The packing of ribosomes in crystals and polysomes

Abstract

Many structures of the bacterial ribosome have been determined at high resolution by x-ray crystallography over the past few years. These structures span a number of steps in the protein elongation and termination cycle, and include a wide variety of conformations of the ribosome. While these structures are landmarks in our understanding of protein synthesis, the very fact that ribosomes crystallize at all is quite striking. Notably, bacterial ribosomes pack in most of the crystal forms by recapitulating the organization of polysomes. While not all bacterial polysomes adopt the same overall architecture, the fact that ribosome crystals recapitulate common forms of polysomes has structural implications for how these polysomes function. We show that ribosome packing in crystals can be correlated with ribosome organization in active polysomes if ribosome dynamics are considered. The packing arrangement also suggests possible means for polysome regulation that may be used in bacteria.