Figure 1

The ribosome synthesis pathway in eukaryotes. The initial stages of ribosome synthesis take place in the nucleolus. The first step is the association of newly synthesized 35S rRNA with 40S processing proteins and 40S ribosomal proteins which form a complex with the future 18S rRNA sequence even before the transcript is completed (the co-transcriptional assembly stage). After completion of rRNA transcription, the 35S rRNA and its associated proteins form a 90S pre-ribosome particle which contains numerous 40S processing factors and 40S ribosomal proteins but very few 60S processing proteins or 60S ribosomal proteins. RNA cleavage releases U3 snoRNP and separates the 90S particle into 40S and 60S pre-ribosome particles. The latter recruits 60S processing proteins and 60S ribosomal proteins, and the separate pre-ribosome complexes are exported out of the nucleolus into the nucleus and cytoplasm. Most of our knowledge about this pathway has been compiled from studies in budding yeast; see text for further details.