Synthesis, maturation and packaging of ribosomal RNAs (rRNAs) into ribosomal particles in eukaryotic cells takes place in the nucleolus. Ribosomal RNA genes are transcribed by RNA polymerase I into long 35/47S precursors (pre-rRNAs) which are processed into mature 18S, 5.8S and 25/28S rRNAs. The maturation process involves a large number of RNA intermediates and cleavage events which may follow alternative pathways. In addition, rRNAs are extensively modified: methylation of the 2'-hydroxyl group of sugar residues (2'-O-methylation) and conversion of uridines to pseudouridines ( PSI ) (pseudouridylation) are by far the most frequent modifications. Processing of pre-rRNA occurs concomitantly with packaging of RNA into ribonucleoprotein structures containing tens of ribosomal proteins and also nucleolar protein associating only transiently with the nascent ribosomes [for review, see 24, 25, 37, 41].
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