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 I-A(GA) CBASS system (PubMed:32839535). Cyclic dinucleotide synthase that catalyzes the synthesis of 3'3'-cyclic GMP-AMP (cGAMP) from GTP and ATP, and of c-di-AMP and c-di-GMP, that are second messengers for cell signal transduction. 2 ATP = 3',3'-c-di-AMP + 2 diphosphate 2 GTP = cyclic di-3',5'-guanylate + 2 diphosphate ATP + GTP = 3',3'-cGAMP + 2 diphosphate Binds 1 Mg(2+) ion per subunit. Belongs to the CD-NTase family. E01 subfamily.