Figure 3.

Ribosomal biogenesis and rRNA processing in eukaryotic cells. The initial pre-rRNA transcript is first transcribed from repetitive ribosomal DNA genes by RNA polymerase I (Pol I) in the nucleolus. rRNA precursors are then processed, chemically modified, and folded in the nucleolus, and ribosomal proteins, which are translated in the cytoplasm and imported into this organelle, concomitantly assemble with pre-rRNAs [40, 46]. There are two alternative pathways for rRNA processing in human HeLa cells [56]. Bystin is likely involved in processing of a 21S intermediate, of which the final product, 18S rRNA, is included in the 40S small subunit [13]. The importance of bystin homolog in 18S rRNA processing has also been shown in budding yeast [8] and in mouse [12].