Figure 5

Models of XerD activation.

(i) XerD is shown as a ribbon, modelled onto DNA by alignment to the Cre-loxP synaptic structure¹⁶, and (ii) shows a close up of the active site residues in their inactive state, as seen in the XerD_C-FtsKγ structure. The arrows in (i) indicate movement of helices in order to re-arrange the active site residues to the active configuration as seen in the Cre recombinase and result in the arrangement seen in (iii) and the active site poised for cleavage of DNA (iv). The helices involved in this movement are helix L (blue) helix M (green) and helix N (blue-green). (B) (i) The Cre synaptic structure is shown with the very C-terminal helices coloured to emphasise their interaction with the partner recombinase in a cyclic manner. (ii) 4 monomers of the “active” XerD conformation from (A) are shown superimposed on the position of the Cre monomers from the synapse in (i). The monomers are coloured to represent XerD (orange) and XerC (blue) in a XerD-active synapse. In order to achieve the same cyclic interactions as seen with Cre the C-terminal N-helices of each XerD monomer must break³⁶ and be donated into the adjacent recombinase partner as indicated by the arrows. (C) (i) A model representing the “active” arrangement of XerD from (A), shown in orange, with the FtsKγ domain in green. The groove in which the C-terminus of XerC is thought to bind is shown by the hatched region and extends to the interaction site of XerD with FtsKγ. (ii) The activated XerD conformation and the XerA structure were overlaid and, the position of the C-terminal tail of XerA is shown (blue helix) occupying the cleft in XerD.