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 (Probable). This CRISPR-Cas system protects bacteria against transformation with plasmids containing DNA homologous to its spacer regions (PubMed:29979631). This subunit is a single-strand-specific endoribonuclease (ssRNase) that is stimulated by cyclic oligoadenylates (cOA) produced by the Cas10 subunit of the Csm effector complex (By similarity). Non-specific ssRNase activity is allosterically activated by cyclic oligoadenylates (cOA), a second messenger produced by Cas10 of the ternary Csm effector complex in the presence of a cognate target RNA. Homodimer; the composite ssRNase active site is formed at the dimer interface. The N-terminal CRISPR-associated Rossman fold (CARF) probably binds the cOA effector. ssRNase activity resides in the C-terminal HEPN domain. Deletion of the entire CRISPR-Cas locus (cas6 to cas2, Rv2824c to Rv2816c) decreases resistance to plasmids encoding spacer elements about 6-fold. Encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Csm6 family. Truncated N-terminus.