse range of effectors, leading to bacterial cell death and thus abortive phage infection. A type III-C(AAA) CBASS system (PubMed:32839535). A cyclic nucleotide-activated dsDNase. In the presence of 3',3',3'-cyclic AMP-AMP-AMP (cAAA), and to a lesser extent 3',3',3'-cyclic AMP-AMP-GMP (cAAG) and cyclic-di-AMP (c-di-AMP), endonucleolytically degrades dsDNA (PubMed:31932165, PubMed:31932164). Binds one cAAA in a pocket on one surface of the trimer; cAAA binding promotes hexamerization, which is necessary for nuclease activation. Also binds c-diAMP or linear di-AMP with lower affinity. The nuclease digests dsDNA to about 50 bp lengths with a 2-base 3' overhang and a consensus recognition site of 5'-Axx|T-3'. 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 and the 2-base overhang (PubMed:31932164). Protects E.coli strain JP313 against bacteriophage lambda cI- infection. When the cdnC-cap7-cap6-nucC operon is transformed into a susceptible strain it confers bacteriophage immunity. Mutations in the sensor (Cap7 also called HORMA) or effector proteins (CdnC, NucC) but not the disassembly protein (Cap6 also called Trip13) no longer confer immunity. The presence of the intact operon leads to culture collapse and cell death which occurs before the phage has finished its replication cycle, thus protecting non-infected bacteria by aborting the phage infection and preventing its propagation. Activated by cAAA and to a lesser extent cAA and cAAG; cAAA and cAA are products of its cognate CD-NTase. Cyclic nucleotide binding causes hexamerization (PubMed:31932165). Cyclic nucleotide binding causes a series of shifts that enclose the cAAA molecule, enable hexamer formation and juxtapose pairs of active sites to allow dsDNA cleavage (PubMed:31932164). Self-oligomerizes (PubMed:31932165). Forms homotrimers; in the presence of cAAA the trimers associate face-to-face to form homohexamers. 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 (PubMed:31932164). Belongs to the NucC endonuclease family.