Figure 4.

Structure of the DNA nuclease Cas3. The type I-E Cas3 crystal structure of Th. fusca (pdb: 4QQW) reveals two tandem RecA-like domains, one HD-type nuclease domain and a CTD located at the top of the ensemble. The core helicase, containing two RecA-like domains, forms a cleft that locates the residues for the binding of NTP, Mg²⁺ ions and the ssDNA substrate. Two Fe(II) ions are located at the catalytic center's HD motif. The 5^′ end of the ssDNA enters Cas3 from the RecA2 side and is further threaded to RecA1 and the HD-type nuclease domain (indicated by a scissor). The CTD is proposed to close the ssDNA channel and to contact the Cascade complex.