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) (Probable). 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. This protein is not part of the Csm effector complex (Probable). A single-strand-specific endoribonuclease (ssRNase) producing free 5'-OH (PubMed:26763118). Activity is approximately 1000-fold stimulated by cyclic oligoadenylate (cOA); only cyclic tetraadenylate (cA4) stimulates the ssRNase activity while linear oligoadenylates do not activate the RNase (PubMed:28663439). Another study showed stimulation by linear tetraadenylate at very high concentrations, but did not examine stimulation by cA4 (PubMed:28722012). Does not require a metal cofactor. Non-specific ssRNase activity is allosterically activated about 1000-fold by cyclic tetraadenylate (cA4), which probably binds to its CARF domain. Homodimer. The protein forms a twisted, head-to-head dimer; the composite ssRNase active site is formed at the dimer interface. The N-terminal CRISPR-associated Rossman fold (CARF) probably binds the cA4 effector. ssRNase activity resides in the C-terminal HEPN domain. Probably encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Csm6 family.