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). The Cas4 region acts as a ssDNA exonuclease, while the Cas1 region acts as a dsDNA endonuclease. Involved in the integration of spacer DNA into the CRISPR cassette (By similarity). exonucleolytic cleavage in the 5'- to 3'-direction to yield nucleoside 3'-phosphates. Binds 1 [4Fe-4S] cluster per subunit. Homodimer, forms a heterotetramer with a Cas2 homodimer. Operon expression begins by 6 hours after starvation has initiated development and is under strong negative regulation by DevS. Part of an operon going from at least MXAN_7266 to MXAN_7259 that includes a CRISPR operon with transcription continuing into the pre-crRNA locus. In the N-terminal section; belongs to the CRISPR-associated exonuclease Cas4 family. In the C-terminal section; belongs to the CRISPR-associated endonuclease Cas1 family.