Comparison of serine transposase dimer and resolvase structures. (a) The dimer formed by the catalytic domains of OrfA from ISC1904 from Sulfolobus solfataricus P2 (PDBid 3ilx). The N-terminal DNA binding domain was removed before crystallization. One subunit is in green, one blue, the active site serines are shown as red spheres, three of the active site arginines are shown as blue sticks, and two negatively charged residues conserved only in IS607-family serine transposases are shown as magenta sticks. (b) Superposition of one subunit from the γδ resolvase dimer shown in Figure 3a (pale yellow) with one subunit of the serine transposase (green). The cores of the catalytic domains, comprising a 4-stranded beta sheet and the first 4 helices, have nearly identical tertiary structures. The structures diverge at a point identified as a flexible hinge in studies of other serine recombinases. (c) The serine transposase dimer’s E helices are packed similarly to those of the activated resolvase tetramer. The tetramer is shown in the same view and colouring as in Figure 3b, but only β strand 4 through helix E of each subunit is shown. The transposase dimer (blue and green) was superimposed on the left half of the tetramer using the E helices as guides. (d) Same as (c), but rotated ~90° about a vertical axis.