Fig. 1.

Ribosome biogenesis is a complicated process that involves all three RNA polymerases. Ribosome biogenesis begins in the nucleolus with the transcription of 47S pre-rRNA by Pol I. The 47S pre-rRNA is processed to yield 18S, 5.8S and 28S rRNAs. 5S rRNA is independently transcribed by Pol III. Ribosomal protein genes are transcribed by Pol II and their mRNAs are translated in the cytoplasm. These ribosomal proteins become incorporated into biogenesis intermediates at various steps. The 47S pre-rRNA, 5S rRNA and many ribosomal proteins form a 90S processome, which is then split into pre-40S and pre-60S subunits. These subunits subsequently move into the nucleoplasm and then to the cytoplasm, where they undergo final maturation steps. Several mechanisms, including mTOR signaling and the transcription factor MYC, regulate ribosome biogenesis on multiple levels.