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. Absolutely required for crRNA production or stability. Upon expression in E.coli endonucleolytically processes pre-crRNA, although disruption and reconstitution experiments indicate that in situ other genes are also required for processing. Yields 5'-hydroxy and 3'-phosphate groups. The Csy ribonucleoprotein complex binds target ssDNA with high affinity but target dsDNA with much lower affinity. Metal-ion independent. Binds processed crRNA. Part of the Csy ribonucleoprotein complex with a probable stoichiometry of Csy1(1),Csy2(1),Csy3(6),Cas6/Csy4(1)-crRNA(1). A Csy3(6),Cas6/Csy4(1)-crRNA(1) subcomplex is also formed. Infection with phage DMS3 inhibits biofilm formation; disrupting this gene restores biofilm formation despite DMS3 infection. Normal biofilm formation in the absence of phage infection. Loss of production of crRNA in the presence or absence of phage. Disruption of the entire Y.pestis-subtype CRISPR region disrupts crRNA production but does not alter phage resistance (possibly OLNs PA14_33350 to PA14_33310, plus the flanking CRISPR loci), indicating this CRISPR is not involved in phage resistance. In this bacteria, Y.pestis-subtype CRISPRs do not confer resistance to phage DMS3 or MP22, but instead are required for DMS3-dependent inhibition of biofilm formation and possibly motility. Belongs to the CRISPR-associated endoribonuclease Cas6 family. Cas6f/Csy4, subtype I-F/Ypest subfamily. Truncated N-terminus.