Figure 2.

Alterations in the helicase domains upon binding of Chi. (A) Superposition of the AddA helicase motor domains with bound ADPNP (sticks), Mg2⁺ (spheres) and ssDNA (tube and sticks) from the binary (grey), ternary (coloured as in Fig. 1D) and Chi-bound (coloured by domain in lighter shades than the ternary structure) crystal structures. The Chi-bound 1A domain matches that in the binary structure whereas the 2A and ssDNA are in between the relative conformations of the binary and ternary states. (B) AddB helicase-like 1A and 2A domains from the Chi-bound complex. At the Chi sequence, the ssDNA flips by 180° to form specific interactions with the 1A domain.