CRISPR (clustered regularly interspaced short palindromic repeat) is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). The type III-A Csm effector complex binds crRNA and acts as a crRNA-guided RNase, DNase and cyclic oligoadenylate synthase; binding of target RNA cognate to the crRNA is required for all activities. In a heterologous host the appropriately targeted Csm effector complex prevents growth of dsDNA phage phiNM1-gamma6. ssDNase activity is stimulated in the ternary Csm effector complex; binding of cognate target RNA activates the ssDNase, as the target RNA is degraded ssDNA activity decreases. This subunit is a single-strand-specific deoxyribonuclease (ssDNase) which digests both linear and circular ssDNA; it has both exo- and endonuclease activity. When associated with the ternary Csm effector complex (the crRNA, Cas proteins and a cognate target ssRNA) synthesizes cyclic oligoadenylates (cOA) from ATP, producing (mostly) cyclic hexaadenylate (cA6). cA6 synthesis occurs in the Csm effector complex and requires cognate target RNA and ATP; other NTPs are not incorporated. cOAs are second messengers that induce an antiviral state important for defense against invading nucleic acids. 6 ATP = cyclic hexaadenylate + 6 diphosphate Synthesis of cA6 requires Mg(2+). Synthesis of cA6 is inhibited by EDTA. Part of the Csm effector complex that includes Cas10, Csm2, Csm3, Csm4 and Csm5. The N-terminal HD domain has ssDNase activity. The C-terminal GGDEF domain has the cOA synthesis activity. Encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Cas10/Csm1 family.