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 this Csm effector complex restricts ssRNA phage MS2, suggesting it may target RNA viruses in vivo. Csm functions as a non-specific ssDNase. Base-pairing between crRNA and target RNA to form a ternary Csm complex activates a ssDNase activity; target RNA cleavage suppresses the ssDNase, a temporal control that prevents uncontrolled DNA degradation. Viral RNA transcripts probably tether the Csm complex to the viral genome, recruiting Cas10 ssDNA activity which is able to degrade DNA in the transcription bubble, spatially controlling the DNase activity. This subunit has a weak ssDNase activity that is dramatically activated by the ternary Csm effector complex (the crRNA, Cas proteins and a cognate target ssRNA). Target RNA and ssDNA are cleaved simultaneously, although RNase activity (of Csm3) is much faster. RNA cleavage by Csm3 is not required for ssDNase activity as Csm complex with inactive Csm3 still has ssDNase activity; however as the cleaved target RNA products dissociate away ssDNase activity decreases. Self-recognition, with subsequent repression of the ssDNase activity, occurs when the 5' handle of the crRNA bases pairs with the 3' flanking sequence of the target RNA (which would occur if the CRISPR locus were transcribed as an anti-pre-crRNA). This protein has low activity on dsDNA which is not stimulated by the Csm complex. 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 cyclic triadenylate (cA3) up to cyclic hexaadenylate (cA6), which is the active cOA. The enzyme is also able to cyclize pppA3 up to pppA6. cOAs are second messengers that induce an antiviral state important for defense against invading nucleic acids. Synthesis of cOA can occur with AMP plus ATP, 2'dATP or 3'dATP (but no other nucleotides), and requires a free 3'-OH ribose moiety. 6 ATP = cyclic hexaadenylate + 6 diphosphate Degradation of ssDNA requires Mn(2+), Co(2+) or Ni(2+); Mg(2+), Ca(2+) or Zn(2+) do not activate ssDNase activity (PubMed:27105119). Formation of cOA requires Mn(2+), Co(2+) or Zn(2+), Mg(2+) is not as efficient (PubMed:28663439). ssDNase activity is activated by target RNA binding to the Csm-crRNA complex and is inhibited by EDTA. Part of the Csm effector complex that includes at least Cas10(1), Csm2(3), Csm3(5), Csm4(1), Csm5(1) and mature crRNA (PubMed:25458845, PubMed:27105119, PubMed:28663439). The Csm complex is elongated and slightly twisted with a maximal length of 215 Angstroms and a diameter of 75-80 Angstroms (PubMed:25458845). It has been modeled to have a central protein filamant of Csm3 subunits along which the dsRNA helix of paired crRNA and target RNA binds. The filament is capped at one end by Cas10 and Csm4 and at the other end by Csm5; ssDNA is thought to bind to the N-terminal HD domain of Cas10 (Probable). Csm with a precursor crRNA does not include Csm5, while Cas6, the enzyme probably involved in pre-crRNA processing, is found associated with a subset of the Csm complex (PubMed:25458845). The N-terminal HD domain has ssDNase activity (PubMed:27105119). The C-terminal GGDEF domain has the cOA synthesis activity (PubMed:28663439). Encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Cas10/Csm1 family.