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) (By similarity). Part of the aCascade ribonucleoprotein complex, minimally composed of 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. Crystallography and cloning were done with strain P2, while interaction experiments were done with genes cloned from strain P2 but over-expressed in P1. The aCascade complex was purified from strain P1. Belongs to the CRISPR-associated protein Cas7/Cst2/DevR family. Subtype I-a/Apern subfamily.