THE SMALL NUCLEOLAR RNAS

The present review summarizes key progress made in characterizing the small nucleolar RNAs (snoRNAs) of eukaryotic cells. Recent studies hav e shown snoRNA populations to be substantially more complex than antic ipated initially. Many newly discovered snoRNAs are synthesized by an intron-processing pathway, which provides a potential mechanism for co ordinating nuclear RNA synthesis. Several snoRNAs and snoRNP proteins are known to be needed for processing of ribosomal RNA, but precise fu nctions remain to be defined. In principle, snoRNAs could have several roles in ribosome synthesis including: folding of pre-rRNA, formation of rRNP substrates, catalyzing RNA cleavages, base modification, asse mbly of pre-ribosomal subunits, and export of product rRNP particles.