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-B(UU) CBASS system (PubMed:32839535). Cyclic dinucleotide synthase that catalyzes the synthesis of 3'3'-cyclic UMP-AMP (cUMP-AMP) from UTP and ATP, a second messenger for cell signal transduction. ATP + UTP = 3',3'-cUAMP + 2 diphosphate Binds 1 Mg(2+) ion per subunit. Belongs to the CD-NTase family. E01 subfamily.