Schematic illustration of the functional cycle of ERA and its role in ribosome biogenesis (Tu, Zhou, Tropea, Austin, Waugh, Court, and Ji. Proc. Natl. Acad. Sci. USA 2009; 106:14843-14848. Full Text). The GTPase domain is represented by a grey rectangle, the KH domain by an orange oval, GTP and GDP by purple cartoons, and the conformations of ERA by numbers in red. The pre-30S particle and 30S r-subunit are represented by larger grey ovals. The pre-16S rRNA and 16S rNRA are represented by a grey line with embedded AUCACCUCC sequence. The unoccupied ERA-binding pocket in the pre-30S particle and that in the 30S r-subunit are indicated in white. The four functional states of ERA, including (A) apo-ERA, (B) ERA-GTP, (C) ERA-GTP-pre-30S, and (D) ERA-GDP, are represented by the apo-ERA (PDB entry 1EGA), ERA-GNP (PDB entry 1WF3), ERA-GNP-RNA (PDB entry 3IEV), and ERA-GDP (PDB entry 3IEU) structures. In panel (C), the cleavage sites of RNase E, RNase G, and the unknown nuclease are indicated with numbered arrows 1, 2, and 3, respectively. (E) The pre-30S particle contains pre-16S rRNA. (F) The mature 30S r-subunit contains 16S rRNA. (G) The mature 30S r-subunit may bind apo-ERA in a distinct conformation 3 (PDB entry 1X1L).