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). In strain P2 toxic when expressed in vivo in the absence of CRISPR loci A-D (strain P2 contains 6 CRISPR loci in all). It causes strong growth inhibition, a significant proportion of cells have degraded or no DNA by 20-24 post-induction (PubMed:25277654). Probably oligomeric upon overexpression; does not interact with a Cas5 (Sso1441)/Cas7 (Sso1442)/crRNA complex (PubMed:23846216). Forms dimer and tetramer-sized oligomers when overexpressed in the absence of CRISPR loci A-D; the oligomers are probably responsible for its toxicity in the absence of CRISPR loci A-D (PubMed:25277654). Part of the aCascade ribonucleoprotein complex, minimally composed of Cas7 (formerly Csa2) and Cas5a, which binds crRNA. Other possible components of aCascade in strain P1 are Cas6b (SSO1437) and Csa5 (SSO1443), while SSO1399, Cas5b (SSO1400) and SSO1401 have sometimes been seen weakly associated. Csa2 is probably the major RNA-binding subunit. The Csa2-Cas5a-crRNA complex also binds target DNA homologous to crRNA, probably forming an R-loop. Purified aCascade forms a filament about 6 nm in width (PubMed:21507944). The aCascade complex was purified from strain P1.