Clusters of multiple different small nucleolar RNA genes in plants are expressed as

and processed from polycistronic pre-snoRNAs 1995; Venema and Tollervey, 1995; Tollervey and Kiss,David J.Leader, Gillian P.Clark, 1997). In addition to these essential snoRNAs, genesJenny Watters, Alison F.Beven1, encoding.80 snoRNAs have been isolated recently andPeter J.Shaw1 and John W.S.Brown2 it is expected that a large number of snoRNAs are present Cell and Molecular Genetics, Scottish Crop Research Institute, in eukaryotes (Balakin et al., 1996; Kiss-Lászlo ́ et al., Invergowrie, Dundee DD2 5DA and 1John Innes Centre, 1996; Nicoloso et al., 1996; Ganot et al., 1997a). Norwich Research Park, Colney, Norwich NR4 7UH, UK All of the snoRNAs characterized to date fall into three 2Corresponding author groups: box C/D, box H/ACA and MRP snoRNAs. Th