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 sequences complementary to antecedent mobile elements and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA), formerly called psiRNA (prokaryotic silencing) in this organism. Part of the Cmr ribonucleoprotein complex which has divalent cation-dependent endoribonuclease activity specific for ssRNA complementary to the crRNA (target RNA), generating 5' hydroxy- and 3' phosphate or 2'-3' cyclic phosphate termini. This is probably the subunit that cleaves the target RNA (PubMed:25280103). Cmr complex does not cleave ssDNA complementary to the crRNA. Cleavage of target RNA is guided by the crRNA; substrate cleavage occurs a fixed distance (14 nt) from the 3' end of the crRNA. In vitro reconstitution shows Cmr1-2 and Cmr5 are not absolutely necessary for target cleavage (PubMed:19945378). Forms oligomers in isolation (PubMed:23370277). Part of the type III-B Cmr ribonucleoprotein (RNP) complex, an elongated RNP with Cmr2 and Cmr3 as the base, with Cmr4 and Cmr5 forming a helical core along the mature crRNA (39 or 45 nt in length), while the complex is capped by Cmr6 and Cmr1. The 5' end of the crRNA is bound to Cmr2 and Cmr3, while Cmr6 and a Cmr1 subunit (Cmr1-1 or Cmr1-2) cap the 3' end of the crRNA. The target RNA lies anti-parallel to the crRNA, with its 5' end near Cmr1 and Cmr6 and its 3' end near Cmr2 and Cmr3; major target RNA cleavage occurs nears the junction of Cmr1/Cmr6 and Cmr4/Cmr5, with minor cleavage occurring at 6 nt intervals which coincide with the proposed spacing of Cmr4 subunits (PubMed:24119404, PubMed:25280103). Interacts with Cmr5 (PubMed:23370277). Interacts with Cmr2, Cmr3, Cmr5 and Cmr6 (PubMed:25280103). Belongs to the CRISPR system Cmr4 family.