A model for ES functions and neighborhood-specific effects in ribosome biogenesis. (A) Structural model of the large ribosomal subunit showing ES color-coded according to the pre-rRNA processing steps in which they function. Green-colored ES are necessary for 27SA processing (early steps) and localize to the “equatorial belt.” Purple-colored ES participate in 27S processing, but not specifically 27SA processing (middle steps). These ES localize near the bottom third of the solvent side of the ribosome, near the polypeptide exit tunnel. Orange-colored ES participate in 7S pre-rRNA processing (late steps) and localize near the central protuberance. (B) The same color scheme (early, green; middle, purple; late, orange) is used to represent previously known r-protein depletion phenotypes. (C) Superimposition of A on B. See Results and Discussion for details of the model. (D) A case study of eukaryote-specific r-protein-rRNA elements. The left panel shows the body of r-protein L8 (in green, consistent with its “early” role in ribosome assembly) and its eukaryote-specific N-terminal extension (in orange, consistent with its “late” role in ribosome assembly). The center panel shows the contact established between the “late”-acting ES31L (shown as an orange-colored mesh) and the extension of L8. The right panel shows the 3′ end of 5.8S rRNA (dark gray) to denote the terminal site of pre-rRNA processing of ITS2 by the exosome. ES31L is in close contact with the 3′ end of 5.8S rRNA.
