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(AAAA) CBASS system (PubMed:32839535). Binds cyclic nucleotide second messengers (synthesized by CdnD, the cognate CD-NTase in the CBASS operon). Ligand binding activates it to endonucleolytically degrade dsDNA to approximately 6 bp length fragments, with a preference for 5'-C or 5'-G cleavage site. The minor product of CdnD is the activating nucleotide; also binds the major product (2',3',3'-cyclic AMP-AMP-AMP) but is not activated by it. Only binds DNA in the presence of ligand. Is not activated by c-di-AMP, c-di-GMP, 3'3'-cyclic GMP-AMP (3'3'-cGAMP) or 3',3',3'-cyclic AMP-AMP-GMP. 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 cdnD-cap2-cap3-cap4. The cyclic nucleotide ligand binds in the C-terminal SAVED domain. DNA binding requires the extreme N-terminus. Belongs to the Cap4 nuclease family.