IS607-family transposases and serine resolvases could form very similar activated tetramers. (a) Diagram of the proposed conformation changes during activation of ISC1904 transposase. One subunit from the crystallized dimer (grey) is superimposed on one subunit from the modelled tetramer (green) via the N-terminal portion of helix E. Arrows show the proposed motions of the catalytic domain core and the C-terminal segment of helix E. These rearrangements do not alter the N-terminal portion of helix E, nor its interactions with its partner subunit within the dimer (not shown here). (b) Model of the ISC1904 transposase tetramer. The DNA binding domain and its interactions with DNA were modelled on SoxR, PDBid 2zhg . The central two base pairs of each duplex were not modelled. (c) The ISC1904 model superimposed on the γδ resolvase tetramer (yellow; see Figure 3b) on which it was based. The misalignment of the lower right (light blue) DNA is due to a deviation of the resolvase structure from strict 222 symmetry.