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). Involved in the integration of spacer DNA into the CRISPR cassette (By similarity). Acts as a ss- and dsDNA-specific endonuclease (PubMed:19523907). Mn(2+) supports cleavage of ss- and dsDNA, while Mg(2+) only supports digestion of dsDNA in vitro. Inhibited by EDTA. Homodimer (PubMed:19523907). This bacteria does not encode Cas2; Cas1 must interact with a different protein to insert spacers (Probable). Belongs to the CRISPR-associated endonuclease Cas1 family.