An Enzyme-Catalyzed Multistep DNA Refolding Mechanism in Hairpin Telomere Formation
Figure 4
Structure of the hairpin telomere bound to TelA.
(A) The compact di-nucleotide hairpin DNA product. All bases except two at the apex (Ade3 and Thy4) form Watson–Crick base-pairs. The DNA strand near the tip of the hairpin loop adopts two alternative conformations as indicated by the red arrows. The two protein subunits and two DNA molecules in the TelA–DNA complex are all colored differently (yellow/green for DNA, and grey/cyan for protein) to highlight cis versus trans interactions made by TelA. (B) Schematic diagram of the hairpin DNA conformation. The red dots represent the scissile phosphates. (C and D) Simulated annealing composite omit 2Fo-Fc densities for the wild-type TelA–hairpin DNA complex (C) and the R255A unligated hairpin complex (D). Electron densities within 2.0 Å from the DNA or active site protein atoms are shown, contoured at 1.5 σ (blue) or 7.0 σ (red) above the mean levels. (E) Two hairpin ends in alternative conformations are packed tightly across the 2-fold axis, interacting with one another as well as with Arg205. Van der Waals radii for Arg205 and Thy4 are shown by dots.