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-A(GA) CBASS system (PubMed:32839535). Catalyzes the synthesis of 3'3'-cyclic GMP-AMP (3'3'-cGAMP) from GTP and ATP, a second messenger in cell signal transduction. Is also able to produce c-di-AMP and c-di-GMP from ATP and GTP, respectively; however, 3'3'-cGAMP is the dominant molecule produced by DncV in vivo, contrary to the 2'3'-cGAMP produced by eukaryotes. Is required for efficient V.cholerae intestinal colonization, and down-regulates the colonization-influencing process of chemotaxis. Is not active with dATP, TTP, UTP, and CTP. Controls the activity of cGAMP-activated phospholipase CapV, a patatin-like lipase that is a direct 3',3'-cGAMP receptor encoded in the dncV operon (PubMed:29891656). Protects E.coli against phage T2 infection. When the CBASS operon (capV-dcnV-cap2-cap3) is introduced in E.coli MG1655 there is a more than 10(3) decrease in the efficiency of T2 plaque formation. Protects 100-fold against phage T5, offers no protection against T7 (PubMed:32544385). When the 4 gene operon capV-dncV-cap2-cap3 is introduced in E.coli MG1655 there is about 100-fold protection against phages P1 and T2 (PubMed:31533127). ATP + GTP = 3',3'-cGAMP + 2 diphosphate Binds 1 Mg(2+) ion per subunit. Enzymatic activity of DncV is inhibited by folate-like molecules, such as 5-methyltetrahydrofolate di-glutamate and 5-methyltetrahydrofolate, suggesting the existence of a signaling pathway that links folate-like metabolism cofactors to the regulation of cyclic dinucleotide second messenger synthesis (PubMed:25201413). Lacks a regulatory loop and is constitutively activated (PubMed:25131990). Monomer. Expression is repressed by the transcriptional regulator VspR (PubMed:22500802). Part of the CBASS operon consisting of capV-dncV-cap2-cap3 (Probable). Cells lacking this gene show a significant defect in intestinal colonization. Overproduction of the cGAMP synthase DncV leads to an increase in intracellular cGAMP, and induces the formation of cells more translucent than normal with a defect in cell membrane integrity, planktonic growth arrest and a small colony phenotype in El Tor V.cholerae. Belongs to the CD-NTase family. A01 subfamily.