One-dimensional disorder in spinach ribulose bisphosphate carboxylase crystals

X-ray diffraction photographs of the crystalline ternary complex of spinach ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) with activators Mg²⁺ and CO₂ diffract to high angle while showing diffraction features symptomatic of disorder. Oscillation photographs about the c* axis taken with a conventional source and collimation show streaks of intensity along c*. These intensity streaks can be resolved into satellite reflections, with underlying diffuse scattering, by employing synchrotron radiation with slits limiting the horizontal beam convergence from the monochromator and fine collimation. The disorder is discussed in terms of stacking faults between layers of molecules. The space group of a regular stack of layers, repeating every layer, would be C2. The observed space group C222₁, repeating every alternate layer, is due to the statistical 'back-to- back' arrangement of cells of C2 symmetry. Three models are used to predict various features of the diffraction by these crystals. The first model, with every layer randomly shifted by either a/39 or -a/39 with respect to the previous layer, predicts constant intensity streaks for the maximally affected reflections. A rectangular modulation of odd layers within a framework of even layers predicts satellite reflections for these reflections. A random distribution of these layers predicts broadening of the affected reflections. The observation of superlattice reflections and broadening is best accounted for by considering an approximate superlattice which produces the satellite reflections with both deviations in period and by additional shifts of ± a/39 leading to the diffuse features.