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Early events in eukaryotic ribosome biogenesis
The ribosome is one of the most important cellular machines. Its unique function, linking phenotype to genotype by translating mRNA into proteins, is conserved in all kingdoms of life. Despite this conserved function, the eukaryotic ribosome is substantially larger than its prokaryotic counterpart [1] and has a considerably more complicated assembly pathway. This process has been the subject of many biochemical studies, which discovered more than 200 eukaryote-specific factors involved in modifying, cleaving and folding of ribosomal RNA (rRNA) [2]. The biogenesis of a eukaryotic ribosome requires not only the concerted action of these factors but also all three RNA polymerases and many small nucleolar RNAs [2]. The maturation events are initiated in the nucleolus, where enzymes and chaperones assemble co-transcriptionally on a large precursor RNA (Figure 1A). During these steps, the small-subunit (SSU) processome is formed (Figure 1B, C). This giant nucleolar particle is essential and unique to eukaryotes and catalyzes the initial steps in the assembly of the ribosomal small-subunit.

Figure 1Assembly and structure of the small-subunit processome, a nucleolar ribosomal precursor. (A) A eukaryotic cell with the rDNA containing nucleolus highlighted. The right panel shows an actively transcribed rDNA locus with growing pre-rRNA and terminal knobs that represent pre-ribosomal particles. (B) Transcription dependent maturation pathway of the SSU processome, a particle containing more than 50 ribosome biogenesis factors. (C) The cryo-EM density of the small-subunit processome is shown with its subunits color-coded as in the schematic assembly pathway in (B).

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This work was done at the Klinge Lab at Rockefeller University.