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). Cas3 plus Cascade participate in CRISPR interference, the third stage of CRISPR immunity. Probably unwinds dsDNA templates, thus providing substrates for CRISPR-associated endonuclease Cas3-HD. Unwinding is strongly increased in the presence of ATP, implying the helicase uses ATP to unwind substrate (By similarity). Can form a Cascade complex with Csa5, Cas7, Cas5a, Cas3' and Cas8a2. Repressed by 5 J/m2 ultraviolet light and 50 mM NaCl, slightly induced by 20 J/m2 ultraviolet light, 100 and 150 mM Nacl. Member of the csa5-cas7-cas5a-cas3-cas3'-cas8a2 operon. Proteins of this family have an N-terminal nuclease domain and a C-terminal helicase/ATPase domain. In some CRISPR/Cas systems the domains are swapped, in others they are encoded separately. Belongs to the CRISPR-associated helicase Cas3 family.