Figure 1.
Reaction scheme of translocation as discussed in the text. EF-G (green) is depicted in three conformations: the GTP-bound form, an intermediate GDP-bound form on the ribosome, and the GDP-bound form that dissociates from the ribosome (step 6); the latter conformation probably is represented in the crystal structures of EF-G-GDP and nucleotide-free EF-G (11, 12). Pi is omitted because it is not known when after GTP hydrolysis it is released. The transition state of the ribosome, formed in step 3, is symbolized by an altered conformation of the small ribosomal subunit; of course, alterations of the large subunit or an altered arrangement of the subunits relative to each other also may be involved. A, P (P*), and E denote the tRNA binding sites on the two subunits and are indicated when occupied.