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). Homodimer, forms a heterotetramer with a Cas2 homodimer. 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 endonuclease Cas1 family.