Figure 4.

Spiraling arrangement of the functional pore loops of the NtrC1 activator. (A) The L1 (GAFTGA) and L2 loops surrounding the pore of the NtrC1 hexamer; the coloring is the same as in A. Electron density is contoured at 1.5σ. (B) Left-handed spiral staircase of loops L1 and L2, with labels for subunits A and F illustrating the change in elevation. (C) GAFTGA loops of subunits F and A. (D) Overlay of subunit A with F, L, R, and X of the S-crystal structure, showing rigid body roll of stem–loops L1 and L2. Subunit A (essentially identical to subunits B–E in all four rings) follows the ATP-bound state of 3M0E, while subunits F and R follow the ADP-bound state of 1NY6 subunits L and X, which are poised between the two extremes. In the Q-crystal structure, the first five subunits of each ring are in the ATP-like state, and subunits F, L, R, and X are like F and R in the S crystal.