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 deoxyribonuclease (ssDNase) which digests both linear and circular ssDNA; it has both exo- and endonuclease activity. 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. When associated with the ternary Csm effector complex (the crRNA, Cas proteins and a cognate target ssRNA) synthesizes cyclic oligoadenylates (cOA) from ATP. cOAs are second messengers that stimulate the ssRNase activity of Csm6, inducing an antiviral state important for defense against invading nucleic acids. Part of the Csm effector complex, that includes Cas10, Csm2, Csm3, Csm4, Csm5 and mature crRNA. The N-terminal HD domain has ssDNase activity. The C-terminal GGDEF domain has the cOA synthesis activity. 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 Cas10/Csm1 family.