Nucleotidyltransferase that catalyzes the formation of cyclic GMP-AMP (2',3'-cGAMP) from ATP and GTP and plays a key role in innate immunity (PubMed:34261127). Acts as a key sensor of double-stranded RNA (dsRNA), the presence of dsRNA in the cytoplasm being a danger signal that triggers the immune responses (PubMed:34261127). Directly binds dsRNA, activating the nucleotidyltransferase activity, leading to synthesis of 2',3'-cGAMP, a second messenger that binds to and activates Sting, thereby triggering the antiviral immune response via activation of the NF-kappa-B transcription factor Rel (Relish) (By similarity). ATP + GTP = 2',3'-cGAMP + 2 diphosphate The enzyme activity is specifically activated by double-stranded RNA (dsRNA). Belongs to the mab-21 family.