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). Processes pre-crRNA into individual crRNA units. Homodimer. Part of the probable Csy ribonucleoprotein complex with cys1, cys2, csy3, cas6f and crRNA. Expressed in late exponential phase (other phases not tested); part of a large cas-CRISPR3 polycistronic operon. Loss of crRNA processing (PubMed:21519197), no chromosomal targeting (PubMed:23637624). If the spacer DNA has a perfect match in the chromosome then toxicity results. Suppression of the toxic effects occurs via mutations in the CRISPR/Cas machinery, or via target deletion, which might contribute to genome plasticity. This CRISPR/Cas system can be used to remove genomic islands, and possibly other genomic regions. Belongs to the CRISPR-associated endoribonuclease Cas6 family. Cas6f/Csy4, subtype I-F/Ypest subfamily.