Partial spontaneous intersubunit rotations in pretranslocation ribosomes.

The two main steps of translation, peptidyl transfer, and translocation are accompanied by counterclockwise and clockwise rotations of the large and small ribosomal subunits with respect to each other. Upon peptidyl transfer, the small ribosomal subunit rotates counterclockwise relative to the large subunit, placing the ribosome into the rotated conformation. Simultaneously, tRNAs move into the hybrid conformation, and the L1 stalk moves inward toward the P-site tRNA. The conformational dynamics of pretranslocation ribosomes were extensively studied by ensemble and single-molecule methods. Different experimental modalities tracking ribosomal subunits, tRNAs, and the L1 stalk showed that pretranslocation ribosomes undergo spontaneous conformational transitions. Thus, peptidyl transfer unlocks the ribosome and decreases an energy barrier for the reverse ribosome rotation during translocation. However, the tracking of translation with ribosomes labeled at rRNA helices h44 and H101 showed a lack of spontaneous rotations in pretranslocation complexes. Therefore, reverse intersubunit rotations occur during EF-G catalyzed translocation. To reconcile these views, we used high-speed single-molecule microscopy to follow translation in real time. We showed spontaneous rotations in puromycin-released h44-H101 dye-labeled ribosomes. During elongation, the h44-H101 ribosomes undergo partial spontaneous rotations. Spontaneous rotations in h44-H101-labeled ribosomes are restricted prior to aminoacyl-tRNA binding. The pretranslocation h44-H101 ribosomes spontaneously exchanged between three different rotational states. This demonstrates that peptidyl transfer unlocks spontaneous rotations and pretranslocation ribosomes can adopt several thermally accessible conformations, thus supporting the Brownian model of translocation.