Sno Storm in the Nucleolus: New Roles for Myriad Small RNPs

Biologists have known for decades that the nucleolus is the compartment of the eukaryotic cell most densely packed with RNA. But few would have guessed that, in addition to the ribosomal RNAs (rRNAs) at various stages of maturation, the nucleolus contains a multitude of discrete small RNA molecules. These small nucleolar RNAs, or snoRNAs, are responsible not only for orchestrating the cleavage events that cut the long pre-rRNA into 18S, 5.8S, and 28S molecules, but also for adding finishing touches to rRNAs as they assemble into the ultimate products of the nucleolus, the ribosomal subunits. These finishing touches include remodeling of certain rRNA uridines into pseudouridines and tagging of numerous ribose moieties with methyl groups. The amazing recent realization is that each of these modifications is directed by its own specific snoRNA that is packaged (as are all small nuclear RNAs) into a ribonucleoprotein (snoRNP) particle.