Starvation in vivo for aminoacyl-tRNA increases the spatial separation between the two ribosomal subunits

Structures in situ of individual ribosomes in E. coli have been determined by computer-aided electron microscope tomography using a tilt series of positively stained embedded cellular sections. Amino acid starvation of a bacterial culture, causing a deficiency for aminoacyl-tRNA, induces a spatial separation between the ribosomal subunits compared with ribosomes in exponentially growing cells. Eight ribosomes from each growth condition were aligned to each other, and the two average structures were determined. Comparison of these suggests that the distance between the two subunits increases by approximately 3 nm upon starvation for aminoacyl-tRNA during protein synthesis. Ribosomes in most other states of the translational elongation cycle in exponentially growing cells show a more compact structure than previously realized.