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. Functions as a ssDNA-dependent ATPase; dsDNA, ssRNA do not stimulate ATPase activity, while other nucleotides (aside from dATP) are not hydrolyzed. Functions as a ssDNA nuclease; activity does not require ATP. Functions as an ATP-dependent helicase; helicase activity requires hydrolysable ATP. Unwinds both DNA/DNA hybrids and RNA/DNA hybrids, moving mostly in a 3' to 5' direction. Mn(2+) or Mg(2+) or Ca(2+). ATPase and nuclease activities are dependent on divalent cations, for ATPase Mn(2+) is marginally preferred over Mg(2+) or Ca(2+). Nuclease activity is inhibited by EDTA. 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. Found in the E.coli-type CRISPR4/cas system. In the N-terminal section; belongs to the CRISPR-associated nuclease Cas3-HD family. In the central section; belongs to the CRISPR-associated helicase Cas3 family.