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. A single-strand deoxyribonuclease (ssDNase) which digests linear and circular ssDNA; has 5'-3' and 3'-5' exonuclease activity as well as a less efficient endonuclease activity. Has a minimal size requirement; 100 nucleotide ssDNA (nt) is more efficiently digested than 50 or 25 nt ssDNA, while 14 nt ssDNA is not cleaved at all. It has no activity on dsDNA or ssRNA. 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. Ni(2+) and Mn(2+), but not Mg(2+), Ca(2+), Zn(2+), Co(2+), or Cu(2+), is required for ssDNase activity. ssDNase activity is inhibited by EDTA. Probably part of the Csm effector complex, that includes Cas10, Csm2, Csm3, Csm4, Csm5 and mature crRNA (By similarity). Will form a homodimer in solution, interacts with Csm4, which is a tighter, better association than the homodimeric Cas10 and uses the same interface for interaction (PubMed:25773141). The N-terminal HD domain has ssDNase activity (PubMed:25773141). The C-terminal GGDEF domain has the cOA synthesis activity (By similarity). Encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Cas10/Csm1 family.