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 (Probable). A type I-B(UU) CBASS system (PubMed:32839535). Cyclic dinucleotide synthase that catalyzes the synthesis of 3',3'-cyclic UMP-UMP (c-di-UMP) as the major product, and of 3',3'-cyclic CMP-UMP as a minor product, that are second messengers for cell signal transduction. 2 UTP = c-di-UMP + 2 diphosphate CTP + UTP = cyclic CMP-UMP + 2 diphosphate Binds 1 Mg(2+) ion per subunit. Belongs to the CD-NTase family. E02 subfamily. The sequence of this protein is available under the NCBI RefSeq accession WP_042646516, but the source is unknown.