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). Cyclic trinucleotide synthase that catalyzes the synthesis of 3',3',3'-cyclic AMP-AMP-GMP as the major product, a second messenger for cell signal transduction. 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. 2 ATP + GTP = cyclic AMP-AMP-GMP + 3 diphosphate Binds 1 Mg(2+) ion per subunit. Part of the CBASS operon consisting of cap4-cdnD-cap2-cap3. Belongs to the CD-NTase family. D02 subfamily.