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. 2015 Nov 30;112(50):15396–15401. doi: 10.1073/pnas.1509761112

Fig. 3.

Fig. 3.

The first six phases of the accretion model of ribosomal evolution. In Phase 1, ancestral RNAs form stem–loops and minihelices. In Phase 2, the LSU catalyzes the condensation of nonspecific oligomers. The SSU may have a single-stranded RNA-binding function. In Phase 3, the subunits associate, mediated by the expansion of tRNA from a minihelix to the modern L shape. LSU and SSU evolution is independent and uncorrelated during Phase 1–3. In Phase 4, evolution of the subunits is correlated. The ribosome is a noncoding diffusive ribozyme in which proto-mRNA and the SSU act as positioning cofactors. In Phase 5, the ribosome expands to an energy-driven, translocating, decoding machine. Phase 6 marks the completion of the common core with a proteinized surface (the proteins are omitted for clarity). The colors of the phases are the same as in Fig. 2. mRNA is shown in light green. The A-site tRNA is magenta, the P-site tRNA is cyan, and the E-site tRNA is dark green.