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). Acts as a dsDNA endonuclease. Involved in the integration of spacer DNA into the CRISPR cassette. Homodimer, forms a heterotetramer with a Cas2 homodimer. In culture expression is high during early log phase (1 hour), decreases and then rises again in late stationary phase (16 hours). During infection of mouse bone marrow-derived macrophages (BMDM) expression is maximal after 1 hour, when the bacteria is expected to be in the phagosome. No effect on expression of bacterial lipoprotein FTN_1103. Bacteria are as virulent in mice as wild-type bacteria. Part of a type II CRISPR-Cas system. Belongs to the CRISPR-associated endonuclease Cas1 family.