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). This protein is a sequence-specific endonuclease that cleaves pre-crRNA at G21 into mature crRNA. Does not cleave pre-crRNA associated with the T.thermophilus strain HB27 Cas5 protein (AC Q746C2) CRISPR locus (PubMed:23006625). The reaction mechanism may proceed by an intramolecular attack of the 2'-hydroxyl group of G21 on the scissile phosphodiester, cutting the precursor 3' to G21 residue yielding 5'-hydroxyl and 2' and/or 3' ends lacking a hydroxyl group (perhaps a 2'/3' cyclic phosphodiester) (Ref.2). Does not require a metal cofactor. Belongs to the CRISPR-associated protein Cas5 family. Subtype I-C/Dvulg subfamily.