Comparing the assembly of activated recombinase tetramers. (a) Cartoons of the γδ resolvase dimer and tetramer structures shown in Figure 3, highlighting the conformational changes that occur during tetramer formation. Red stars denote active sites. (b) Proposed pathway for conversion of an IS607-family transposase dimer to an activated tetramer. The first panel (top left) cartoons the catalytic domain crystal structures, with a DNA binding domain added to the N-termini. In the second panel (top right), the green subunit’s DNA binding domain and the blue subunit’s E-helix bind the same DNA half site (DNA carrying specific binding sites for the transposase is green; this segment represents the junction of the left and right ends in the circular form, as cartooned in Figure 1a). Unbending of the E-helix so that its C-terminal segment can bind DNA exposes a hydrophobic surface, which is satisfied by interactions with a second dimer (third panel; bottom left). A tetramer is thus formed on one DNA duplex. The conformational changes required to form this tetramer prearrange the remaining DNA binding moieties (the blue DBDs and the C-termini of the green subunits), which would lower the energy barrier to their interacting non-specifically with a target DNA of near-random sequence.