Initial biochemical studies of snoRNAs in S. cerevisiae yielded excitement at the large variety of RNA species, but also perplexity at their possible functions [Riedel et al., 1986]. In contrast to the abundant (> 200,000 copies / cell), essential U1-U6 snRNAs already being studied in metazoans, these RNAs were of low abundance (100-1000 copies/cell), and most were found to be nonessential [Parker et al., 1988]. One strain in which five different snoRNA genes were disrupted showed no change in growth from the wild-type strain [Parker et al., 1988]. A small number of yeast snoRNAs were found to be essential (U3, U14, snR30, RNase MRP) or temperature sensitive (snR10), and all these are involved in various pre-rRNA cleavage steps. Their precise molecular roles in cleavage are still unclear, although U3 and U14 contain rRNA complementary regions that are essential for rRNA processing and viability [Beltrame & Tollervey, 1995,Liang & Fournier, 1995].
The main function of all other characterized snoRNAs is mediating ribosomal RNA modifications. Specifically, the H/ACA box snoRNAs are involved in guiding rRNA pseudouridylations [Ni et al., 1997,Gannot et al., 1997], and most C/D box snoRNAs are involved in guiding rRNA ribose methylations [Kiss-Laszlo et al., 1996,Ni, 1998]. A chronology of reviews over the past four years detailing these recent discoveries reflects the substantial progress and excitement in the field [Maxwell & Fournier, 1995,Bachellerie et al., 1995,Maden & Hughes, 1997,Smith & Steitz, 1997,Tollervey & Kiss, 1997,Bachellerie & Cavaille, 1997,Bachellerie & Cavaille, 1998,Ofengand & Fournier, 1998,Weinstein & Steitz, 1999].