In eukaryotes, ribosome biogenesis starts in the nucleolus where Pol I transcribes a long rRNA precursor, which comprises rRNA species of both ribosomal subunits. Assembly of ribosomal proteins and trans-acting factors onto the rRNA leads to the formation of a large pre-ribosomal particle. Cleavage of the rRNA precursor at a specific site yields a pre-40S and pre-60S particle, which subsequently undergo separate maturation in the nucleolus and nucleoplasm before they are exported to the cytoplasm. There, final maturation steps occur and both ribosomal subunits join to form the translational competent 80S ribosome. Pol III transcribes the 5S rRNA of the 60S subunit independently in the nucleoplasm. Pol II transcribes mRNAs encoding for ribosomal proteins, which are translated in the cytoplasm and most ribosomal proteins are imported into the nucleus for incorporation into pre-ribosomal particles. The whole process is assisted by more than 200 trans-acting factors that are recycled for following rounds of ribosome synthesis. For simplicity, the contribution of snoRNPs to ribosome biogenesis is not illustrated.
To shed light on the cellular repertoire of factors involved in ribosome synthesis in mammals, we have performed a genome-wide siRNA screen on 40S subunit biogenesis in HeLa cells using a visual assay relying on RPS2-YFP localization as read-out.