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 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. This protein is not part of the Csm complex. 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. A single-strand-specific endoribonuclease (ssRNase) that is approximately 1000-fold stimulated by cyclic oligoadenylate (cOA); although several species of cOA are synthesized by this organism only cyclic hexaadenylate (cA6) stimulates the ssRNase activity. Cleaves preferentially within GA or AA dinucleotides, although the presence of cA6 broadens the preference. Linear oligoadenylates do not activate the RNase. Non-specific ssRNase activity is allosterically activated about 1000-fold by cyclic hexaadenylate (cA6), a second messenger produced by Cas10 of the ternary Csm effector complex in the presence of a cognate target RNA. ssRNase activity is inhibited by physiological concentrations of ATP (1 mM), activity is restored by cOA. Homodimer (PubMed:28663439). The composite ssRNase active site is formed at the dimer interface (By similarity). The N-terminal CRISPR-associated Rossman fold (CARF) probably binds the cA6 effector. ssRNase activity resides in the C-terminal HEPN domain. Encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Csm6 family.