Extended Data Fig. 3: IS621 recombinase structure.

(a) Structural comparison of the RuvC domains of IS621 and Cas9 (PDB: 7S4X). The catalytic residues are shown as stick models. The core α-helices and β-strands are labeled. The RuvC active site of IS621 plays a role in coordinating a Mg²⁺ ion that stabilizes the 5′-phosphoserine DNA intermediates, thereby facilitating DNA cleavage and religation. In contrast, most RuvC domains, such as that of Cas9, bind two Mg²⁺ ions and catalyse the DNA cleavage reaction (i.e., the nucleophilic attack of an activated water molecule on the scissile phosphodiester bond in a substrate DNA). (b) Structures of the four IS621 protomers. The disordered S241 loops in IS621.1 and IS621.3 are indicated by dotted lines. (c) Superimposition of the RuvC domains in the four IS621 protomers. (d) Structure of the IS621 tetramer. The active sites formed by RuvC.1–Tnp.4 and RuvC.3–Tnp.2 are indicated by blue and magenta circles, respectively.