FUNCTION AND SYNTHESIS OF SMALL NUCLEOLAR RNAS

Eukaryotic cells contain an extraordinarily complex population of smal l nucleolar RNAs (snoRNAs). During its brief lifetime, each human pre- rRNA molecule will transiently associate with similar to 150 different snoRNA species, In the past year our understanding of snoRNAs has bee n clarified by the recognition that the snoRNA population can be divid ed into a small number of groups which are structurally and functional ly distinct, The two largest groups of snoRNAs direct the site-specifi c modification of the pre-rRNA at positions of 2'-O-methylation and ps eudouridine formation. Other groups of snoRNAs function in pre-rRNA cl eavage and in the formation of the correct structure of the pre-rRNA.