Figure 1. Eukaryotic ribosome assembly. (a) Schematic representation of the assembly of eukaryotic ribosomes. Transcription by RNA polymerase I of the multiple, identical rDNA genes into precursor-rRNA (pre-rRNA) and transcription of the ribosomal protein genes (r-protein genes) as well as the genes encoding the trans-acting factors by RNA polymerase II, takes place in the nucleolus and nucleoplasm, respectively. The pre-RNA is processed into the mature 18S, 5.8S and 25–28S rRNA species in an ordered series of steps, involving modification of specific nucleotides and removal of the spacer regions. The mRNAs for r-proteins and trans-acting factors are exported to the cytoplasm, where they are translated by the ribosomes.The protein products are then imported into the nucleus where they associate in a strict order with the various intermediates of the pre-mRNA processing pathway. The trans-acting factors dissociate after having completed their task (see Figure 4). (b) The genetic organization of eukaryotic ribosomal RNA genes. Top: S. cerevisiae. Bottom: metazoan cells. Each rDNA unit encompasses the sequences for the mature rRNA species (dark blue), flanked by the external and internal transcribed spacers (ETS and ITS; light blue). Neighbouring rDNA units are separated by intergenic spacers (grey). In S. cerevisiae, but not in metazoans, each intergenic spacer also contains a gene for 5S rRNA, which is transcribed separately by RNA polymerase III.

Figure 2. Generalized models of snoRNA structure and function in pre-rRNA modification. (a) Box C/D snoRNAs guiding ribose (2¢-O-) methylation. Some C/D snoRNAs contain a second, internal copy of box D (D¢) also preceded by a ‘guide’ sequence, which allows the snoRNA to select two different ribose methylation sites. The site of modification is always the fifth nucleotide of the sequence complementary to the guide sequence. (b) Box H/ACA snoRNAs guiding formation. The pair of guide sequences used in selecting the U to be modified (indicated in pink) can occur in either hairpin. In some cases, both hairpins contain guide sequences allowing the snoRNA to select two different sites. Box H and Box ACA are located ~14 nucleotides downstream from the 3¢ member of the pair of guide sequences.

Figure 3. Pre-rRNA processing pathways in lower (panel A; S. cerevisiae) and higher (panel B) eukaryotic cells. At the top, the primary transcript including the processing sites is depicted with the subsequent intermediates below. Regions corresponding to the mature rRNAs are in green; spacer regions are in brown. Light-coloured intermediates are short-lived and not detectable under normal conditions. Vertical and horizontal scissors indicate endonucleolytic cleavages and exonucleolytic trimming, respectively. Blue scissors in the panel B indicate that the nature of the processing step is unknown but, by analogy with yeast, is likely to be exonucleolytic. Where known, the enzyme(s) involved in a particular processing step is identified. A question mark indicates that the enzyme is still unknown.

Figure 4. The pre-rRNA processing/assembly machinery in S. cerevisiae. The different intermediate pre-ribosomal particles so far identified are shown with their S value and, in parentheses, their pre-rRNA constituents. Note that the number of intermediates is tentative as is the identity of the components that associate or dissociate at the various stages. Only those trans-acting factors that can be assigned to a specific functional class are included in the diagram. The inset shows an EM picture of the ‘terminal balls’ thought to correspond to the early 80S particle (photograph by Dr Yvonne Osheim, University of Virginia).