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). Functions as a ssRNA-specific endoribonuclease. Involved in the integration of spacer DNA into the CRISPR cassette. Plasmid targeted by CRISPR locus P1 transform wild-type cells very poorly (PubMed:22767603). Homodimer, forms a heterotetramer with a Cas1 homodimer. Loss of the 8 Cas genes in this locus (cas1, cas2, cas3, cas4, cas5, cas6, cas7 and cas8b) leads to loss of CRISPR interference against plasmid targeted by this CRISPR locus, i.e. plasmid is not destroyed by CRISPR. There are 3 CRISPR RNA loci in this organism and a single cas gene locus. A CRISPR-Cas type I-B system. Belongs to the CRISPR-associated endoribonuclease Cas2 protein family.