Structure and biogenesis of small nucleolar RNAs acting as guides for ribosomal RNA modification

Maturation of pre-ribosomal RNA (pre-rRNA) in eukaryotic cells takes place in the nucleolus and involves a large number of cleavage events, which freq uently follow alternative pathways. In addition, rRNAs are extensively modi fied, with the methylation of the 2'-hydroxyl group of sugar residues and c onversion of uridines to pseudouridines being the most frequent modificatio ns. Both cleavage and modification reactions of pre-rRNAs are assisted by a variety of small nucleolar RNAs (snoRNAs), which function in the form of r ibonucleoprotein particles (snoRNPs). The majority of snoRNAs acts as guide s directing site-specific 2'-O-ribose methylation or pseudouridine formatio n. Over one hundred RNAs of this type have been identified to date in verte brates and the yeast Saccharomyces cerevisiae. This number is readily expla ined by the findings that one snoRNA acts as a guide usually for one or at most two modifications, and human rRNAs contain 91 pseudouridines and 106 2 '-O-methyl residues. In this article we review information about the biogen esis, structure and function of guide snoRNAs.