X-ray study of the paracrystalline nature of crystallized 70Sribosome lamellae

Abstract

If a mixed organic solvent is added to a ribosome solution and stored at 277 K in a crystallization chamber, thin lamellae of a few μm diameter can be obtained. Under the electron microscope, tetragonal face-centred macrocells of b = 40 and c = 55 nm are visible. These findings are substantiated by X-ray small- and wide-angle diffraction which leads to macrocells of b = 42 and c = 52 nm and establishes the third dimension of the macrocell with a = 42 nm. Mainly 0kl reflections appear. From their line widths, it follows that each lamella consists of at least 15 × 15 × 15 macrocells with small paracrystalline distortions gs - 1.5%, whilst their 6 × 8 × 12 subcells with a0 = 7.1, b0 = 5.3 and c0 = 4.3 nm have large fibrillary distortions in the bc plane, similar to the aggregation of microparacrystals in stretched and annealed polymers. With a new method, the folding root of the Q function projected on the b axis can be analysed. Two compounds are placed in each macrocell, each consisting of about 136 cylinders of about 2.0 nm diameter and 7 nm length. They are aligned to fibrils of different lengths orthogonal to the bc plane and occupy about 47% of the macrocell. The residual volume is filled up with an 'amorphous phase' as in semicrystalline polymers. A comprehensive study of the existing literature leads to the conclusion that the 'crystalline phase' consists of two 70S ribosome tetramers and that the cylinders are obviously parts of two-stranded helices, whilst most of the RNA proteins are in the 'amorphous phase'. The biological significance of the ribosome crystals and the paracrystalline sublattice is described in detail in the Discussion.