Eukaryotic cells contain many fibrillarin-associated small nucleolar RNAs (snoRNAs) that possess long complementarities to mature rRNAs. Characterization of 21 novel antisense snoRNAs from human cells followed by genetic depletion and reconstitution studies on yeast U24 snoRNA provides evidence that this class of snoRNAs is required for site-specific 2'-O-methylation of preribosomal RNA (pre-rRNA). Antisense sno-RNAs function through direct base-pairing interactions with pre-rRNA. The antisense element, together with the D or D' box of the snoRNA, provide the information necessary to select the target nucleotide for the methyltransfer reaction. The conclusion that sno-RNAs function in covalent modification of the sugar moieties of ribonucleotides demonstrates that eukaryotic small nuclear RNAs have a more versatile cellular function than earlier anticipated.

One of the most important tasks of any cell is to synthesize ribosomes. In eukaryotes, this process occurs sequentially in the nucleolus, the nucleoplasm and the cytoplasm. It involves the transcription and processing of pre-ribosomal RNAs, their proper folding and assembly with ribosomal proteins and the transport of the resulting pre-ribosomal particles to the cytoplasm where final maturation events occur. In addition to the protein and RNA constituents of the mature cytoplasmic ribosomes, this intricate process requires the intervention of numerous protein and small RNA trans-acting factors. These transiently interact with pre-ribosomal particles at various stages of their maturation. Most of the constituents of pre-ribosomal particles have probably now been identified and research in the field is starting to unravel the timing of their intervention and their precise mode of action. Moreover, quality control mechanisms are being discovered that monitor ribosome synthesis and degrade the RNA components of defective pre-ribosomal particles.