FIGURE 2.

Simplified schematic of ribosome assembly in yeast. Ribosomal protein mRNAs are transcribed by RNA polymerase II (Pol II) and translated in the cytoplasm. A large number of ribosomal proteins are then imported into the nucleus, while a few ribosomal proteins assemble into late cytoplasmic ribosome biogenesis intermediates. In the nucleolus, Pol I and Pol III transcribe the 35S pre-rRNA and 5S rRNA precursors, respectively. The 35S pre-rRNA is incorporated into the SSU processome, where the U3 snoRNP-dependent A₂ cleavage separates the nascent large and small subunits, which subsequently undergo maturation and nuclear export independently. The exact timing of the incorporation of the pre-assembled 5S RNP into the nascent large subunit has not yet been determined. Final maturation takes place in the cytoplasm. Shuttling factors that travel from the nucleus to the cytoplasm bound to pre-ribosomal particles are recycled into the nucleus to participate in new rounds of ribosome biogenesis. See text for further details.