CLUSTERS OF MULTIPLE DIFFERENT SMALL NUCLEOLAR RNA GENES IN PLANTS ARE EXPRESSED AS AND PROCESSED FROM POLYCISTRONIC PRE-SNORNAS

Small nucleolar RNAs (snoRNAs) are involved in many aspects of rRNA pr ocessing and maturation. In animals and yeast, a large number of snoRN As are encoded within introns of protein-coding genes, These introns c ontain only single snoRNA genes and their processing involves exonucle olytic release of the snoRNA from debranched intron lariats. In contra st, some U14 genes in plants are found in small clusters and are expre ssed polycistronically. An examination of U14 flanking sequences in ma ize has identified four additional snoRNA genes which are closely link ed to the U14 genes, The presence of seven and five snoRNA genes respe ctively on 2.05 and 0.97 kb maize genomic fragments further emphasizes the novel organization of plant snoRNA genes as clusters of multiple different genes encoding both box C/D and box H/ACA snoRNAs. The plant snoRNA gene clusters are transcribed as a polycistronic pre-snoRNA tr anscript from an upstream promoter. The lack of exon sequences between the genes suggests that processing of polycistronic pre-snoRNAs invol ves endonucleolytic activity. Consistent with this, U14 snoRNAs can be processed from both non-intronic and intronic transcripts in tobacco protoplasts such that processing is splicing independent.