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 II-C(AAG) CBASS system (PubMed:32839535). Binds second messenger 3',3',3'-cyclic AMP-AMP-GMP (cAAG). In the presence of 3',3',3'-cyclic AMP-AMP-GMP (synthesized by the cognate CD-NTase protein in the CBASS operon), endonucleolytically degrades dsDNA to approximately 17 bp length fragments, with a preference for 5'-C|NG sites. Only binds DNA in the presence of cAAG. Not activated by c-di-AMP, c-di-GMP, 3'3'-cyclic GMP-AMP (3'3'-cGAMP) or the second messenger of A.baumanii strain ATCC 27244. Protects E.coli against phage T2 infection. When the cdnD-cap2-cap3-cap4 operon is introduced in E.coli there is a more than 10(3) decrease in the efficiency of T2 plaque formation. The operon does not protect against phage T5 and only about 10-fold against T7. DNase activity is activated upon ligand binding. Inhibited by EDTA. A monomer in the absence of ligand, in its presence it forms oligomers. Part of the CBASS operon consisting of cap4-cdnD-cap2-cap3. The cyclic nucleotide ligand binds in the C-terminal SAVED domain. DNA binding requires the extreme N-terminus. Belongs to the Cap4 nuclease family.