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

these snoRNAs. Yet, as more snoRNAs were discovBiologists have known for decades that the nucleolus ered, not all exhibited the hallmarks of U3, U8, and U13: is the compartment of the eukaryotic cell most densely some possessed an unmodified terminal 59 monophospacked with RNA. But few would have guessed that, in phate instead of the TMG cap and some lacked the addition to the ribosomal RNAs (rRNAs) at various characteristic box C and D sequences (Table 1). Progstages of maturation, the nucleolus contains a multitude ress in assigning functions was made by deletion/rescue of discrete small RNA molecules. These small nucleolar experiments in yeast and in Xenopus oocytes. Certain RNAs, or snoRNAs, are responsible not only for orchessnoRNAs were found to be essential for growth (U3, trating the cleavage events that cut the long pre-rRNA U14, snR10 [temperature sensitive], and snR30) in yeast into 18S, 5.8S, and 28S molecules, but also for adding and/or for specific cleavage steps in pre-rRNA profinishing touches to rRNAs as they assemble into the cessing (U3, U8, and U22) in Xenopus. Curiously, deleultimate products of the nucleolus, the ribosomal subtion of other yeast snoRNA genes had no detectable units. These finishing touches include remodeling of cereffect on growth rate or rRNA maturation (reviewed by tain rRNA uridines into pseudouridines and tagging of Maxwell and Fournier, 1995). numerous ribose moieties with methyl groups. The A completely unanticipated mode of biogenesis for amazing recent realization is that each of these modifithe particularly small (60–90 nucleotides) and less abuncations is directed by its own specific snoRNA that is dant (z104 copies per cell) members of the vertebrate packaged (as are all small nuclear RNAs) into a ribobox C/D snoRNA family then emerged (reviewed by nucleoprotein (snoRNP) particle. Maxwell and Fournier, 1995). Rather than being tranSuggesting some important function, methylated and scribed from theirown genes, these snoRNAs are intronpseudouridylated residues are confined to the most encoded. Liberated by exonucleolytic processing of exhighly conserved portions of rRNA sequences and are cised introns, these stable intronic fragments possess absent from the discarded regions of pre-rRNA. Verte59 monophosphates instead of TMG caps. SnoRNA host brate rRNAs contain approximately 100 methylated suggenes most commonly specify proteins involved in ars, 95 pseudourdines, and 10 methylated bases (Matranslation or ribosome biogenesis; some prominent exden, 1990), whereas yeast rRNAs exhibit about half as amples are ribosomal proteins, nucleolin, and translamany modifications. Eubacteria display a larger number tion factors. A surprising exception is the U22 host gene of elaborate base modifications and only a few sugar(UHG), whose spliced exons do not appear to produce methylated and pseudouridylated residues. The true a protein product; from its introns, however, are released purpose of these myriad modifications has been unclear U22 and seven other fibrillarin-associated snoRNAs no though it has been postulated that rRNA modifications longer than 85 nucleotides (Tycowski et al., 1996a). contribute in subtle ways to ribosome function. Ribose Characteristic of the shorter box C/D snoRNAs is the methylation may stabilize rRNA by increasing hydrophopresence of extensive sequence complementarity (rangbic interaction surfaces; isomerization of uridine into ing from 10 to 21 nucleotides) to highly conserved repseudouridine creates the potential for an additional gions of rRNA. Thus, a snoRNA:rRNA duplex could theohydrogen bond at the N-1 position, which may contribretically form upstream of either box D or an internal ute to rRNA folding. Also mysterious has been the mechbox D sequence, termed D9 (Figure 1A). Following the anism by which specific rRNA sites are selected for discovery of UHG, a number of approaches—creation modification since no obvious signals, either consensus of a human intron-encoded RNA library (Kiss-Laszlo et sequences or local secondary structures, are apparent. al., 1996), database searches for species with boxes C The answer, we now know, lies in simple base pairing. and D as well as complementarity to rRNA (Nicoloso et A snoRNA exhibits extensive complementarity to the al., 1996), and electrophoretic isolation from HeLa cells rRNA sequence flanking the nucleotide to be modified (Tycowski et al., 1996b)—were employed to identify over and directs, according to its class, either sugar methyla30 new examples of the so-called antisense snoRNAs.